Birthday problem - in javascript \o/ cf http://en.wikipedia.org/wiki/Birthday_problem

birthday-problem.js

// PARADOX ! Inception-style
var paradox = function(setSize,p){
  Math.sqrt(2*setSize*Math.log(1/(1-p)))
};
// So that's the generic formula.

// Applied to the usual "over 50% of prob to have 2 people with the same birthday"
paradox(365, 0.5); // => 22.49

var paradox = function(setSize,p){
  return Math.sqrt(2*setSize*Math.log(1/(1-p)))
};