a(n) = 4^n for n = 0..3; for n > 3, a(n) = 4*a(n-1) + a(n-4).

Terms

a(0) =1a(1) =4a(2) =16a(3) =64a(4) =257a(5) =1032a(6) =4144a(7) =16640a(8) =66817a(9) =268300a(10) =1077344a(11) =4326016a(12) =17370881a(13) =69751824a(14) =280084640a(15) =1124664576a(16) =4516029185a(17) =18133868564a(18) =72815558896a(19) =292386900160