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

Terms

a(0) =1a(1) =0a(2) =0a(3) =6a(4) =36a(5) =150a(6) =540a(7) =1806a(8) =5796a(9) =18150a(10) =55980a(11) =171006a(12) =519156a(13) =1569750a(14) =4733820a(15) =14250606a(16) =42850116a(17) =128746950a(18) =386634060a(19) =1160688606a(20) =3483638676a(21) =10454061750a(22) =31368476700a(23) =94118013006a(24) =282379204836a(25) =847187946150