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

Terms

a(0) =-1a(1) =0a(2) =6a(3) =30a(4) =114a(5) =390a(6) =1266a(7) =3990a(8) =12354a(9) =37830a(10) =115026a(11) =348150a(12) =1050594a(13) =3164070a(14) =9516786a(15) =28599510a(16) =85896834a(17) =257887110a(18) =774054546a(19) =2322950070a(20) =6970423074a(21) =20914414950a(22) =62749536306a(23) =188261191830a(24) =564808741314