53189708domain: NAppears in sequencesa(n) = 2*a(n-1) + a(n-2) - a(n-3), with a(0) = a(1) = 0, a(2) = 1.at n=24A006054Expansion of g.f.: 1/(1 - 2*x - x^2 + x^3).at n=22A106805