Another spin-off of the Noga-poster-formula-competition is a MathOverflow question: Important formulas in combinatorics.

### The question collects important formulas representing major progress in combinatorics.

So far there are 31 formulas and quite a few were new to me. There are several areas of combinatorics that are not yet represented. As is natural, many formulas come from enumerative combinatorics. Don’t hesitate to contribute (best – on MathOverflow) more formulas!

Advertisements

The second expression from a bottom is not a formula.

How would you call it?