TITLE: 'Plus' Operator analog of the factorial function?
QUESTION [6 upvotes]: Possible Duplicate:
What is the term for a factorial type operation, but with summation instead of products? 

Is there a similar function for the addition operator as there is the factorial function for the multiplication operator?
For factorials it is 5! = 5*4*3*2*1, is there a function that would do 5+4+3+2+1?
Thanks,

REPLY [9 votes]: As far as I know we haven't given that expression a name since we may write it explicitly as
$$
\sum_{i=1}^ni = {n+1 \choose 2} = \frac{n(n+1)}{2}.
$$