a(n) = n^5 - n^2*(n^2 - 1)/2.

Terms

a(0) =0a(1) =1a(2) =26a(3) =207a(4) =904a(5) =2825a(6) =7146a(7) =15631a(8) =30752a(9) =55809a(10) =95050a(11) =153791a(12) =238536a(13) =357097a(14) =518714a(15) =734175a(16) =1015936a(17) =1378241a(18) =1837242a(19) =2411119a(20) =3120200a(21) =3987081a(22) =5036746a(23) =6296687a(24) =7797024a(25) =9570625a(26) =11653226a(27) =14083551a(28) =16903432a(29) =20157929