Binary products in a category are denoted $A_1 \times A_2$, while arbitrary products are denoted $\prod_{i \in I} A_i$. Binary coproducts are (sometimes) denoted $A_1 + A_2$,* so I expected that arbitrary coproducts would (sometimes) be denoted $\sum_{i \in I} A_i$.**
As an explicit example (in $\textbf{Set}$), the binary disjoint union of sets is sometimes denoted by $A_1 + A_2$, and the arbitrary disjoint union of sets by $\sum_{i \in I} A_i$, but I wasn't able to find a reference where $\sum$ is used for coproducts in a general category (I checked Mac Lane's and Awodey's books on category theory, as well as doing a fairly comprehensive Google search).
So my question is: does anybody know of a reference where $\sum$ is used for coproducts? (I would be interested to know if anyone reading this uses this notation personally, even if they do not know of any reference.)
Thanks
*although the notations $A_1 \sqcup A_2$ and $A_1 \coprod A_2$ are perhaps more common (and according to wikipedia $A_1 \oplus A_2$ is also used).
**the notations $\bigsqcup_{i \in I} A_i$ and $\coprod_{i \in I} A_i$ are common (and according to wikipedia $\bigoplus_{i \in I} A_i$ is also used).
Best Answer
In the interest of providing an answer, I'll restate my comments.
Yes, $\sum A_i$ is used to denote a coproduct by many people (myself included). I've seen this used in plenty of talks, and there was never any confusion.
For an explicit instance of coproducts being introduced this way in print, see the section on "Sums" in Leinster's Basic Category Theory. In the linked edition, it's page 127.
I hope this helps ^_^