a(n) = (2 + 7*4^n)/3.

Terms

a(0) =3a(1) =10a(2) =38a(3) =150a(4) =598a(5) =2390a(6) =9558a(7) =38230a(8) =152918a(9) =611670a(10) =2446678a(11) =9786710a(12) =39146838a(13) =156587350a(14) =626349398a(15) =2505397590a(16) =10021590358a(17) =40086361430a(18) =160345445718a(19) =641381782870