a(n) = 4*3^n+4^n.

Terms

a(0) =5a(1) =16a(2) =52a(3) =172a(4) =580a(5) =1996a(6) =7012a(7) =25132a(8) =91780a(9) =340876a(10) =1284772a(11) =4902892a(12) =18902980a(13) =73486156a(14) =287567332a(15) =1131137452a(16) =4467154180a(17) =17696429836a(18) =70269158692a(19) =279526952812