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

Terms

a(0) =5a(1) =18a(2) =59a(3) =184a(4) =561a(5) =1694a(6) =5095a(7) =15300a(8) =45917a(9) =137770a(10) =413331a(11) =1240016a(12) =3720073a(13) =11160246a(14) =33480767a(15) =100442332a(16) =301327029a(17) =903981122a(18) =2711943403a(19) =8135830248a(20) =24407490785a(21) =73222472398a(22) =219667417239a(23) =659002251764