a(n) = ((n-1)^2*n^2*(n+1)^2)/6 - 2*Sum_{l=2..n}Sum_{k=2..n}(n-k+1)*(n-l+1)*(k-1)*(l-1).

Terms

a(0) =0a(1) =0a(2) =4a(3) =64a(4) =400a(5) =1600a(6) =4900a(7) =12544a(8) =28224a(9) =57600a(10) =108900a(11) =193600a(12) =327184a(13) =529984a(14) =828100a(15) =1254400a(16) =1849600a(17) =2663424a(18) =3755844a(19) =5198400a(20) =7075600a(21) =9486400a(22) =12545764a(23) =16386304a(24) =21160000a(25) =27040000a(26) =34222500a(27) =42928704a(28) =53406864a(29) =65934400