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

Terms

a(0) =0a(1) =1a(2) =4a(3) =28a(4) =272a(5) =3376a(6) =51012a(7) =908608a(8) =18640960a(9) =432891136a(10) =11225320100a(11) =321504185344