a(n) = (5^n + 3)/4.

Terms

a(0) =1a(1) =2a(2) =7a(3) =32a(4) =157a(5) =782a(6) =3907a(7) =19532a(8) =97657a(9) =488282a(10) =2441407a(11) =12207032a(12) =61035157a(13) =305175782a(14) =1525878907a(15) =7629394532a(16) =38146972657a(17) =190734863282a(18) =953674316407