A378278
Cogrowth sequence of the 20-element Frobenius group Frob20 = .
1, 0, 0, 0, 1, 1, 3, 7, 15, 27, 51, 99, 199, 403, 819, 1651, 3303, 6579, 13107, 26163, 52327, 104755, 209715, 419635, 839271, 1678131, 3355443, 6710067, 13420135, 26841907, 53687091, 107377459, 214754919, 429503283, 858993459, 1717973811, 3435947623
Offset: 0
Formula
G.f.: (x^6+x^4+x^3-3*x^2+3*x-1) / ((2*x-1) * (x+1) * (x^2+1)/(2*x^2-2*x+1)).