a(n) = 2^n + 4^n.

Terms

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