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What is the term for a factorial type operation, but with summation instead of products?

I am looking for the name of an operation similar to factorial.

Factorial would be like: 6! = 6*5*4*3*2*1

but if the operator is "+" and not "*", like: 6+5+4+3+2+1

What would I call this?

Edit: Sorry, it seems this was a duplicate question. The answer appears to be "nth triangular number". Although that's really not as satisfying and concise as the word "factorial".

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logicbird
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  • $\displaystyle \sum_{i=1}^6 n$ Maybe? –  Sep 15 '11 at 23:51
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    We do not need an additional terminology to describe this sum, because it can be summed to give a "closed-form answer", i.e., an answer that can be written using simpler functions like $+$ and $\times$ and $/$. The sum is equal to $\frac{n(n+1)}{2}$. – Srivatsan Sep 15 '11 at 23:52
  • Instead of $n!$ why not write $n\triangle$? – GEdgar Sep 16 '11 at 00:23

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