Turkish NMO First Round - 2012 Problem - 04 {Combinatorics}
Source:
July 1, 2012
Problem Statement
How many f:A→A are there satisfying f(f(a))=a for every a∈A={1,2,3,4,5,6,7}?<spanclass=′latex−bold′>(A)</span>1<spanclass=′latex−bold′>(B)</span>106<spanclass=′latex−bold′>(C)</span>127<spanclass=′latex−bold′>(D)</span>232<spanclass=′latex−bold′>(E)</span>None