Originally posted by CZekebut still, it's sigma - upper case for the summation and lower for integral. Yeah, it looks like latin s, but can be also read as greek letter.
That's sort of a stretch, though. The integral sign isn't a sigma, it's an S. A lowercase sigma looks like an S, but only when it's at the end of a word; more to the point, Leibniz, who came up with this notation, was surely thinking of the Latin alphabet. He used d for derivatives, after all.
Originally posted by AllderdiceActually, my question was a joke. "What's the delta?" Greek, delta, difference, differential. (sigh)
uppercase delta looks loke a triangle with one side extended and is a symbol for difference in conditions (e. g. final-initial), lowercase delta is like an "o" with a '~' on top of it and a lone connecting the two. It's also used to show the difference, but only in chemistry - it shows, which end of a molecule is polar + and which is polar -. The uppercase is used mostly in physics.