How to write sigma? (As in Σ) is it sum() ?

"Sigma" is used to denote a sum

The symbol can be found here : http://math.typeit.org/

As an example of its use......suppose we have

 3

∑   N + 1

1

This tells us to compute the sum (1 + 1) + (2 + 1) + (3 + 1)  =   2 + 3 + 4  =  9

On this calculator...it looks like it might be possible to make a sum..but I can't quite figure out how...

https://web2.0calc.com/help/number-theory

When I copy and paste the example..it says  " TODO !! "

I agree with hectictar......sums can probably be computed with the calculator, but I don't know the proper syntax.......maybe one of our "advanced" mods, Alan, knows the key !!!

Meanwhile.....WolframAlpha will easily compute sums : 

https://www.wolframalpha.com/input/?i=sum+(x+%2B1),+from+x+%3D1+to+x+%3D+10