a(n) = 4^n - 2*3^(n-1)*(n+3) + 2^(n-2)*(n^2+3*n+4).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =6a(5) =80a(6) =650a(7) =4172a(8) =23310a(9) =119016a(10) =571122a(11) =2621828a(12) =11651222a(13) =50536928a(14) =215219706a(15) =903799548a(16) =3754755102a(17) =15469272536a(18) =63320624642a(19) =257886717812