a(n) = n*(n+1)*(2*n+1)*(n^2+n+3)/30.

Terms

a(0) =0a(1) =1a(2) =9a(3) =42a(4) =138a(5) =363a(6) =819a(7) =1652a(8) =3060a(9) =5301a(10) =8701a(11) =13662a(12) =20670a(13) =30303a(14) =43239a(15) =60264a(16) =82280a(17) =110313a(18) =145521a(19) =189202a(20) =242802a(21) =307923a(22) =386331a(23) =479964a(24) =590940a(25) =721565a(26) =874341a(27) =1051974a(28) =1257382a(29) =1493703