10460235105domain: NAppears in sequencesa(1) = a(2) = 1, a(3) = 9; for n > 3, a(n) = 3*a(n-1) - 3*a(n-2) + 9*a(n-3).at n=21A101990Sum 3^max(k,n-k),k=0..n.at n=20A107660Binomial transform of A131666.at n=23A135254