a(n) = a(n-1) - 2*a(n-2) - 3*a(n-3) - ... - (n-1)*a(1), a(1) = a(2) = 3, a(3) = -3.

Terms

a(0) =3a(1) =3a(2) =-3a(3) =-18a(4) =-33a(5) =-15a(6) =84a(7) =261a(8) =333a(9) =-138a(10) =-1557a(11) =-3315a(12) =-2436a(13) =6153a(14) =24009a(15) =36390a(16) =1431a(17) =-129639a(18) =-323292a(19) =-318819a(20) =400725a(21) =2149686a(22) =3807795a(23) =1476405a(24) =-10310388a(25) =-30697599a(26) =-37588047a(27) =20103078a(28) =186854271a(29) =384871329