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\magnification=\magstep1
\baselineskip=4ex

\def \page {\vfill \eject}
\def \bndry {\partial}
\def \s {\sigma}
\def \sp {{\sigma^\prime}}
\def \ht {\tilde H}
\def \no {\noindent}
\def \ker {T(s,\s)}
\def \sker {t(s_B,\{\s_i\}_{i \in B})}
\def \skero {t(s_\bk,\sbk)}
\def \l {\Lambda}
\def \lp {{\Lambda^\prime}}
\def \bli {{\bndry \Lambda_\infty}}
\def \blpi {{{\bndry \Lambda^\prime}_\infty}}
\def \bl {{\bndry \Lambda}}
\def \blp {\bndry {\Lambda^\prime}}
\def \bg {{\bndry G}}
\def \bk {{B_0}}
\def \sbk {{\{\s_i\}_{i \in \bk}}}
\def \ra {\rightarrow}
\def \sgn {sgn}
\def \qed {\vrule height5pt width5pt}
\def \implies {\Rightarrow}
\def \half {{1 \over 2}}
\def \contains {\ni}
\def \t {\tau}
\def \o {\omega}
\def \dt {{d \over dt}}
\def \dszero {{d \over d s}_{s=0} }
\def \sumn {\sum_{n=2}^\infty}
\def \sumnorm {{\sum}^N}

% numbers in the examples  
\def \maxbetadecone { 1.3004}
\def \maxbetadectwo { 1.3645}
\def \alphadeconebetac {0.6667 }
\def \alphadectwobetac {0.3530 }
\def \pcrit {0.6585 }
\def \alphapcbetac {  0.9044 }
\def \maxbetapc {1.0453}
\def \plow {0.6128}
\def \phigh {0.7839}
% curve in figure6 asymptotes to this as p goes to infinity
\def \maxbetamr {0.3283}

%  lots of decimal places
%\def \maxbetadecone { 1.30048806}
%\def \maxbetadectwo { 1.36450541835 }
%\def \alphadeconebetac {0.6666666667 }
%\def \alphadectwobetac {0.3529796358 }
%\def \pcrit {0.658478948463 }
%\def \alphapcbetac {  0.9043965604 }
%\def \maxbetapc {1.045353}
%\def \plow {0.61276}
%\def \phigh {0.78391}
% curve in figure 6 asymptotes to this as p goes to infinity
%\def \maxbetamr {0.32828}

% figures 
\def \figtypes {7} 

% references

\def  \refbmo {1}
\def \refbry {2}
\def \refcg {3} 
\def  \refds {4} 
\def  \refgpa {5}
\def  \refgpb {6} 
\def  \refg {7}
\def  \refi {8}
\def  \refk {9}
\def \reftk {10}
\def \refsim {11}
\def \refmoa {12}
\def  \refmob {13}
\def \refnv {14}
\def \refo {15}
\def \refop {16}
\def \refol {17}
\def  \refve {18}
\def  \refvefk {19}
\def \refvefs {20}


\hyphenation {re-norm-al-iz-a-tion}

\bigskip \bigskip \bigskip
\bigskip \bigskip \bigskip

\centerline{ \bf Absence of renormalization group pathologies}
\centerline{ \bf near the critical temperature - two examples}

\bigskip \bigskip 
\bigskip 
\bigskip \bigskip 
\bigskip 

\centerline{Karl Haller${}^1$, Tom Kennedy${}^2$}

\bigskip \bigskip \bigskip 
\bigskip \bigskip \bigskip 

\noindent {\bf Abstract:} We consider real space renormalization 
group transformations for Ising type systems which are formally defined
by 
$$e^{-H^\prime(\s^\prime)} = \sum_\sigma T(\s,\s^\prime) e^{-H(\s)}
$$
where $T(\s,\s^\prime)$ is a probability kernel, i.e.,
$\sum_{\s^\prime} T(\s,\s^\prime)=1$, for every configuration $\s$.
For each choice of the block spin configuration $\s^\prime$, let 
$\mu_{\s^\prime}$ be the measure on spin configurations $\s$ which
is formally given by taking the probability of $\s$ to be proportional to 
$T(\s,\s^\prime) e^{-H(\s)}$.
We give a condition which is sufficient to imply that the 
renormalized Hamiltonian $H^\prime$ is defined. Roughly speaking, the 
condition is that the collection of measures $\mu_{\s^\prime}$ are
in the high temperature phase uniformly in the block spin configuration
$\s^\prime$. The proof of this result uses methods of Olivieri and Picco.
We use our theorem to prove that the first iteration of the renormalization
group transformation is defined in the following two examples:
decimation with spacing $b=2$ on the square lattice with 
$\beta < 1.36 \beta_c$ 
and the Kadanoff transformation with parameter $p$ 
on the triangular lattice in a subset of the $\beta,p$ plane that 
includes values of $\beta$ greater than $\beta_c$. 

\bigskip

\hrule 

\smallskip

\no ${}^1$ 
Program in Applied Mathematics, Bldg \# 89,
University of Arizona,
Tucson, AZ 85721, USA. Email: haller@math.arizona.edu

\no ${}^2$ 
Department of Mathematics, Bldg \# 89,
University of Arizona,
Tucson, AZ 85721, USA. Email: tgk@math.arizona.edu

\page

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%%%%%%%%%%%%%%%%  SECTION 1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\noindent {\bf 1. Introduction } 

\bigskip

Many recent papers have shown that 
position space renormalization group transformations which act on discrete
spin systems on a lattice are not defined in a variety of settings. 
Formally these transformations map a Hamiltonian $H$ into a ``renormalized''
Hamiltonian $H^\prime$ by the equation 
$$e^{-H^\prime(\s^\prime)} = \sum_\sigma T(\s,\s^\prime) e^{-H(\s)}
\eqno(1.1)
$$
The spins in the original system are denoted by $\s$. 
The ``block'' spins are denoted by $\sp$. 
Here $T(\s,\s^\prime)$ is a probability kernel, i.e.,
$\sum_{\s^\prime} T(\s,\s^\prime)=1$, for every configuration $\s$.
The spins take on a discrete set of values, e.g., $\{-1,+1\}$. 
Examples of such transformations include majority rule, decimation,
and the Kadanoff transformation. 
(Introductory discussions of these transformations may be found in 
[\refnv,\refvefs].)

The above equation defines $H^\prime$ in a finite volume, but the existence
of the infinite volume limit of $H^\prime$ is nontrivial.
Thus the renormalization group map $H \ra H^\prime$ may not even be defined.
If the temperature is very high or there is a large magnetic field
then the transformation is known to be defined [\refgpb,\refi,\refk].
At low temperatures heuristic arguments were given by Griffiths and 
Pearce [\refgpa,\refgpb,\refg] and by Israel [\refi] that the 
transformation is not defined.
Rather than using the above equation, one can define the
renormalization group transformation as a map on probability measures.
Then the hard question is whether or not the renormalized measure is
the Gibbs measure of some Hamiltonian.
Van Enter, Fern{\'a}ndez and  Sokal proved that the renormalized measure
is not Gibbsian for a variety of models at low temperature [\refvefs].
Further examples of renormalization group pathologies may be found in 
[\refmoa,\refmob,\refve,\refvefk].

A key tenet of the  renormalization group is that while the correlation 
length of the original system diverges at  a second order transition,
the introduction of the block spins should make the correlation length
finite. More precisely, if one fixes a choice of the block spin 
configuration, then the system of original spins conditioned on 
this block spin configuration will not have a phase transition at the 
point where the original system does. 
The introduction of the block spins should shift the location
of the critical point to a lower temperature.
While this belief may be obvious in momentum space transformations
in which one integrates out a slice of momentum, it is not obvious
for these position space transformations. 
For three particular choices of the block spin configuration, including
the checkerboard configuration, 
Kennedy proved that for the majority rule in two dimensions with two by two
blocks, the critical temperature is indeed lowered by conditioning on 
the block spin configuration [\reftk]. 
Benfatto, Marinari, and Olivieri did a Monte Carlo  study
of a renormalization group
transformation in which the block spin is equal to the sum of the spins
in the block [\refbmo]. 
(So the block spins do not just take on two values as 
the original spins do.) 
They considered the block spin configuration in which all the block spins are
zero. They found that the introduction of this block spin 
configuration does indeed lower the critical temperature, but only 
by about 10\% . However, preliminary Monte Carlo calculations
of Ould-Lemrabott on the two dimensional Ising model with majority rule
indicate that the introduction of the block spins lowers the critical
temperature by at least a factor of two [\refol]. 

To state our main theorem we need some assumptions and definitions.
We consider only finite range, translation invariant Hamiltonians. 
We only consider Ising type systems, i.e., the spin space at each site
is $\{-1,+1\}$.
The renormalization group kernel $T(\s,\s^\prime)$ is a product over
blocks of a local function of the block spin and the spins in the original
lattice in that block. We assume that $T(\s,\s^\prime)$ is always 
greater than zero. At first glance this last assumption appears to 
rule out many examples, in particular decimation and majority rule. 
However, it is often possible to reformulate the original system in 
such a way that $T(\s,\s^\prime)$ is never zero. Consider decimation.
Usually one takes $T(\s,\s^\prime)$ to be 1 if for every
site $i$ at which there is both an original spin $\s_i$ and a block spin 
$\s^\prime_i$ we have $\s_i=\s^\prime_i$. 
However, we can instead just think of 
$\s$ as consisting only of the original spins that live at sites without
a block spin. All the other original spins are just set equal to the
corresponding block spin. With this reduced $\s$, $T(\s,\s^\prime)$ is 
always 1. 
For majority rule we can obtain an equivalent system with 
$T(\s,\s^\prime)>0$ by first summing out some of the spins in the 
original system. We do not provide the details of this procedure 
since we have not been able to verify the hypothesis of 
our main theorem for majority rule.

Finally, we give the definitions needed for our theorem.
Let $V$ be a finite set of sites in the original lattice, and 
$\tau$ a boundary condition for $V$, i.e., a spin
configuration on the sites outside of $V$.
Let $\sp$ be a block spin configuration. Then we define a probability
measure which depends on $V, \tau$ and $\sp$ by 
$$\mu_{\sp,V,\tau}(F) = {\sum_\s F(\s) T(\s,\s^\prime) e^{-H(\s)}
\over \sum_\s T(\s,\s^\prime) e^{-H(\s)}}
\eqno(1.2)
$$
Here $F(\s)$ is a function on the original spin configurations $\s$
and $\mu(F)$ denotes the expectation of such a function with respect
to the probability measure $\mu$. 
The sums over $\s$ are over the spin configurations on $V$. 
The Hamiltonian $H(\s)$ is the Hamiltonian for the volume $V$ using
the boundary condition $\tau$ outside of $V$. 
The kernel $T(\s,\s^\prime)$ is the product over blocks which 
intersect the volume $V$ of the kernel for that block.
Note that the block spin configuration is fixed throughout the above.

\medskip

\no {\bf Theorem 1.1:} Suppose there exist constants $c<\infty$ and $m>0$
such that for every finite subset $V$ of the lattice, every 
two sites $i,j \in V$, every boundary condition $\tau$ {\it and every
block spin configuration $\sp$}
$$| \mu_{\sp,V,\tau}(\s_i \s_j) 
- \mu_{\sp,V,\tau}(\s_i) \, \mu_{\sp,V,\tau}(\s_j) |
\le c \exp(-m |i-j|) \eqno(1.3) $$
Then the infinite volume limit of the renormalized Hamiltonian 
$H^\prime(\sp)$ exists. It may be written in the form
$$H^\prime(\sp) = \sum_X H^\prime_X(\sp) \eqno(1.4)
$$ 
where $H^\prime_X(\sp)$ only depends on the block spins in $X$
and the sum is over finite sets of block spin sites.
Furthermore, there is a $\mu>0$ such that
$$\sum_{X \ni 0} e^{\mu ||X||} 
||H^\prime_X||_\infty < \infty  \eqno(1.5)
$$
where $||X||$ denotes the cardinality of the 
smallest connected set of block spin sites which contains $X$.

\bigskip

We will show that 
if the Dobrushin uniqueness condition is satisfied uniformly in the 
block spins, then the hypothesis of our theorem is satisfied.
We verify numerically that the Dobrushin condition is satisfied
uniformly in two examples.
The first example is decimation for the two dimensional Ising model 
with scale factor $b=2$. 
Van Enter, Fern{\'a}ndez and  Sokal proved that the renormalized measure
is not 
Gibbsian in this example for $T < T_c /1.73$. We find that the 
Dobrushin condition is satisfied uniformly, 
and hence the renormalized Hamiltonian
is defined, for $T>T_c/1.36$. In particular it is defined in a neighborhood
of the critical point.
The second example is the Kadanoff transformation for the triangular
lattice in two dimensions. The Kadanoff transformation contains a 
parameter $p$. In the limit $p \ra \infty$, the Kadanoff transformation 
becomes the majority rule transformation. We find that there is an
interval of values of $p$ for which the Dobrushin condition
is satisfied uniformly in the block spins for temperatures slightly
below the critical temperature. Thus there are values of $p$ for 
which the Kadanoff transformation is defined in a neighborhood of the 
critical point.

The hypothesis of the theorem is similar to one of Dobrushin and Shlosman's
many equivalent definitions of completely analytic interactions [\refds].
Indeed, if for every block spin configuration 
the interaction (including the renormalization group
kernel) is completely analytic with the constants that appear in this
property independent of the block spin configuration, then the hypothesis 
of the theorem holds. We prove theorem 1.1 by developing a convergent 
polymer expansion. Such an expansion was developed for completely 
analytic interactions by Olivieri  and Picco [\refo,\refop]. We do not
rely on any of their results, but instead give a different development
of the expansion. Our motivation for doing this, besides the maxim
that a good theorem deserves more than one proof, is to make this paper
as self-contained as possible. We assume the reader knows polymer 
expansions, and for the examples we assume familiarity with the 
Dobrushin uniqueness theorem and related results at the level of sections
V.1 and V.2 of [\refsim]. We do not assume familiarity with 
Dobrushin and Shlosman's work on completely analytic potentials
[\refds] or with the work of Olivieri and Picco [\refo,\refop]. 

To prove that an expansion converges one must usually require that some
parameter, e.g., the inverse temperature, be small. 
For an expansion that will work for all completely analytic interactions,
one must obtain this smallness from something besides the inverse
temperature. Following Olivieri and Picco, our expansion involves a 
length scale $L$ which will be chosen to be large compared to the
correlation length of the system. Correlations between observables
separated by at least a distance $L$ are then very small. This is
the smallness that drives the expansion.

Our main theorem gives a sufficient condition for the existence of the 
renormalized Hamiltonian. It is conceivable that this condition is 
much stronger than what is needed for existence of the renormalized
Hamiltonian. For example, there might be some block spin configurations
for which the hypothesis of our theorem is not satisfied, but 
these block spin configurations could have probability zero in the
renormalized measure and so not cause any problems. 
For a renormalization group transformation which is somewhat different
from those considered in this paper, it has been suggested that
if the introduction of one particular block spin configuration 
puts the system in a high temperature phase, then the renormalized 
Hamiltonian exists [\refcg]. However, there is an example 
which shows that this is not true in general [\refve]. 

In this paper we have only used the Dobrushin uniqueness theorem 
to show in particular examples that the hypothesis of our theorem 
is satisfied. This approach does not work in a neighborhood of the
critical point in an important example, the majority rule for 
two by two blocks in two dimensions. One can hope that the more
general methods of Dobrushin and Shlosman [\refds] might work in
examples like this. 

Our two examples are discussed in section two. Section three contains
the proof of the main theorem. 
The appendix shows how to use the Dobrushin uniqueness condition to
verify the hypothesis of the main theorem using only results presented in 
[\refsim]. 

\bigskip
\bigskip



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%  SECTION 2 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\def \eqx {2.1}
\def \eqy {2.2}
\def \eqa {2.3}
\def \eqb {2.4}


\no {\bf 2. Examples }

\bigskip

In this section we consider two examples in which our theorem may be used
to prove that the first iteration of the renormalization group is 
defined in a neighborhood of the critical point. 
We verify the hypothesis of the theorem 
by showing that Dobrushin's condition for 
uniqueness of the Gibbs state is satisfied uniformly in the block 
spin configuration.
We begin by reviewing Dobrushin's condition. We follow the notation 
and exposition of section V.1 of [\refsim] closely.
For each site $j$, $\mu_j(\s,\s_j)$ is the measure on the spin space 
$\{-1,+1\}$ at the site $j$. The $\s$ denotes the values of the spins 
at all the sites other than $j$. 
Define  
$$
\eqalign{
\rho_{ij} &= \half \sup \{ ||\mu_j(\s,\cdot) - \mu_j(\omega,\cdot)|| \, : \,
\s_k=\omega_k \, \, if \, \, k \ne i \} 
} \eqno (\eqx)
$$
Then $\rho_{ij}$ measures the amount of change in the distribution of the 
spin at site $j$ when we flip the spin at site $i$. 
Next define
$$
\eqalign{
\rho_j &=\sum_{i \ne j} \rho_{ij} \cr
\alpha &= \sup_j \rho_j \cr
}  \eqno (\eqy)
$$
Then Dobrushin's theorem says that if $\alpha <1$ then there is a unique 
Gibbs state.
For a finite range Hamiltonian the above condition leads to much
more, e.g., exponential decay of the truncated correlations.
In our setting $\alpha$ depends on the block spin configuration $\sp$.
If $\alpha<1$ uniformly in $\sp$, then we get uniform exponential 
decay of the correlations.
This decay can be used to prove hypothesis (1.3) of the main theorem.
The argument is given in the appendix.
The resulting proposition  is as follows.

\medskip

\no {\bf Proposition 2.1 :} Define $\alpha$ as above. If 
$\sup_\sp \alpha <1$,
then hypothesis (1.3) of theorem 1.1 follows. 

\bigskip

\no{\bf Remark:} In our examples, translation invariance will imply that
$\sup_{\sp} \rho_j$ is independent of $j$. 
So $\sup_{\sp} \alpha=\sup_\sp \rho_j$ for any choice of $j$. 
We will label the site $j$ that we use by 0 in the examples.

\bigskip

The first example we consider
is decimation on the square lattice with $b=2$, 
i.e., the spins on a sublattice
with spacing 2 are considered as the block spins and the rest of the 
spins are summed out. We prove that this transformation is defined
for $\beta < \maxbetadectwo  \beta_c$.


Decimation with $b=2$ is equivalent to two iterations of the decimation
transformation with $b= \sqrt 2$. The first iteration is trivial;
the renormalized Hamiltonian, $H_1$, may be computed explicitly. The four
nearest neighbors of each spin that must be summed over are block spins,
so the sum may be done explicitly. (See figure 1.) 
$$\eqalign{\sum_{\s_0} \exp[\beta \s_0 & (\s_1+\s_2+\s_3+\s_4)] \cr
& = \exp[a(\s_1 \s_2+\s_1 \s_3 + \s_1 \s_4 + \s_2 \s_3 + \s_2 \s_4 + \s_3 \s_4)
+ b \s_1 \s_2 \s_3 \s_4 + c] 
} \eqno(\eqa)
$$
with
$$\eqalign { 
a &= { 1 \over 8} \ln ( \cosh(4 \beta)) \cr
b &= { 1 \over 8} \ln ( \cosh(4 \beta)) -  { 1 \over 2} \ln ( \cosh(2 \beta))
 \cr
}
$$
There is a similar formula for $c$, but it plays no role in the renormalized 
Hamiltonian. The lattice that remains after this first iteration of decimation
with $b=\sqrt 2$ has spacing $\sqrt 2$. We rescale it so that it has spacing
1.  Then the terms $\s_1 \s_2$, $ \s_1 \s_4 $, $ \s_2 \s_3 $ and 
$ \s_3 \s_4$
are nearest neighbor terms in the renormalized Hamiltonian, and the 
terms $\s_1 \s_3 $ and $ \s_2 \s_4 $ contain lattices sites that are
a distance $\sqrt 2 $ apart. The term $\s_1 \s_2 \s_3 \s_4 $ is the 
product of the four spins in a plaquette. Thus the renormalized Hamiltonian
from the first iteration of the $b = \sqrt 2$ decimation transformation is
$$H_1 = 2 a \sum_{<i,j>:|i-j|=1} \s_i \s_j 
+ a \sum_{<i,j>:|i-j|=\sqrt 2} \s_i \s_j
+ b \sum_P \prod_{i \in P} \s_i
\eqno(\eqb)
$$
where the first sum is over nearest neighbor bonds, the second sum is over
diagonal bonds and the third sum is over plaquettes $P$. 
The factor of 2 in the first term appears because for each bond in 
this sum there is a contribution from two different sums of the form (\eqa).
(In all of the above sums, each bond is only summed over once.)

The second iteration of the $b = \sqrt 2$ decimation transformation 
must be applied to the Hamiltonian $H_1$, and so it is not trivially 
computable. We use our main theorem to prove that it is defined, and we use the
proposition at the  start of this section to verify the hypothesis of the
main theorem.
Considering the types of terms that appear in $H_1$, the spins we need
to consider to test the Dobrushin condition are shown in figure 2.
The site at which we test the condition is $0$. The spins at sites
$1,2,3,4$ are block spins. Following our convention of denoting block
spins by $\sp$ and original spins by $\s$, these four spins are denoted
$\s^\prime_1,\s^\prime_2,\s^\prime_3,\s^\prime_4$.  


The terms in $H_1$ that involve the spin at
site 0 are 
$$\eqalign{ H & = 2 a (\s^\prime_1+\s^\prime_2+\s^\prime_3+\s^\prime_4) \s_0 \cr
 & + (a+b \s^\prime_1 \s^\prime_2) \s_0 \s_5
 + (a+b \s^\prime_2 \s^\prime_3) \s_0 \s_6
 + (a+b \s^\prime_3 \s^\prime_4) \s_0 \s_7
 + (a+b \s^\prime_1 \s^\prime_4) \s_0 \s_8
} 
$$
At $\beta=\beta_c$ we find that $\sup_\sp \alpha=\alphadeconebetac$. 
This implies that $\sup_\sp \alpha<1$
in a neighborhood of $\beta_c$. In fact we find that $\sup_\sp \alpha<1$ for 
$\beta< \maxbetadecone  \beta_c$.


We can extend the interval of $\beta$ for which we can prove that
$b=2$ decimation is defined with a little more work. In the above we
did not test the Dobrushin condition on the original system. Indeed, 
such a test must fail at $\beta_c$. 
With $b=2$, some of the sites have two nearest neighbors that are block
spins and two that are original spins, but there are also spins that
have four nearest neighbors that are original spins. For such sites $j$
the quantity $\rho_j$ is the same as it would be in the Ising model
with no decimation, and so $\rho_j$ cannot be $<1$ at $\beta_c$. 
What was crucial in the above was that we first summed out some of the 
original spins before we tested the Dobrushin condition. 
(``Original spins'' refers to spins in the original Hamiltonian, i.e., 
non-block spins.) 
The subset of
spins we summed out was a sublattice with spacing $\sqrt 2$. 
Consider figure 3. 
The block spins are indicated by B's and the original
spins by circles  and X's. We do the sum over the original spins by first
summing over the spins indicated by circles, and then over those indicated by 
X's. The spins in the first category break up into groups of five spins
where each spin in the group has a nearest neighbor interaction
only with other spins in the group, block spins and spins in the second
category. Thus the sum over these five spins is a finite calculation.
The result of this calculation is a new Hamiltonian for the spins
in the second category and the block spins. Having summed out the spins
in the first category, we use our theorem for the sum over the spins in 
the second category. Computing the effective Hamiltonian that results
from the sum over the spins in the first category 
and then testing that the Dobrushin
condition for the spins in the second category holds uniformly in 
the block spins is a bit of computation.
At $\beta=\beta_c$ we find $\sup_\sp \alpha=\alphadectwobetac$. 
The Dobrushin condition $\sup_\sp \alpha<1$ holds for 
$\beta< \maxbetadectwo  \beta_c$. 

\bigskip

The second example we consider is the Kadanoff transformation on
the triangular lattice. For the triangular lattice the blocks are 
triangles containing three sites. So the block spins live on a lattice
with spacing $\sqrt 3$. 
The blocking of the triangular lattice is shown in figure 4.
\no If $\s_1,\s_2,\s_3$ are the three spins in a block and $\sp$ is the 
block spin, then the kernel for the Kadanoff transformation for a single
block is
$$t(\s_1,\s_2,\s_3,\sp)= 
   { \exp[p \sp (\s_1+\s_2+\s_3)] \over
2 \cosh[p(\s_1+\s_2+\s_3)] }
$$
where $p>0$ is a parameter. As $p \ra \infty$ we obtain the majority rule. 
The Kadanoff transformation may be defined in a much more general setting.
Given a blocking of a lattice, the kernel is given by the above formula
with $\s_1+\s_2+\s_3$ replaced by the sum of the spins in the block.
For the hypercubic lattice in two or more dimensions, 
van Enter, Fern{\'a}ndez and Sokal [\refvefs] proved that for all $p>0$, 
the renormalized measure is 
non Gibbsian at sufficiently low temperature. 

We can rewrite the kernel as 
$$t(\s_1,\s_2,\s_3,\sp)= 
    \exp[p \sp (\s_1+\s_2+\s_3) + q (\s_1 \s_2+\s_1 \s_3+\s_2 \s_3)
+c]
$$
where $q$ and $c$ are functions of $p$. They are determined by 
$$
q (\s_1 \s_2+\s_1 \s_3+\s_2 \s_3) + c =
- \log (2 \cosh [p(\s_1+\s_2+\s_3)]) 
$$
which after a little algebra implies 
$$q = - {1 \over 4} [\log(\cosh(3 p))-\log(\cosh(p))] $$
Note that $q$ is negative, so these terms in the kernel have the 
opposite sign of the nearest neighbor interactions in the Hamiltonian.
Thus there is some cancellation between terms in the renormalization group
kernel and nearest neighbor interactions for which the two spins are in 
the same block.
One third of the nearest neighbor interactions are between spins in the 
same block. 

We now ask if there are any values of $p$ for which we can prove that the
renormalized Hamiltonian is defined in a neighborhood of $\beta=\beta_c$.
To apply our theorem we can try testing the Dobrushin condition. 
Even if we choose $p$ so that $q= - \beta_c$ to get the most possible 
cancellation between the kernel and terms from the original Hamiltonian,
we find that at $\beta=\beta_c$ the Dobrushin condition is not
satisfied. Instead we do something similar to what we did for decimation.
We sum out a subset of the original spins before we apply our theorem.
The subset that we sum out consists of all the original spins that
are at the top of the triangular block that they are in. (So the subset
contains one third of the original spins.) 
Each spin being 
summed out interacts only with spins not being summed out and with 
block spins. Thus we can do this initial summation explicitly. 
This initial summation then produces a new Hamiltonian for the remaining
original spins. We then check the Dobrushin condition for this 
new Hamiltonian. Figure 5 shows the spins that are involved in 
checking the condition. 
We want to check the condition for the spin 
at site 0. The spins at sites 1,2 and 3 are in the group that is summed 
out initially. For example, the sum over the spin at site 1 amounts to 
computing $f(\s_0,\s_4,\s_7,\s_8,\s_9,\s_5,\s^\prime_1)$
where 
$$\eqalign{
 &\sum_{\s_1} \, \exp[(\beta+q)\s_1 \s_0  +(\beta+q)\s_1 \s_4 
+ \beta \s_1 \s_7 + \beta \s_1 \s_8 + \beta \s_1 \s_9 + \beta \s_1 \s_5 \cr
& \quad \quad \quad \quad \quad 
+ p \s^\prime_1 \s_1+ p \s^\prime_1 \s_4+ p \s^\prime_1 \s_0] 
=\exp[f(\s_0,\s_4,\s_7,\s_8,\s_9,\s_5,\s^\prime_1)]
}
$$
Here $\s^\prime_1$ is the block spin which is denoted by B1 in figure 5. 

The choice of $p$ which yields $q= -\beta_c$ is $p=\pcrit$. For this 
value of $p$ we find $\sup_\sp \alpha=\alphapcbetac$ at $\beta=\beta_c$ and 
$\sup_\sp \alpha<1$ for $\beta<\maxbetapc \beta_c$. 
With $\beta=\beta_c$ we find
that $\sup_\sp \alpha<1$ for $\plow < p < \phigh$.
Figure 6 shows the region in the $\beta,p$ plane for which 
$\sup_\sp \alpha < 1$, and hence for which 
the first iteration of the Kadanoff transformation is defined.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%  SECTION 3 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% eq numbers 
\def \eqz {3.1}
\def \eqa {3.2}
\def \eqfe {3.3}
\def \eqaa {3.4}
\def \eqb {3.5}
\def \eqd {3.6}
\def \eqx {3.7}
\def \eqy {3.8}
\def \eqt {3.9}
\def \eqc {3.10}
\def \eqcc {3.11}
\def \eqe {3.12}
\def \eqbb {3.13}
\def \eqg {3.14}
\def \eqh {3.15}
\def \eqi {3.16}
\def \eqj {3.17}
\def \eqhh {3.18}
\def \eqk {3.19}
\def \eqdd {3.20}
\def \eqm {3.21}
\def \eqw {3.22}
\def \eqee {3.23}
\def \eqo {3.24}
\def \eqff {3.25}
\def \eqgg {3.26}
\def \eqr {3.27}

\def \lemmaa {3.2 \ }
\def \lemmab {3.3 \ }
\def \lemmac {3.4 \ }


\def \bigo {O}
\def \bigol {\bigo (L^p e^{-mL})}
\def \wei {W}

\bigskip


\noindent {\bf 3. Proof of the main result}

\bigskip

Throughout this section we will use a Fourier series representation of 
functions of Ising spins. If $V$ is a finite set of sites, then 
every function $F(\s)$ of the spins in $V$ may be written in the form
$$F(\s)= \sum_X c_X \s^X \eqno(\eqz)$$
where $X$ is summed over all subsets of $V$ (including the empty set),
$\s^X = \prod_{i \in X} \s_i$, and the constants $c_X$ are given by
$$c_X = \sum_\s \, \s^X \, F(\s) \eqno(\eqa) $$
The sum is over the spin configurations $\s$ on $V$.
This sum is normalized so that $\sum_\s \, 1 =1 $, i.e., we include 
a factor of $2^{-|V|}$ in the definition of the sum. 
Throughout this section all sums over spin configurations will be normalized
so that the sum of 1 is 1. 

As is standard for expansion methods, we 
work in a finite volume $V$, but all our estimates will be uniform 
in the volume. The existence of the infinite volume limit will follow
by the usual arguments. 
In the following most quantities depend
on the finite volume $V$, the choice of boundary condition outside 
of it and the block spin configuration, but we do not make this dependence
explicit.
In the following the partition function we want to compute is of the form
$$Z=  \sum_\sigma T(\s,\s^\prime) e^{-H(\s)}
$$
where $T(\s,\s^\prime)$ is the renormalization group kernel. This kernel
is a product over blocks of a local kernel, and in the above we only include 
the terms corresponding to the blocks in $V$. 
Recall that we are assuming that the kernel $T(\s,\s^\prime)$ 
is always greater than zero. 
Thus we may take its logarithm and simply include it in the 
Hamiltonian. 
So in the following, $e^{-H(\s)}$ will actually stand for
$T(\s,\s^\prime) e^{-H(\s)}$. We will usually just denote this by 
$e^{-H}$.  
(Since the kernel is a product over blocks of a function
of the spins in that block, the logarithm is a finite range interaction.)

The first step in the proof is to show that hypothesis (1.3) of the 
theorem implies a condition  on free energies. By free energy we will
always mean minus the logarithm of the partition function. Of course
this differs from the usual definition by a factor of $\beta$. 

\medskip

\no {\bf Lemma 3.1:} Suppose that hypothesis (1.3) holds.
For a finite volume $V$, a boundary condition $\tau$ outside of $V$ 
and a block spin configuration $\sp$, let $F_{\sp,V,\tau}$ be 
the free energy, i.e., minus the logarithm of the partition function.
Then there is a 
constant $c$ such that for every finite volume $V$, every boundary 
condition $\tau$, every block spin configuration $\sp$ and 
every two sites $i,j \notin V$ we have
$$| \sum_{\tau_i,\tau_j} \, \tau_i \tau_j \, F_{\sp,V,\tau} | \le c e^{-m|i-j|}
\eqno(\eqfe)$$
Here $m$ is the same constant that appears in (1.3), but $c$ is a new
constant. 

\bigskip

\no {\bf Proof:} 
When we compute $F_{\sp,V,\tau}$, the Hamiltonian we use consists of
those terms in the infinite volume Hamiltonian whose support intersects $V$. 
To make a connection with the correlation functions that appear in 
hypothesis (1.3),
we now define a slightly different free energy for the volume $V$. 
Let $W$ be $V \cup \{i,j\}$. Let $F_{\sp,V,\tau}^\prime$ be the free 
energy for the volume $V$ computed using the Hamiltonian that consists 
of all the terms in the infinite volume Hamiltonian whose support 
intersects $W$. Note that in both $F_{\sp,V,\tau}$ and 
$F_{\sp,V,\tau}^\prime$ we sum over the spins in $V$. The only difference
is that in the latter we include some additional terms in the Hamiltonian.
But since these terms do not involve any spins in $V$, the relationship
between $F_{\sp,V,\tau}$ and $F_{\sp,V,\tau}^\prime$ is trivial. Their 
difference equals the sum of the terms in the Hamiltonian whose 
support intersects $W$ but not $V$. In particular
$$ \sum_{\tau_i,\tau_j} \, \tau_i \tau_j \, F_{\sp,V,\tau}^\prime
= \sum_{\tau_i,\tau_j} \, \tau_i \tau_j \, F_{\sp,V,\tau}
$$
if $|i-j|$ is large enough (depending on the range of the Hamiltonian).
So it suffices to prove (\eqfe) with $F_{\sp,V,\tau}$ replaced by 
$F_{\sp,V,\tau}^\prime$.

In (\eqfe), $V$, $\sp$ and $\tau$ do not change except 
for $\tau_i$ and $\tau_j$. 
So we will denote  $F_{\sp,V,\tau}^\prime$ by simply $F(\tau_i,\tau_j)$.
Define 
$$\eqalign{A &= \exp[-F(+1,+1)] \cr
           B &= \exp[-F(-1,-1)] \cr
           C &= \exp[-F(+1,-1)] \cr
           D &= \exp[-F(-1,+1)] \cr
}
$$
Then 
$$\sum_{\tau_i,\tau_j} \tau_i \tau_j \, F_{\sp,V,\tau} = - \ln(AB/CD) 
$$
$\tau$ gives a boundary condition
for the volume $W$ in an obvious way; we simply drop $\tau_i$ and $\tau_j$. 
We will apply (1.3) to the volume
$W$. A little computation shows that
$$\mu_{\sp,W,\tau}(\s_i \s_j) -\mu_{\sp,W,\tau}(\s_i)
\, \mu_{\sp,W,\tau}(\s_j) = {4(AB-CD) \over (A+B+C+D)^2 }
$$
We will show $\ln(AB/CD)$ is small by showing that $AB/CD$ is close to 1. 
We start with
$$ \eqalign{
\left| {AB \over CD} -1 \right| &= {|AB-CD| \over CD}  \cr
& = {|AB-CD| \over (A+B+C+D)^2} {(A+B+C+D)^2 \over CD }   \cr
}
$$
Our assumptions on the Hamiltonian and $T(\s,\sp)$ easily imply
that the change in the free energy when any 
single boundary spin is flipped is bounded by a constant. Hence there
is a constant $M$ such that the ratio of any two of $A,B,C$ and $D$ 
is bounded above by $M$. 
This implies 
$${(A+B+C+D)^2 \over CD }  \le 16 M$$
So we have
$$ \left| {AB \over CD} -1 \right| 
   \le {|AB-CD| \over (A+B+C+D)^2} 16 M = 4 M 
|\mu_{\sp,W,\tau}(\s_i \s_j) -\mu_{\sp,W,\tau}(\s_i)
\mu_{\sp,W,\tau}(\s_j)|
$$
and so (1.3) implies (\eqfe). 
\qed

We divide the lattice into blocks which are $L$ sites long on each side 
and so contain $L^\nu$ sites. These blocks are not the blocks used by
the renormalization group transformation. When we wish to emphasize this
fact we will refer to these blocks as $L$-blocks. 
Whenever we refer to $L$-blocks, we mean only those $L$-blocks that appear
in this partitioning of the lattice. There are other blocks 
with side $L$ that are not part of this partition, but they will never appear
in our proof. 
$L$ will be chosen large and we also 
choose it so that our $L$-blocks are commensurate with the blocks in
the renormalization group transformation, i.e., each renormalization group
block is a subset of an $L$-block.
To keep the notation under
control, we now restrict our attention to two dimensions; the generalization
to higher dimensions is straightforward.
(In a few places where the dependence of a quantity on the number of 
dimensions is significant, we will state the result for an arbitrary 
number of dimensions, denoting the number of dimensions by $\nu$. )
We divide the $L$-blocks into four types labelled by $i=1,2,3,4$
as shown in figure \figtypes. 
(In $\nu$ dimensions there would be $2^\nu$ types of blocks.) 
The crucial property is that the distance between any two $L$-blocks of the
same type is at least $L$. 
Let $\sum_i$ denote the summation over the spins in $V$ 
which are in a type $i$ $L$-block. Then we trivially have
$$Z= \sum_4 \sum_3 \sum_2 \sum_1 \exp(-H) \eqno (\eqaa)$$

We will use $b$ to denote an $L$-block. The notation $b:i$ means that
$b$ is a type $i$ block. For example, $\prod_{b:i}$ is the product over
all blocks $b$ of type $i$. Let $\s_b$ denote the spin configuration on $b$.
Then we have
$$\sum_i = \prod_{b:i} \sum_{\s_b}$$
We start by considering $\sum_1 e^{-H}$. 
Write the Hamiltonian in its Fourier representation
$$H= \sum_X c_X \s^X
$$
Given a set of sites $X$ we define $\bar X$ to be the union of all the
$L$-blocks that contain at least one site in $X$. We think of $\bar X$ as
the support of the term $c_X \s^X$ viewed on the scale $L$. 
We use $B$ to denote a union of $L$-blocks. In everything that follows, 
{\it the only $B$'s that appear are those that are small enough that they are
contained in some $3L$ by $3L$ block}. For such a $B$ we define
$$H_B=\sum_{X:\bar X=B} c_X \s^X$$
Since $H$ is finite range, if $L$ is chosen large enough, then every term 
in $H$ will be in exactly one $H_B$ and so $H = \sum_B H_B$. 
Also, no term in $H$ can contain sites from two different type 1 blocks. 
So $H_B=0$ if $B$ contains more than one type 1 block.
Thus we have
$$H=\sum_{B:no \, 1} H_B + \sum_{b:1} \sum_{B:b \subset B} H_B \eqno(\eqb)$$
where $B:no \, 1$ means that $B$ does not contain any type 1 blocks.
Define $F^1$ by 
$$\exp(-F^1) = \sum_1 \exp(-H) \eqno(\eqd)$$
So $F^1$ is a function of the spins in blocks of types $2,3$ and $4$.
Equation (\eqb) implies that the sum 
in (\eqd) factors into a product over
type 1 blocks of the sum over the spins in that block. 
Thus we have
$$F^1=\sum_{B:no \, 1} H_B  + \sum_{b:1} F^{1,b} \eqno(\eqx)
$$
where for type 1 blocks $b$ we define
$$F^{1,b}= - \ln[\sum_{\s_b} \exp(- \sum_{B:b \subset B} H_B)] \eqno(\eqy)
$$

Next we try to compute $\sum_2 \exp(-F^1)$. However, $F^1$ can contain terms
which involve spins in more than one type 2 block. Thus this sum does not
factor into a product of independent sums over the type 2 blocks. 
Note that the terms in $F^1$ which prevent the factorization
are supported on sets of sites with diameter greater than $L$. 
To proceed we need to distinguish these long range terms that prevent
the factorization from the short range terms in $F^1$ that do not.
We do this by looking at the supports of the terms in $F^1$ on scale $L$. 
Write $F^1$ in its Fourier representation.
$$F^1 = \sum_X c^1_X \s^X
$$
and define for sets $B$ which are a union of $L$-blocks,
$$F^1_B = \sum_{X:\bar X=B} c^1_X \s^X \eqno(\eqt)
$$ 
So $F^1 = \sum_B F^1_B$.
The definition of $F^{1,b}$ implies that $F^{1,b}$ is supported in 
a neighborhood of $b$, and so is supported in the $3L$ by $3L$ square
centered about $b$. Thus $F^1_B$ is nonzero only if $B$ is a subset of
some $3L$ by $3L$ block.
We say $B$ is long range (LR) if it 
contains two $L$-blocks which are separated by a distance of at least $L$.
Otherwise we say $B$ is short range (SR). 
The terms that prevent the factorization are the $F^1_B$'s for $B$'s that 
contain at least two type 2 blocks. Such $B$'s are long range. 
So if we define
$$\eqalign{ F^1_{SR} &= \sum_{B:SR} F^1_B  \cr
            F^1_{LR} &= \sum_{B:LR} F^1_B  \cr
} \eqno(\eqc) 
$$
then $\sum_2 \exp(-F^1_{SR})$ will factor into a product over the
type 2 blocks. If $F^1_{LR}$ is small, then we can hope to use the 
factorization that occurs when $F^1_{LR}=0$ to develop a polymer expansion.

We define $F^2$ by
$$\exp(- F^2)= \sum_2 \exp(- F^1_{SR})$$
We will show that the computation of $F^2$ is a local operation, just as the
computation of $F^1$ was. 
The short range $B$ contain at most one type 
2 block. Thus we may write $F^1_{SR}$ as  
$$F^1_{SR}=\sum_{B:SR} F^1_B 
=\sum_{B:SR, \, no 2} F^1_B + \sum_{b:2} \sum_{B:SR, \, b \subset B} F^1_B 
$$
and so
$$F^2 =  \sum_{B:SR, \, no 2} F^1_B + \sum_{b:2} F^{2,b}
$$
where for type 2 blocks $b$ we define
$$F^{2,b}= - \ln[\sum_{\s_b} \exp(- \sum_{B:SR, \, b \subset B} F^1_B)]
$$

We continue the above definitions inductively. Given $F^i$ we decompose
it as $F^i = \sum_B F^i_B$ where $F^i_B$ contains the terms in $F^i$
whose support $X$ satisfies $\bar X=B$. Then
$F^i = F^i_{SR} + F^i_{LR} $ with 
$$\eqalign{ F^i_{SR} &= \sum_{B:SR} F^i_B  \cr
            F^i_{LR} &= \sum_{B:LR} F^i_B  \cr
} \eqno(\eqcc) 
$$
The terms in $F^i_{LR}$ prevent the sum over spins in type $i+1$ blocks
from factoring, so we define $F^{i+1}$ by
$$\exp(- F^{i+1})= \sum_{i+1} \exp(- F^i_{SR}) \eqno(\eqe)
$$
As before the computation of $F^{i+1}$ is local:
$$F^{i+1} =  \sum_{B:SR, \, no \, (i+1)} F^i_B + \sum_{b:{i+1}} F^{{i+1},b}
$$
where for type $i+1$ blocks $b$ we define
$$F^{{i+1},b}= - \ln[\sum_{\s_b} \exp(- \sum_{B:SR, \, b \subset B} F^i_B)]
$$

In this construction $F^i$ is a function of the spins in type
$j$ blocks for $j>i$. In particular, $F^4$ will not depend on the spins
inside the finite volume $V$, only on the boundary spins outside of $V$. 
However, $F^4$ is not the free energy for this 
volume since we have dropped all the long range terms in the above.
To get the true free energy we proceed as follows.
The expectation that is associated with the Hamiltonian $H$ is 
$$<f> = Z^{-1} \sum_4 \sum_3 \sum_2 \sum_1 \, \exp(-H) \, f 
$$
We define a modified expectation $E$ by
$$E \,f  = \exp(F^4) \sum_4 \sum_3 \sum_2 \sum_1 \, 
\exp(-H+F^1_{LR}+F^2_{LR}+F^3_{LR}) \, f \eqno(\eqbb)
$$
It is straightforward to use our definitions to check that $E 1=1$, and
that the partition function is given by
$Z= \exp(-F^4) \bar Z$ where 
$$\bar Z= E \exp(-F^1_{LR}-F^2_{LR}-F^3_{LR}) \eqno(\eqg)$$

Now we develop an expansion for $\bar Z$.
For each allowable long range $B$ we define
$$K(B)=\exp(-F^1_B-F^2_B-F^3_B) - 1 \eqno(\eqh)$$
(``Allowable'' means that $B$ is a union of $L$-blocks and is small enough
to fit inside some $3L$ by $3L$ square.) 
Then
$$\exp(-F^1_{LR} - F^2_{LR} - F^3_{LR}) = \prod_B [K(B)+1]$$
where the product is over all allowable long range $B$. This equals 
$$\sum_{n=0}^\infty {1 \over n!} \sum_{B_1,\cdots,B_n: distinct} K(B_1) 
\cdots K(B_n)$$
The sum is over distinct $B_1,\cdots,B_n$. This does not mean they must
be disjoint, only different. Inserting the above in (\eqg) we have
$$\bar Z=\sum_{n=0}^\infty {1 \over n!} \sum_{B_1,\cdots,B_n: distinct} 
E K(B_1) \cdots K(B_n) \eqno(\eqi)
$$

The next ingredient we need for our expansion is a factorization property
for the expectation $E$. More precisely, we seek a condition on 
functions $f$ and $g$ of the spins which implies
$$E \, f \, g = E \, f \, \, E \, g \eqno(\eqj)$$
Since $E$ is not simply the measure in which
all the spins are independent, this property is not trivial.
We claim that there is a constant $c$ which depends only on 
the number of dimensions
such that if $dist(supp \, f,supp \, g) > c L$, 
then (\eqj) holds. To prove this we first consider the computation of
$E f$. It begins with $\sum_1 f e^{-H}$. The support of $f$ may involve
more than one type 1 block. So the inclusion of $f$ in this sum 
messes up the factorization, but only in a region near the support 
of $f$. More precisely, it is easy to see that there is a function $f^1$
such that 
$$\sum_1 f e^{-H} = f^1 e^{-F^1}$$
and the support of $f^1$ is contained in the set of sites within 
a distance $c_1 L$ of the support of $f$, where $c_1$ is a constant. 
In general, 
$$\sum_i f e^{-F^{i-1}_{SR}} = f^i e^{-F^i}$$
where $f^i$ is supported on the sites within a distance $c_i L$ of the 
support of $f$. The analogous statement holds for $g$. Thus if the supports
of $f$ and $g$ are sufficiently well separated, then the supports
of $f^i$ and $g^i$ will not overlap and (\eqj) follows. 

We define a collection $B_1,B_2,\cdots, B_n$ to be connected
if for every $B_i$ and $B_j$ we can find $B_{k_1},B_{k_2},\cdots,B_{k_l}$
in the list $B_1,\cdots,B_n$ such that $B_{k_1}=B_i$, $B_{k_l}=B_j$, and
for $m=1,\cdots,l-1$, the distance between $B_{k_m}$ and $B_{k_{m+1}}$
is at most $c_1 L$. Geometrically, $B_1,\cdots,B_n$ are connected if
when we ``fatten'' each set up by a boundary of width $c_1 L/2$, then the
union of the fattened sets is a connected set.
A connected collection $\{B_1,B_2,\cdots,B_n\}$ will be called a 
{\it polymer} and denoted typically by $P$.  
Two polymers $P_1$ and $P_2$ are said to be connected if $P_1 \cup P_2$ 
is connected; otherwise they are said to be disconnected. 
The weight of a polymer $P=\{B_1,\cdots,B_n\}$
is defined to be 
$$ \wei(P)= E K(B_1) \cdots K(B_n) \eqno (\eqhh) $$
If $P$ and $P^\prime$ are disconnected polymers, then the factorization
property (\eqj) and the definition of disconnectedness imply that 
$\wei(P \cup P^\prime)= \wei(P) \ \wei(P^\prime)$. 
Thus we have 
$$\bar Z= \sum_{n=0}^\infty {1 \over n!} \sum_{P_1,\cdots,P_n: disconnected}
\quad \wei(P_1) \cdots \wei(P_n)  \eqno(\eqk)
$$
where the sum is over collections of polymers $P_1,\cdots,P_n$ such 
that each pair $P_i,P_j$ is disconnected when $i \ne j$. 

To obtain a convergent expansion for $\log \bar Z$, we need to show that
$\wei(P)$ is small. 
We bound it by
$$|\wei(P)| \le \prod_{i=1}^n ||K(B_i)||_\infty \eqno(\eqdd)$$
If $||F^i_B||_\infty$ is small, then $||K(B)||_\infty$ will be 
small. The smallness of the former should come from condition (\eqfe)
in lemma 3.1. 
However, there is a counting problem that must be overcome.
Fix a finite volume and consider the free energy $F$ associated  with it.
So $F$ is a function of the boundary spins. Write $F$ in its Fourier
representation
$$F=\sum_X c_X \s^X$$
Here the sets $X$ range over subsets of the boundary spins. 
We have 
$$c_X = \sum_\s \s^X F$$
where $\s$ is summed over boundary spin configurations. 
(Recall that sums over spins configurations are normalized so that
$\sum_\s \ 1 = 1$.)
If $X$ is long range
then there are two sites $i$ and $j$ in $X$ with $|i-j| \ge L$. 
So condition (\eqfe) implies that $|c_X| \le c e^{-mL}$. However, the 
number of subsets of even a single $L$-block grows as $2^{L^2}$ and so
overwhelms the smallness of $e^{-mL}$. 
Thus we cannot hope to bound $F$ by using $|F| \le \sum_X |c_X|$. 
A second problem that we must deal with is that condition (\eqfe) applies to
free energies and $F^i$ is not 
exactly a free energy since we dropped long range terms. 
The following lemma handles the counting problem.

\no {\bf Lemma \lemmaa:} Suppose that condition (\eqfe) holds.
Then there is a constant $c_0$ such that for 
any finite volume $V$ and any boundary condition $\s_\bndry$ and any long range $B$, 
$$||F_B(V,\s_\bndry)||_\infty \le c_0 L^{\nu |B|} e^{-mL}
$$ 
where $F(V,\s_\bndry)$ is the free energy for the volume $V$ with boundary
condition $\s_\bndry$. 
$|B|$ is the number of $L$-blocks in $B$. 
The constant $c_0$ depends only on the constant $c$ in
condition (\eqfe) and the number of dimensions. The constant $m$ is the same
constant $m$ that appears in condition (\eqfe)
(The definition of $F_B(V,\s_\bndry)$, which is probably obvious 
at this point, may be found at the start of the proof.)

\medskip 

\no {\bf Remark:} The lemma applies to any volume $V$. 
The volumes we apply it to are rather unusual, in particular they are
not connected. For example, to obtain 
bounds on $F^1$ we would use a volume which consists of the $L$-blocks of 
type 1 that are contained in our original finite volume. 

\medskip

\no {\bf Proof:}
Write $F(V,\s_\bndry)$ in its Fourier representation
$$F(V,\s_\bndry)= \sum_X c_X \s^X
$$
$F_B(V,\s_\bndry)$ contains those terms $c_X \s^X$ such that 
$X \subset B$ and $X$ contains at least one site from each $L$-block in $B$. 
We can extract precisely these terms from $F(V,\s_\bndry)$ 
by the following rather complicated operation. 
Let $b_1,b_2,\cdots,b_l$ be the $L$-blocks in $B$. 
(So $l=|B|$.)
Since $B$ is long range we can order them so that $b_1$ and $b_2$ 
are at least a distance $L$ apart. Label the spins in $b_k$ by
$\s^k_i$ with $i=1,2,\cdots,L^2$. Let $i_1,i_2,\cdots,i_l$ be integers 
between $1$ and $L^2$ and consider 
$$
\sum_{\s_{B^c}} \, \, 
\sum_{\s^1_1,\s^1_2,\cdots,\s^1_{i_1}} \, \, 
\sum_{\s^2_1,\s^2_2,\cdots,\s^2_{i_2}} \cdots
\sum_{\s^l_1,\s^l_2,\cdots,\s^l_{i_l}} 
\s^1_{i_1} \s^2_{i_2} \cdots \s^l_{i_l} 
\, F(V,\s_\bndry)  \eqno(\eqm)
$$
This operation is designed to wipe out lots of the terms
in $F(V,\s_\bndry)$.
The first sum is over all the spin configurations outside of $B$. 
This wipes out all the terms $c_X \s^X$ for which $X$ is not a subset of 
$B$. Keeping in mind that there is a factor of $\s^1_{i_1}$ in the
above, the second sum wipes out a term unless $\s^X$ does not contain 
$\s^1_j$ for $j<i_1$ and does contain $\s^1_{i_1}$.
Together with the remaining sums we see that the only terms that 
survive the summation in (\eqm)
are those that have $X \subset B$ and $X$ contains 
at least one site in each of $b_1,\cdots,b_l$ with $i_1,...,i_l$ being the 
first such site in each of the respective blocks. 
The $L$-blocks $b_1$ and $b_2$ are at least a distance $L$ apart, so
the spins $\s^1_{i_1}$ and  $\s^2_{i_2}$ are at least a distance $L$ apart.
Since (\eqm) contains
$$\sum_{\s^1_{i_1},\s^2_{i_2}} \s^1_{i_1},\s^2_{i_2} F(V,\s_\bndry)$$ 
and all of the sums are normalized,
the $|| \quad||_\infty$ of (\eqm) is bounded by 
$c \exp(-m L)$ by (\eqfe). To obtain all the terms in 
$F_B(V,\s_\bndry)$ 
we must sum each of $i_1,i_2,\cdots,i_l$ from $1$ to $L^2$,
which produces a factor of $L^{2l}$. 
\qed

\medskip

\no {\bf Remark:} Since $B$ must be a subset of a hypercube with 
side $3L$, $|B|$ is bounded by a dimension dependent constant. 
For the purposes of this paper the value of this constant does not
really matter because of the factor $e^{-mL}$. However, if one 
wants to extend these methods to infinite range Hamiltonians with
a power law decay, then the value of this constant becomes very important.
In the analogous estimate in [\refop],
Olivieri and Picco have $L^{2 \nu}$, a better bound than
our lemma. However, this would still require that the Hamiltonian 
decay faster than $1/r^{2 \nu}$, while decay that is faster than 
$1/r^\nu$ should be sufficient. It appears that there is no hope 
of proving this optimal sort of result with the methods of this 
paper.

\medskip

The above lemma only applies to free energies, and the quantities
$F^i$ are not quite free energies. To address this problem we first
need the following technical lemma which says that the operation of 
extracting the long range part is continuous in some sense. 

\medskip

\no {\bf Lemma \lemmab:} For any function $F(\s)$ and any $B$
which is a union of $L$-blocks,
$$||F_B||_\infty \le c(|B|) ||F||_\infty
$$ 
where $c(|B|)$ is a constant which depends only on $|B|$, the number of 
$L$-blocks in $B$.

\medskip

\no {\bf Proof:} 
Write $F(\s)$ in its Fourier representation
$$F(\s)= \sum_X c_X \s^X
$$
Let 
$$\bar F(\s) = \sum_{\s_{B^c}} F(\s)
$$
where the sum is over all spin configurations on the complement of $B$.
We have 
$$||\bar F||_\infty \le ||F||_\infty \eqno(\eqw)$$
This sum over $\s_{B^c}$ 
kills any term in the Fourier representation that is not
supported inside $B$. So 
$$\bar F(\s) = \sum_{X: X \subset B} c_X \s^X  
$$
Thus $\bar F(\s)$ contains all the terms that go into $F_B$. Unfortunately,
it contains some additional terms, those whose support is a proper 
subset of $B$. In fact, we have 
$$\bar F(\s) = \sum_{B^\prime: B^\prime \subset B} F_{B^\prime}
$$
and so
$$F_B(\s) = \bar F(\s) - \sum_{B^\prime: B^\prime \subset B, B^\prime \ne B} 
F_{B^\prime}
$$
Together with (\eqw) this implies 
$$||F_B||_\infty \le ||F||_\infty 
+ \sum_{B^\prime: B^\prime \subset B, B^\prime \ne B} 
||F_{B^\prime}||_\infty
$$
The lemma now follows by induction on $|B|$. 
\qed

\medskip

\no {\bf Lemma \lemmac:} 
Suppose that condition (\eqfe) holds. Then there are constants
$c^\prime,p,L_0$ such that 
$$||F^i_B||_\infty \le c^\prime L^p e^{-mL}$$
if $L \ge L_0$ and $B$ is LR. Here $m$ is the same constant that appears in 
(\eqfe)

\medskip

\no {\bf Proof:} 
Fix a $B_0$ that is LR. $F^{1,b}$ only depends on terms $H_X$ in the 
Hamiltonian for which $X \cap b \ne \emptyset$. $F^{i,b}$ only depends
on $F^{i-1,B}$ for $B$ with $b \subset B$. Thus there is a constant $c$ such
that $F^i_B$ depends on a term in $H_X$ only if $X$ is within a distance $cL$ 
of $B$. This implies that we can change the volume $V$ and the boundary
condition outside of the set 
$$\Lambda(B_0)= \{i:dist(i,B_0) \le c L \}
$$
and $F^i_{B_0}$ will be unchanged. In particular, when computing $F^i_{B_0}$
we can replace $V$ by $V \cap \Lambda(B_0)$, and replace the boundary condition
by any boundary condition that agrees with the original one inside
$\Lambda(B_0)$. Thus we can just assume $V \subset \Lambda(B_0)$.

We define $\tilde F^i$ by
$$\eqalign{\exp(- \tilde F^i) &= \sum_i \sum_{i-1} \cdots \sum_1 \exp(-H) \cr
&=\sum_i \exp(- \tilde F^{i-1})
} \eqno(\eqee)
$$
Note that each $\tilde F^i$ is the free energy of some volume, albeit
a rather strange one. 
In the following, when we say that a quantity is $\bigol$ 
we will mean that there is a constant $c^\prime$ and an integer $L^\prime$
such that the $||\quad ||_\infty$ of the quantity is bounded by 
$c^\prime L^p e^{-mL}$ for $L \ge L^\prime$. 
We now argue by induction on $i$. 
The inductive assumption is that 
$$\tilde F^i_B - F^i_B = \bigo (L^p e^{-mL})
$$
for all $B$, both SR and LR. 
Since $\tilde F^i$ is the free energy of some volume, lemma 
\lemmaa implies 
$$\tilde F^i_B = \bigo (L^p e^{-mL})
$$
if $B$ is LR. Thus proving the inductive assumption will prove the lemma. 
The inductive assumption is trivially true for $i=1$ since $\tilde F^1=F^1$.

Assume the inductive claim is true for $i-1$. 
The number of $L$-blocks in $\Lambda(B_0)$ is bounded by a constant that 
only depends on the number of dimensions.
Since $V \subset \Lambda(B_0)$,
the number of $B$ such that $F^i_B \ne 0$ is also bounded by a constant which 
depends only on the number of dimensions. By the inductive assumption this
implies 
$$\sum_{B:SR} (F^{i-1}_B - \tilde F^{i-1}_B ) = \bigol
$$
Lemma \lemmaa implies  
$$\sum_{B:LR} \tilde F^{i-1}_B = \bigol
$$
Thus 
$$\left( \sum_{B:SR} F^{i-1}_B \right) - \tilde F^{i-1} = 
\sum_{B:SR} (F^{i-1}_B - \tilde F^{i-1}_B ) - \sum_{B:LR} \tilde F^{i-1}_B 
= \bigol
$$
And so 
$$\eqalign{ F^i &= - \ln \left( \sum_i 
   \exp[- \sum_{B:SR} F^{i-1}_B] \right) \cr
&= - \ln \left( \sum_i \exp[- \tilde F^{i-1}+ \bigol] \right) \cr
&= - \ln \left( \sum_i \exp(- \tilde F^{i-1}) \right) + \bigol \cr
&= \tilde F^i + \bigol
}
$$
By lemma \lemmab this implies 
$F^i_{B_0} - \tilde F^i_{B_0} = \bigol$.
\qed

\medskip

Lemma \lemmac implies that there is a function $\epsilon(L)$ such that
$$|\wei(P)| \le \epsilon(L)^{|P|}
$$
with $\epsilon(L) \ra 0$ as $L \ra \infty$. (Here $|P|$ denotes the
number of $B$'s in the polymer $P$.) 
The weight of a polymer $\wei(P)$ is a function of the block spins
$\sp$. All of the above estimates are uniform in the block spin
configuration $\sp$, so we have in fact shown that 
$$\sup_{\sp} |\wei(P)| \le \epsilon(L)^{|P|} \eqno (\eqo)
$$
The representation (\eqk) says that $\bar Z$ is a gas of polymers with
a two body interaction - each pair of polymers must be ``disconnected.''
Standard results on the polymer expansion [\refbry] then imply that 
if we choose $L$ sufficiently large, then we have a convergent expansion
for $\ln \bar Z$.
$$\ln \bar Z = \sum_{P_1,\cdots,P_n} 
\psi_c(P_1,\cdots,P_n) \wei(P_1) \cdots \wei(P_n) \eqno(\eqff)
$$
where $\psi_c$ is the connected part of our two body interaction.
In particular $\psi_c(P_1,\cdots,P_n)$ vanishes whenever $\cup_i P_i$ 
is not connected in the sense of connectedness that we  defined for the 
polymers

The weight $\wei(P)$ will depend on the block spins in $P$. It can also
depend on some of the block spins outside of $P$, but we will now
argue that there is a constant $a$ such that $\wei(P)$ depends only on 
the block spins $\s^\prime_i$ with $i$ within a distance $aL$ of $P$. 
The following statements follow from the definitions of the various 
quantities.
$F^{1,b}$ only depends on the block spins in $b$. 
$F^{2,b}$ only depends on the block spins in $b$ and the two type 1
$L$-blocks adjacent to $b$. 
By induction we then see that 
there is a constant $a_i$ such that $F^{i,b}$
only depends on the block spins within a distance $a_iL$ of $b$. 
Thus $K(B)$ only depends on the block spins within a distance $a_0 L$ of $B$
for some constant $a_0$. Recall that if $P=B_1,\cdots,B_n$, then the
weight of $P$ is $E \, K(B_1) \cdots K(B_n)$. The quantity 
$K(B_1) \cdots K(B_n)$ only depends on the block spins within a distance
$a_0 L$ of $P$. However, the expectation $E$ also depends on the block
spins. So $\wei(P)$ may depend on more block spins than 
$K(B_1) \cdots K(B_n)$ did. However, it follows by an argument similar to
that which proved (\eqj) that the expectation $E$ can only extend the
range of dependence  on the block spins by a finite amount, i.e., 
there is a constant $a$ such that $\wei(P)$ only depends on the block spins 
within a distance $a L$ of $P$. 

The renormalized Hamiltonian $H^\prime$ is equal to 
$-\ln Z = F^4 - \ln \bar Z$.
$F^4$ is a local function of the block spins.
Define $supp(P_1 \cup \cdots \cup P_n)$ to be the set of block spin sites that 
are within a distance $a L$ of $P_1 \cup \cdots \cup P_n$. 
Since $\psi_c(P_1,\cdots,P_n)$ vanishes if $P_1 \cup \cdots \cup P_n$ is not 
connected, $supp(P_1 \cup \cdots \cup P_n)$ will be a connected 
set of block spins.
The results of the previous paragraph show that a term 
$-\psi_c(P_1,\cdots,P_n) \wei(P_1) \cdots \wei(P_n)$ in the expansion
of $- \ln \bar Z$ only depends on the block spins 
in $supp(P_1 \cup \cdots \cup P_n)$. 
For each finite set of block spin sites, define 
$H^\prime_X(\sp)$ to be the sum of the terms with 
$supp(P_1 \cup \cdots \cup P_n)=X$. 
$F^4$ is a sum of terms, each of which only 
depends on a finite number of block spins. We include each of these terms
in the appropriate $H^\prime_X$. 
We have shown that the renormalized Hamiltonian 
may be written in the form
$$H^\prime(\sp) = \sum_X H^\prime_X(\sp) \eqno(\eqgg)
$$ 
where $H^\prime_X(\sp)$ only depends on the block spins in $X$.
Our expansion shows that each $H^\prime_X$ has an infinite volume 
limit and they satisfy
$$\sum_{X \ni 0} ||H^\prime_X||_\infty < \infty \eqno(\eqr)
$$
Let $|X|$ denote the number of block spin sites in $X$. Then
there is an $L$ dependent constant $M$ such that for every term in (\eqff)
with $supp(P_1 \cup \cdots \cup P_n)=X$
we have $|X| \le M \sum_{i=1}^n |P_i|$. It follows that we can 
even include a factor of $\exp(\mu |X|)$ in (\eqr),  and the sum will still
be finite if $\mu>0$ is small enough.
We organized things above so that the sets $X$ are connected. Thus
we have completed the proof of theorem 1.1.

\bigskip
\bigskip
\bigskip

\no {\bf Acknowledgements:} 
The authors thank Christian Maes, Enzo Olivieri and Aernout van Enter 
for useful discussions and comments. 
This work was supported in part by NSF grant DMS-9303051

\bigskip
\bigskip
\bigskip

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%  APPENDIX  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\no {\bf Appendix }

\bigskip

In this appendix we prove proposition 2.1. We will show that if 
$ \sup_{\sp} \alpha < 1 $, then hypothesis (1.3) of theorem 1.1 holds.
Recall that the original Hamiltonian is finite range and translation 
invariant.
Since $H$ is finite range,
for the terms with $\rho_{ij} \ne 0$, the distances 
$|i-j|$ are bounded by a constant.
Thus we can choose $\gamma < 1$ and 
$\epsilon>0$ sufficiently small, so that 
$$\sum_{i: i \ne j} e^{\epsilon |i-j|} \, \rho_{ij} \le \gamma < 1
\eqno (A.1)$$
for all $\sp$. 

Fix a block spin configuration $\sp$. The finite volume measures
$\mu_{\sp,V}$ were defined in eq. (1.2). 
Since the hypothesis of the Dobrushin uniqueness theorem is satisfied,
for every $\sp$ we get a unique infinite volume Gibbs state $\mu_\sp$.
By condition (A.1) and theorem V.2.1 of [\refsim], there are constants
$c>0, m< \infty$ such that for every block spin configuration $\sp$
$$|\mu_\sp(\s_i \s_j) -\mu_\sp(\s_i ) \mu_\sp(\s_j) | \le c e^{-m|i-j|}
 \eqno(A.2)
$$
This tells us that for every block spin configuration, the infinite 
volume measure of the constrained system has exponentially decaying 
correlations, and the decay is uniform in the block spin configuration.
However, for condition (1.3) to hold, we need the same decay 
for all finite volumes and boundary conditions. This follows easily
as we now show.

The finite volume measure $\mu_{\sp,V}$ may be thought of as an
infinite volume measure which is supported entirely on spin configurations
that agree with the boundary condition $\tau$ outside of $V$. 
Fix a $V,\tau$ and $\sp$. For each site $j$, we define a measure
$\nu_j$ on the spin space $\{-1,1\}$. For $j \in V$,
$\nu_j=\mu_{\sp,\{j\}}$. For $j \notin V$, $\nu_j$ is the measure 
which assigns probability 1 to $\tau_j$ and probability 0 to $-\tau_j$.
For a function $f$ on spins configurations, $\nu_j(\omega,f)$ denotes the 
expectation of $f$ with respect to $\nu_j$. 
It is a function of $\omega$, 
the spin configuration on the set of sites different from $j$. 
Clearly then, 
$$\mu_{\sp,V}(\tau,\nu_j(\omega,f)) =  
\mu_{\sp,V}(\tau,f)
$$
$\rho_{ij}$ is defined by (2.1). We define $\tilde \rho_{ij}$ by the 
same equation with $\mu_j$ replaced by $\nu_j$. It follows easily
from the definition of $\nu_j$ that 
$$\tilde \rho_{ij} =  \cases{ \rho_{ij} &if $j \in V$ \cr
		     0 &if $j \notin V $ \cr}
$$
So $\tilde \rho_{ij} \le \rho_{ij}$. Hence
$$\sup_j \sum_{i: i \ne j} e^{\epsilon |i-j|} \, \tilde \rho_{ij} \le 
\gamma <1
$$
The infinite volume measure corresponding to the $\nu_j$'s is the
meausure $\mu_{\sp,V}$. 
So theorem V.2.1 of [\refsim] implies (A.2) holds with $\mu_{\sp}$
replaced by $\mu_{\sp,V}$. Thus the hypothesis of the main theorem
is satisfied.

\page

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%  REFERENCES  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\def \jsp {{\it J. Stat. Phys.} }

\centerline {\bf References}

\bigskip

\no  \refbmo. G. Benfatto, E. Marinari, E. Olivieri,
Some numerical results on the block spin transformation for
the 2$D$ Ising model at the critical point, 
\jsp {\bf 78}, 731 (1995). 
\smallskip

\no \refbry. D. Brydges, A short course on cluster expansions, in:
{\it Critical phenomena, random systems, gauge theories}, 
K. Osterwalder, R. Stora (eds.) (Elsevier, 1986).
\smallskip

\no \refcg. M. Cassandro, G. Gallavotti, The Lavoisier law and the critical
point, {\it Nuovo Cimento} {\bf B25}, 691 (1975).

\no  \refds. R. L. Dobrushin, S. B. Shlosman, Completely analytic Gibbs fields,
Constructive criterion for the uniqueness of Gibbs field, both articles appear 
in {\it Statistical Physics and Dynamical Systems}, Birkhauser, 1985; 
Completely analytical interactions: constructive description, 
\jsp {\bf 46}: 983 (1987).
\smallskip

\no  \refgpa. R.B. Griffiths, P.A. Pearce, Position-space 
renormalization group transformations: some proofs and some problems, 
{\it Phys. Rev. Lett.} {\bf 41}, 917 (1978).
\smallskip

\no  \refgpb. R.B. Griffiths, P.A. Pearce, 
Mathematical properties of position-space renormalization group 
transformations, 
\jsp {\bf 20}, 499 (1979).
\smallskip

\no  \refg. R.B. Griffiths, Mathematical properties of renormalization group
transformations, {\it Physica} {\bf 106A}, 59 (1981). 
\smallskip
 
\no  \refi. R. B. Israel,
Banach algebras and Kadanoff transformations,
in: {\it Random Fields (Esztergom, 1979), vol.\ II}, 
J. Fritz, J. L. Lebowitz, D. Sz{\'a}sz, (eds.), 
(North-Holland,1981).
\smallskip

\no  \refk. I. A. Kashapov, 
Justification of the renormalization - group method,
{\it Theor. Math. Phys.} {\bf 42}, 184 (1980).
\smallskip

\no \reftk. T. Kennedy, 
Some rigorous results on majority rule renormalization 
group transformations near the critical point, 
\jsp {\bf 72}, 15 (1993).
\smallskip

\no \refsim. B. Simon, {\it The statistical mechanics of lattice
gases, vol. 1} (Princeton University Press, 1993).
\smallskip

\no \refmoa. F. Martinelli, E. Olivieri, Some remarks on pathologies
of renormalization group transformations, \jsp {\bf 72}, 1169 (1993). 
\smallskip

\no  \refmob. F. Martinelli, E. Olivieri, Instability of 
renormalization group pathologies under decimation,
\jsp {\bf 79}, 25 (1995). 
\smallskip

\no \refnv. Th. Niemeijer, M. J. van Leeuwen, Renormalization theory for 
Ising-like spin systems, in: 
{\it Phase Transitions and Critical Phenomena, vol. 6}, 
C. Domb, M. S. Green (eds.) (Academic Press, 1976). 
\smallskip

\no \refo. E. Olivieri, On a cluster expansion for lattice spin systems:
a finite size condition for the convergence, \jsp {\bf 50}, 1179 (1988).
\smallskip

\no \refop. E. Olivieri, P. Picco,
Cluster expansion for $D$-dimensional lattice systems and finite volume
factorization properties, \jsp {\bf 59}, 221 (1990).
\smallskip

\no \refol. M. Ould-Lemrabott, private communication.

\no  \refve. A. C. D. van Enter, Ill-defined block-spin transformations
at arbitrarily high temperatures. Preprint. 

\no  \refvefk. A. C. D. van Enter, R. Fern{\'a}ndez, R. Koteck\'y, 
Pathological behavior of renormalization group maps at high fields and above 
the transition temperature, \jsp {\bf 79}, 969 (1995).
\smallskip

\no \refvefs. A. C. D. van Enter, R. Fern{\'a}ndez, A.D. Sokal,
Renormalization transformations in the vicinity of first-order phase
  transitions: What can and cannot go wrong,
{\it Phys. Rev. Lett.} {\bf 66}, 3253 (1991);
Regularity properties and pathologies of position-space
  renormalization group transformations,
{\it Nucl. Phys. B (Proc. Suppl.)} 
{\bf 20}, 48 (1991);
Regularity properties and pathologies of position-space renormalization - group
transformations: scope and limitations of Gibbsian theory,
\jsp {\bf 72}, 879 (1993).
\smallskip

\page

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%  FIGURE CAPTIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\centerline {\bf Figure Captions}

\bigskip

\no Fig. 1. For decimation with $b=\sqrt{2}$, each spin in the 
original lattice that is summed out has only block spins as nearest neighbors.
The spins at sites 1,2,3 
and 4 are block spins and hence fixed. 
Thus the sum over the spin at site 0 may be done explicitly.

\bigskip

\no Fig. 2. After one iteration of the $b=\sqrt{2}$ decimation transformation,
we check the Dobrushin condition for site 0. The effective Hamiltonian
(2.4) couples the spin at this site to all the spins shown in the figure.
The spins at sites 1,2,3 and 4 are block spins.  

\bigskip

\no Fig. 3. The original spins are denoted by circles and X's, the block spins 
by B's. The circles form clusters of five sites with no nearest neighbor
interactions between two such clusters. So the sum over the spins at the 
circles may be done explicitly.

\bigskip

\no Fig. 4. The blocking of the triangular lattice is shown with the block 
spins denoted by B. Each block contains the three sites in the original 
lattice that are adjacent to the block spin. 
The block spins live on a triangular lattice indicated with dashed lines.

\bigskip

\no Fig. 5. We test the Dobrushin condition at site 0. The spins at sites 
1,2 and 3 are first summed out explicitly. 
The resulting effective Hamiltonian couples the 
spin at 0 to all of the spins shown.


\bigskip

\no Fig. 6. $p$ is the parameter in the Kadanoff transformation for 
the triangular lattice. 
The Dobrushin condition is satisfied uniformly in the block spins, and 
hence the renormalized Hamiltonian exists, in the region to the left 
of the curve. 

\bigskip

\no Fig. 7. The division of the lattice into large $L$ by $L$ blocks is shown. 
These blocks are then grouped into four types as indicated by 1,2,3,4.
The spins in the blocks are then summed over in this order.

\end


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