Sextuple factorials, 6-factorials, n!!!!!!, n!6.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =4a(5) =5a(6) =6a(7) =7a(8) =16a(9) =27a(10) =40a(11) =55a(12) =72a(13) =91a(14) =224a(15) =405a(16) =640a(17) =935a(18) =1296a(19) =1729a(20) =4480a(21) =8505a(22) =14080a(23) =21505a(24) =31104a(25) =43225a(26) =116480a(27) =229635a(28) =394240a(29) =623645