class MoreMath::Permutation

an even number of transpositions.
transpositions (cycles of length 2), or even if it can be represented of
A permutation is odd if it can be represented by an odd number of

an even permutation.
It's -1 if this permutation is odd and 1 if it's
Returns the signum of this Permutation instance.

def signum
  s = 1
  cycles.each do |c|
    c.size % 2 == 0 and s *= -1
  end
  s
end