Subordination
If $T_t$ is a strongly continuous semigroup of contraction operators on a Banach space and $\mu_t$ is a semigroup of sub-probabilistic measures on $[0, \infty)$, then \[ \tilde{T}_t u = \int_{[0, \infty)} T_r u \mu_t(\mathrm d r) \] defines another semigroup of operators $\tilde{T}_t$, which is said to be subordinate (in the sense of Bochner) to $\tilde{T}_t$.[1]
Relation to Bernstein functions
For some Bernstein function $f$, \[ \int_{[0, \infty)} e^{-r z} \mu_t(\mathrm d r) = e^{-t f(z)} . \] Conversely, for any Bernstein function $f$ there exists a semigroup of sub-probabilistic measures $\mu_t$ satisfying the above equality.
One often writes $\tilde{L} = f(L)$, where $-L$ and $-\tilde{L}$ are the generators of $T_t$ and $\tilde{T}_t$, respectively. This notation agrees with spectral-theoretic definition of $f(L)$ if the underlying Banach space is a Hilbert space.
References
- ↑ Schilling, R. (1998), "Subordination in the sense of Bochner and a related functional calculus", J. Aust. Math. Soc. Ser. A 64: 368–396, http://www.austms.org.au/Publ/Jamsa/V64P3/abs/p86/
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