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

Terms

a(0) =0a(1) =-4a(2) =-8a(3) =-4a(4) =16a(5) =60a(6) =136a(7) =252a(8) =416a(9) =636a(10) =920a(11) =1276a(12) =1712a(13) =2236a(14) =2856a(15) =3580a(16) =4416a(17) =5372a(18) =6456a(19) =7676a(20) =9040a(21) =10556a(22) =12232a(23) =14076a(24) =16096a(25) =18300a(26) =20696a(27) =23292a(28) =26096a(29) =29116