a(0)=1, a(1)=3,a(n)=6*a(n-2)-a(n-1).

Terms

a(0) =1a(1) =3a(2) =3a(3) =15a(4) =3a(5) =87a(6) =-69a(7) =591a(8) =-1005a(9) =4551a(10) =-10581a(11) =37887a(12) =-101373a(13) =328695a(14) =-936933a(15) =2909103a(16) =-8530701a(17) =25985319a(18) =-77169525a(19) =233081439a(20) =-696098589a(21) =2094587223a(22) =-6271178757a(23) =18838702095a(24) =-56465774637a(25) =169497987207a(26) =-508292635029