TITLE: What is the term for a factorial type operation, but with summation instead of products?
QUESTION [95 upvotes]: (Pardon if this seems a bit beginner, this is my first post in math - trying to improve my knowledge while tackling Project Euler problems)
I'm aware of Sigma notation, but is there a function/name for e.g.
$$ 4 + 3 + 2 + 1 \longrightarrow 10 ,$$
similar to $$4! = 4 \cdot 3 \cdot 2 \cdot 1 ,$$ which uses multiplication? 
Edit: I found what I was looking for, but is there a name for this type of summation?

REPLY [43 votes]: Donald Knuth in The Art of Computer Programming calls the $n$-th triangular number the "termial function", and denotes it
$$n? = 1 + 2 + ... + n = \sum_{k=1}^n k $$
(Third edition, Volume 1, page 48).