a(n) = (7*4^n - 1)/3.

Terms

a(0) =2a(1) =9a(2) =37a(3) =149a(4) =597a(5) =2389a(6) =9557a(7) =38229a(8) =152917a(9) =611669a(10) =2446677a(11) =9786709a(12) =39146837a(13) =156587349a(14) =626349397a(15) =2505397589a(16) =10021590357a(17) =40086361429a(18) =160345445717a(19) =641381782869