a(n) = (3^n+6*(-4)^n)/7.

Terms

a(0) =1a(1) =-3a(2) =15a(3) =-51a(4) =231a(5) =-843a(6) =3615a(7) =-13731a(8) =57111a(9) =-221883a(10) =907215a(11) =-3569811a(12) =14456391a(13) =-57294123a(14) =230770815a(15) =-918300291a(16) =3687550071a(17) =-14707153563a(18) =58957754415a(19) =-235443597171a(20) =942936650151