a(n) = ((n + 1)^2 * (5*n + 4)*n) / 12.

Terms

a(0) =0a(1) =3a(2) =21a(3) =76a(4) =200a(5) =435a(6) =833a(7) =1456a(8) =2376a(9) =3675a(10) =5445a(11) =7788a(12) =10816a(13) =14651a(14) =19425a(15) =25280a(16) =32368a(17) =40851a(18) =50901a(19) =62700a(20) =76440a(21) =92323a(22) =110561a(23) =131376a(24) =155000a(25) =181675a(26) =211653a(27) =245196a(28) =282576a(29) =324075