a(n) = (5^n - 1)/4.

Terms

a(0) =0a(1) =1a(2) =6a(3) =31a(4) =156a(5) =781a(6) =3906a(7) =19531a(8) =97656a(9) =488281a(10) =2441406a(11) =12207031a(12) =61035156a(13) =305175781a(14) =1525878906a(15) =7629394531a(16) =38146972656a(17) =190734863281a(18) =953674316406