This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A322939 #6 Jan 06 2019 11:25:07 %S A322939 1,1,1,9,25,81,273,889,2921,9601,31521,103529,340025,1116721,3667633, %T A322939 12045529,39560841,129928801,426722241,1401474249,4602830425, %U A322939 15116972561,49648333393,163058906169,535530702761,1758831457601,5776490648161,18971598480169,62307994735225 %N A322939 a(n) = [x^n] (4*x^2 + x - 1)/(4*x^3 + 3*x^2 + 2*x - 1). %F A322939 a(n) = 4*a(n-3) + 3*a(n-2) + 2*a(n-1) for n >= 3. %p A322939 gf := (4*x^2 + x - 1)/(4*x^3 + 3*x^2 + 2*x - 1): ser := series(gf, x, 22): %p A322939 seq(coeff(ser, x, n), n = 0..20); %p A322939 a := proc(n) option remember; %p A322939 `if`(n < 3, [1, 1, 1][n+1], 4*a(n-3) + 3*a(n-2) + 2*a(n-1)) end: %Y A322939 Cf. A322940. %K A322939 nonn %O A322939 0,4 %A A322939 _Peter Luschny_, Jan 06 2019