At uni it once took me hours to work out that "." was used for function application in one particular paper. "." was also used for multiplication and (in some example code) had the usual object oriented meaning in the same paper. It was incredibly frustrating.
It's at the start of like... every book ever. Pretty much any book on mathematics will start off with a fairly in-depth list of symbols.
It's the math equivalent of expecting you know what a 'while' loop is when you go reading through the documentation for a library (reading a paper) - basic programming literacy is assumed.
I see a lot of 'n' for a number, 'i', 'j', 'k' for loop indexes, 'a' and 'b' for the variables in a swap function, 'f' and 'g' for various things involving function composition.
The case of knowing what a capital pi or sigma means, however, is much more like knowing what a "while" loop is.
I would love to see a wiki style website that aims to produce such a wealth of information.
I don't use LaTeX but now I'm looking for an excuse :)
Worse, they also like to use the same symbol to denote different things in different fields. I'm sure it's extremely convenient to have a shorthand when working and sharing "code" with some peers in the know but for stuff like wikipedia articles it makes things appear more complex and obscure than they should.
I don't mind if people call a local function variable "n" in some code, it's usually non ambiguous. But if you export a variable "n" in an external API you will be screamed at. Why is it ok for maths?
To give a quick example, the letter R in maths can mean the set of real numbers. It can also (with a different typeface!) mean Ramanujan summation. Oh wait, you're doing physics? Then R is the gas constant, silly.
Also, i is the imaginary unit. Except in physics, then it's j, because i is used for currents. Makes sense.