The geometric series with ratio and constant
is the following infinite sum:
This series converges if and only if , and its sum is:
- If
, the series diverges.
- If
, the series oscillates between
and
.
- If
, the series oscillates between 0 and
.
Proof.
Let be the partial sum of the series
Thus,
Multiplying by
:
We observe that contains all the terms of
except for the first term
and adds a new term,
.
That is, . We have obtained an equation for
, so solving for
:
As long as , because otherwise, if
, then
would not be defined. We need a well-defined expression for
to determine the sum for all values of
, that is,
.
We have successfully expressed the terms of the sum in a general equation. This is very useful because the series is the limit of
as
, and since we obtained an explicit expression for
, we just need to compute this limit.
As you can see, we used the restriction because otherwise,
is not finite.