a(n) = (n^6 + n^3)/2.

Terms

a(0) =0a(1) =1a(2) =36a(3) =378a(4) =2080a(5) =7875a(6) =23436a(7) =58996a(8) =131328a(9) =266085a(10) =500500a(11) =886446a(12) =1493856a(13) =2414503a(14) =3766140a(15) =5697000a(16) =8390656a(17) =12071241a(18) =17009028a(19) =23526370a(20) =32004000a(21) =42887691a(22) =56695276a(23) =74024028a(24) =95558400a(25) =122078125a(26) =154466676a(27) =193720086a(28) =240956128a(29) =297423855