

1 − 2 + 3 − 4 + · · ·  =  1/4 - solipsist
http://en.wikipedia.org/wiki/1_%E2%88%92_2_%2B_3_%E2%88%92_4_%2B_%C2%B7_%C2%B7_%C2%B7

======
makecheck
It may be better to look at a series in general:

<http://en.wikipedia.org/wiki/Convergent_series>

<http://en.wikipedia.org/wiki/Divergent_series>

In particular there are many interesting sums on the convergent series page.

