a(n) = (29*3^n - 18(n + 3)*2^n + 6n^2 + 24n + 27)/12.

Terms

a(0) =0a(1) =0a(2) =6a(3) =46a(4) =228a(5) =930a(6) =3406a(7) =11682a(8) =38412a(9) =122806a(10) =385182a(11) =1192254a(12) =3656452a(13) =11141178a(14) =33791934a(15) =102161962a(16) =308156748a(17) =928008846a(18) =2791497262a(19) =8390220006a(20) =25203689700a(21) =75680274610a(22) =227185526766a(23) =681858569586