Road Map
The central question is to determine whether an infinite series converges.
If it converges, then there are various ways to compute its sum. Whereas, if it diverges, then it makes no sense.
We already have one criterion:
If $\displaystyle\lim_{n\to\infty}a_n\ne0$, then $\sum a_n$ diverges. But what if $a_n$ does shrink? How can we tell if it shrinks fast enough?
The main techniques are:
- The Integral Test:
Certain infinite sums can be compared to improper integrals. The sums converge if and only if the integrals converge.
- Comparison Tests:
If $\sum a_n$ converges and {$b_n$} is better behaved than {$a_n$}, then $\sum b_n$ converges. If $\sum a_n$ diverges and {$b_n$} is worse behaved than {$a_n$}, then $\sum b_n$ diverges.
One of the goals will be to understand what "better behaved" and "worse behaved" mean mathematically.
- Alternating Series:
Series whose terms go back and forth between positive and negative have some special properties. The cancellation between positive and negative terms gives more convergence than you might expect from the size of the terms.
- The Ratio Test and the Root Test:
These are the big guns that will allow us to understand radii of convergence for power series.
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