a(n) = 6*a(n-1) - 9*a(n-2) + 2*a(n-3) for n>3, a(1)=0, a(2)=-1, a(3)=0.

Terms

a(0) =0a(1) =-1a(2) =0a(3) =9a(4) =52a(5) =231a(6) =936a(7) =3641a(8) =13884a(9) =52407a(10) =196768a(11) =736713a(12) =2754180a(13) =10288199a(14) =38415000a(15) =143404569a(16) =535268812a(17) =1997801751a(18) =7456200336a(19) =27827523881a(20) =103854943764a(21) =387594348327