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Wolfram Alpha continues to impress me.

But not me

Adding a '+' to the end, before the '..' makes it work again, maybe it needs an operand and the '..' to show that it is an infinite series, maybe !

Although I'm not a fan of using i in a mathematical formula like this. When I first saw it, it looked like they meant sqrt(-1).

hmmm ... for whatever reasons the link was broken. Trying it again: "http://www.wolframalpha.com/input/?i=1+-+2+%2B+3+-+4+%2B+5+-...

here is a PDF snapshot.