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

Terms

a(0) =1a(1) =4a(2) =18a(3) =86a(4) =418a(5) =2022a(6) =9650a(7) =45334a(8) =209730a(9) =956870a(10) =4312402a(11) =19228662a(12) =84948962a(13) =372287398a(14) =1620178674a(15) =7008019670a(16) =30150864514a(17) =129107299206a(18) =550530654866