a(n) = (n^7 - n)/6.

Terms

a(0) =0a(1) =0a(2) =21a(3) =364a(4) =2730a(5) =13020a(6) =46655a(7) =137256a(8) =349524a(9) =797160a(10) =1666665a(11) =3247860a(12) =5971966a(13) =10458084a(14) =17568915a(15) =28476560a(16) =44739240a(17) =68389776a(18) =102036669a(19) =148978620a(20) =213333330a(21) =300181420a(22) =415726311a(23) =567470904a(24) =764411900a(25) =1017252600