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

Terms

a(0) =0a(1) =54a(2) =648a(3) =4860a(4) =29160a(5) =153090a(6) =734832a(7) =3306744a(8) =14171760a(9) =58458510a(10) =233834040a(11) =911952756a(12) =3482001432a(13) =13057505370a(14) =48212327520a(15) =175630621680a(16) =632270238048