MathDB
Probability of n events not happening

Source: Kürschak 2012, problem 3

July 6, 2014
probabilityexpected valuecombinatorics unsolvedcombinatorics

Problem Statement

Consider nn events, each of which has probability 12\frac12. We also know that the probability of any two both happening is 14\frac14. Prove the following. (a) The probability that none of these events happen is at most 1n+1\frac1{n+1}. (b) We can reach equality in (a) for infinitely many nn.