77883687150domain: NAppears in sequencesThe (1,1)-entry in the 3 X 3 matrix M^n, where M = [1,2,1 / 2,2,0 / 1,0,0].at n=19A100191Expansion of g.f.: x^2*(1-2*x) / (1-3*x-3*x^2+2*x^3).at n=21A123950