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I've seen this around, even on Wolfram|Alpha, and I don't know what it means.

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General tip for interpreting mathematical notation: if information is omitted, it should be clear from the context. – Isaac Solomon Mar 25 '12 at 19:02
Depends on the context. I see you tagged it as a "calculus" question and "algebra-calculus" question. In those areas, this sigma is probably being used as a sum. The fact that it lacks the startking index and limit is just that those are being ommited - they can be deduced from the rest of the text, from previous equations and so on. It's just an abbreviation. – Marra Mar 25 '12 at 19:03
writing down the entire formula would be helpful for a correct interpretation – Andrea Mori Mar 25 '12 at 19:21
up vote 2 down vote accepted

The sign $\sum$ denotes an indefinite summation, that is $g(n) = \sum f(n)$ (summation over $n$ is implied), such that $g(n+1) - g(n) = f(n)$.

For example $\sum n = \frac{n^2 - n}{2}$, since $\frac{(n+1)^2 - n-1}{2} -\frac{n^2-n}{2} = n$.

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Nice. This makes me think of $\sum n$ as if it were $\int n$ and infer its "${\rm d}n$" from context. – user2468 Mar 25 '12 at 19:11
Sometimes $\sum$ may mean indefinite summation. But the author should say so, if it does. In "calculus" or "precalculus" (the labels in this question) it probably does not mean indefinite summation. – GEdgar Mar 25 '12 at 19:15

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