An even permutation is a permutation obtainable from an even number of two-element swaps, For initial set 1,2,3,4, the twelve even permutations are those with zero swaps: (1,2,3,4); and those with two swaps: (1,3,4,2, 1,4,2,3, 2,1,4,3, 2,3,1,4, 2,4,3,1, 3,1,2,4, 3,2,4,1, 3,4,1,2, 4,1,3,2, 4,2,1,3, 4,3,2,1). etc.
For a set of n elements and n>2, there are n! / 2 even permutations, which is the same as the number of odd permutations