a(n) = -3*a(n-1) + a(n-3), with a(0)=0, a(1)=6, and a(2)=-15.

Terms

a(0) =0a(1) =6a(2) =-15a(3) =45a(4) =-129a(5) =372a(6) =-1071a(7) =3084a(8) =-8880a(9) =25569a(10) =-73623a(11) =211989a(12) =-610398a(13) =1757571a(14) =-5060724a(15) =14571774a(16) =-41957751a(17) =120812529a(18) =-347865813a(19) =1001639688a(20) =-2884106535a(21) =8304453792a(22) =-23911721688a(23) =68851058529a(24) =-198248721795a(25) =570834443697