4875913domain: NAppears in sequencesa(n) = a(n-1) + 3*a(n-2) for n > 1, a(0) = a(1) = 1.at n=19A006130Expansion of 1/(1 - 7*x + 9*x^2).at n=9A099459a(n) = Sum_{k=0..floor(n/2)} binomial(n-k, k-1) * 3^(k-1).at n=20A099579a(n) = (-1)*a(n-1) + 3*a(n-2) with a(1)=-1 and a(2)=1.at n=19A140167