a(1) = a(2) = 1; for n>2, a(n) = 4*a(n-1) + 3*a(n-2).

Terms

a(0) =1a(1) =1a(2) =7a(3) =31a(4) =145a(5) =673a(6) =3127a(7) =14527a(8) =67489a(9) =313537a(10) =1456615a(11) =6767071a(12) =31438129a(13) =146053729a(14) =678529303a(15) =3152278399a(16) =14644701505a(17) =68035641217a(18) =316076669383