Which geometric series diverges?

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

If |r| > = 1 then the above geometric series diverges. If the above series converges, then the remainder RN = S - SN (where S is the exact sum of the infinite series and SN is the sum of the first N terms of the series) is bounded by 0< = RN < = (N ...) f (x) dx.