a(n) = (1 + 2^n*(3 + 2*(-1)^n))/3.

Terms

a(0) =2a(1) =1a(2) =7a(3) =3a(4) =27a(5) =11a(6) =107a(7) =43a(8) =427a(9) =171a(10) =1707a(11) =683a(12) =6827a(13) =2731a(14) =27307a(15) =10923a(16) =109227a(17) =43691a(18) =436907a(19) =174763a(20) =1747627a(21) =699051a(22) =6990507a(23) =2796203a(24) =27962027a(25) =11184811a(26) =111848107a(27) =44739243a(28) =447392427a(29) =178956971