Divergent geometric series
is divergent if and only if | r | ≥ 1. Methods for summation of divergent series are sometimes useful, and usually evaluate divergent geometric series to a sum that agrees with the formula for the convergent case
In increasing order of difficulty to sum:
- 1 − 1 + 1 − 1 + · · ·, whose common ratio is −1
- 1 − 2 + 4 − 8 + · · ·, whose common ratio is −2
- 1 + 2 + 4 + 8 + · · ·, whose common ratio is 2
- 1 + 1 + 1 + 1 + · · ·, whose common ratio is 1.
Motivation for study 
It is useful to figure out which summation methods produce the geometric series formula for which common ratios. One application for this information is the so-called Borel-Okada principle: If a regular summation method sums Σzn to 1/(1 - z) for all z in a subset S of the complex plane, given certain restrictions on S, then the method also gives the analytic continuation of any other function f(z) = Σanzn on the intersection of S with the Mittag-Leffler star for f.
Summability by region 
Open unit disk 
Ordinary summation succeeds only for common ratios |z| < 1.
Closed unit disk 
Larger disks 
The series is Borel summable for every z with real part < 1. Any such series is also summable by the generalized Euler method (E, a) for appropriate a.
Shadowed plane 
- Korevaar p.288
- Moroz p.21