Quarter-square tetrahedrals: a(n) = k*(k - 1)*(k - 2)/6, k = A002620(n).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =4a(5) =20a(6) =84a(7) =220a(8) =560a(9) =1140a(10) =2300a(11) =4060a(12) =7140a(13) =11480a(14) =18424a(15) =27720a(16) =41664a(17) =59640a(18) =85320a(19) =117480a(20) =161700a(21) =215820a(22) =287980a(23) =374660a(24) =487344a(25) =620620a(26) =790244a(27) =988260a(28) =1235780a(29) =1521520