TITLE: Unknown syntax with multiplication?
QUESTION [8 upvotes]: So I have an assignment and this formula showed up:
$$\prod_{i<j} \;[\,p(j) - p(i)\,]$$
I have seen the multiplication sign before, I know what it is, but I don't know what '$i<j$' means in this context.
EDIT: I've looked all around the web for it, and I haven't found anything.

REPLY [12 votes]: It means "the product over all pairs $i, j$ where $i < j$."  For example, if $p(i)$ is defined for $i = 1, 2, 3$, then
$$
\prod_{i<j} \;[\,p(j) - p(i)\;] = [ p(2) - p(1) ] [p(3) - p(1)] [p(3) - p(2)].
$$

REPLY [8 votes]: It means the product of every version of the expression in which $i <j $ and $i $ and $j $ are in some set implied from context.