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

Terms

a(0) =1a(1) =2a(2) =8a(3) =20a(4) =68a(5) =188a(6) =596a(7) =1724a(8) =5300a(9) =15644a(10) =47444a(11) =141308a(12) =425972a(13) =1273820a(14) =3829652a(15) =11472572a(16) =34450484a(17) =103285916a(18) =309988820a(19) =929704316a(20) =2789637236a(21) =8367863132a(22) =25105686548a(23) =75312865340a(24) =225946984628a(25) =677824176668