Duplicate of A002492.

Terms

a(0) =0a(1) =4a(2) =20a(3) =56a(4) =120a(5) =220a(6) =364a(7) =560a(8) =816a(9) =1140a(10) =1540a(11) =2024a(12) =2600a(13) =3276a(14) =4060a(15) =4960a(16) =5984