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

Terms

a(0) =0a(1) =0a(2) =2a(3) =18a(4) =194a(5) =2550a(6) =39962a(7) =730002a(8) =15257090a(9) =359376750a(10) =9424209002a(11) =272385029466