470016domain: NAppears in sequencesNumber of invertible 2 X 2 matrices mod n.at n=33A000252G.f. satisfies A(x) = exp( Sum_{k>=1} (-1)^(k+1) * A(x^k) * (4*x)^k/k ).at n=7A363443Number of chordless cycles (of length > 3) in the complement of the n-hypercube graph.at n=8A364370