a(n) = 4*a(n-1) + a(n-2); a(0)=1, a(1)=7.

Terms

a(0) =1a(1) =7a(2) =29a(3) =123a(4) =521a(5) =2207a(6) =9349a(7) =39603a(8) =167761a(9) =710647a(10) =3010349a(11) =12752043a(12) =54018521a(13) =228826127a(14) =969323029a(15) =4106118243a(16) =17393796001a(17) =73681302247a(18) =312119004989