a(n) = (3^(2*n+1) + 2^(n+2))/7.

Terms

a(0) =1a(1) =5a(2) =37a(3) =317a(4) =2821a(5) =25325a(6) =227797a(7) =2049917a(8) =18448741a(9) =166037645a(10) =1494336757a(11) =13449026717a(12) =121041232261