Some say that numbers aren't as important as actual functions, but we say they are! :-) For anyone who has wondered why 13 is unlucky, why you can't eat pi, or why 1 is the multiplicative identity, this is the category you've been searching for since grade school. Whatever intellectual curiosity has been nagging at you since you first wrote the number 5, you will be likely to find an answer to your question here. And if not, submit your own! Also: Definitions of important constants, formulas to compute them and formulas that use them.