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

Terms

a(0) =1a(1) =3a(2) =8a(3) =22a(4) =63a(5) =185a(6) =550a(7) =1644a(8) =4925a(9) =14767a(10) =44292a(11) =132866a(12) =398587a(13) =1195749a(14) =3587234a(15) =10761688a(16) =32285049a(17) =96855131a(18) =290565376a(19) =871696110a(20) =2615088311a(21) =7845264913a(22) =23535794718a(23) =70607384132a(24) =211822152373a(25) =635466457095