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

Terms

a(0) =1a(1) =4a(2) =17a(3) =74a(4) =323a(5) =1400a(6) =6005a(7) =25478a(8) =107015a(9) =445556a(10) =1841273a(11) =7561922a(12) =30897227a(13) =125714672a(14) =509767421a(15) =2061390206a(16) =8317305359a(17) =33498803948a(18) =134727010049a(19) =541232563130