It may be better to look at a series in general:<p><a href="http://en.wikipedia.org/wiki/Convergent_series" rel="nofollow">http://en.wikipedia.org/wiki/Convergent_series</a><p><a href="http://en.wikipedia.org/wiki/Divergent_series" rel="nofollow">http://en.wikipedia.org/wiki/Divergent_series</a><p>In particular there are many interesting sums on the convergent series page.