Combinatorics
Source: Danube Mathematical Competition,Seniors #2
October 29, 2016
combinatorics
Problem Statement
A bank has a set S of codes formed only with 0 and 1,each one with length n.Two codes are 'friends' if they are different on only one position.We know that each code has exactly k 'friends'.Prove that:
1)S has an even number of elements
2)S contains at least codes