a(n) = (9*5^n-1)/4.

Terms

a(0) =2a(1) =11a(2) =56a(3) =281a(4) =1406a(5) =7031a(6) =35156a(7) =175781a(8) =878906a(9) =4394531a(10) =21972656a(11) =109863281a(12) =549316406a(13) =2746582031a(14) =13732910156a(15) =68664550781a(16) =343322753906