a(n) = n^5/5 + n^3/3 + 7*n/15.

Terms

a(0) =0a(1) =1a(2) =10a(3) =59a(4) =228a(5) =669a(6) =1630a(7) =3479a(8) =6728a(9) =12057a(10) =20338a(11) =32659a(12) =50348a(13) =74997a(14) =108486a(15) =153007a(16) =211088a(17) =285617a(18) =379866a(19) =497515a(20) =642676a(21) =819917a(22) =1034286a(23) =1291335a(24) =1597144a(25) =1958345a(26) =2382146a(27) =2876355a(28) =3449404a(29) =4110373