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Today I had some Hungarian mathematical logic notes to type in Latex, and again I had to dig up the various references to get the proper symbols I needed.
I used the following pages:

amsmath is unique in that for example it explains how to use
\begin{cases}

this one is a collection of lot of symbols, I found
\mathcal
here 
An article from Wikipedia about logic symbols

The Math chapter of the wellknown LaTeX wikibook

A set of macros making easy to declare lemmas, definitions, proofs.