a(n) = 21*a(n-2) + 7*a(n-3), with a(0)=a(1)=0, a(2)=9.

Terms

a(0) =0a(1) =0a(2) =9a(3) =0a(4) =189a(5) =63a(6) =3969a(7) =2646a(8) =83790a(9) =83349a(10) =1778112a(11) =2336859a(12) =37923795a(13) =61520823a(14) =812757708a(15) =1557403848a(16) =17498557629a(17) =38394784764a(18) =378371537145a(19) =928780383447