Looking for a dictionary of math/CS notation
You can use this sheet. It has a lot of handy formulas and some definitions.

You can also use Wikipedia's list of mathematical symbols propsed by Kyle Jones in comments.

There is also Worfram MathWorld which is a great resource, but you need to know what are you looking for.

When it comes to printed resources, each volume of The Art of Computer Programming has a great reference of mathematical notations, especially volume 1.