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

Terms

a(0) =7a(1) =39a(2) =126a(3) =310a(4) =645a(5) =1197a(6) =2044a(7) =3276a(8) =4995a(9) =7315a(10) =10362a(11) =14274a(12) =19201a(13) =25305a(14) =32760a(15) =41752a(16) =52479a(17) =65151a(18) =79990a(19) =97230a(20) =117117a(21) =139909a(22) =165876a(23) =195300a(24) =228475a(25) =265707a(26) =307314a(27) =353626a(28) =404985a(29) =461745