12335653674domain: NAppears in sequencesExpansion of (1-x)*(1+x)/(1-2*x-x^2+x^3).at n=29A052534a(n) = 2a(n-1) + a(n-2) - a(n-3); a(0)=4, a(1)=9, a(2)=20.at n=27A109110