a(-1) = 1; for n >= 0, a(n) = 2^n + 4^n = 2^n*(1 + 2^n).

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =72a(5) =272a(6) =1056a(7) =4160a(8) =16512a(9) =65792a(10) =262656a(11) =1049600a(12) =4196352a(13) =16781312a(14) =67117056a(15) =268451840a(16) =1073774592a(17) =4295032832a(18) =17180000256a(19) =68719738880a(20) =274878431232