a(n) = (4^(n+1) + 6n + 5)/9.

Terms

a(0) =1a(1) =3a(2) =9a(3) =31a(4) =117a(5) =459a(6) =1825a(7) =7287a(8) =29133a(9) =116515a(10) =466041a(11) =1864143a(12) =7456549a(13) =29826171a(14) =119304657a(15) =477218599a(16) =1908874365a(17) =7635497427a(18) =30541989673a(19) =122167958655a(20) =488671834581