a(n) = 3!*n*S(n-1,3), where S denotes the Stirling numbers of second kind.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =24a(5) =180a(6) =900a(7) =3780a(8) =14448a(9) =52164a(10) =181500a(11) =615780a(12) =2052072a(13) =6749028a(14) =21976500a(15) =71007300a(16) =228009696a(17) =728451972a(18) =2317445100a(19) =7346047140a(20) =23213772120a(21) =73156412196a(22) =229989358500a(23) =721474964100