a(n) = (2 + (-1)^n + 3^n)/4.

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =21a(5) =61a(6) =183a(7) =547a(8) =1641a(9) =4921a(10) =14763a(11) =44287a(12) =132861a(13) =398581a(14) =1195743a(15) =3587227a(16) =10761681a(17) =32285041a(18) =96855123a(19) =290565367a(20) =871696101a(21) =2615088301a(22) =7845264903a(23) =23535794707a(24) =70607384121a(25) =211822152361a(26) =635466457083