a(n) = (10*2^n + 3 - (-1)^n)/6.

Terms

a(0) =2a(1) =4a(2) =7a(3) =14a(4) =27a(5) =54a(6) =107a(7) =214a(8) =427a(9) =854a(10) =1707a(11) =3414a(12) =6827a(13) =13654a(14) =27307a(15) =54614a(16) =109227a(17) =218454a(18) =436907a(19) =873814a(20) =1747627a(21) =3495254a(22) =6990507a(23) =13981014a(24) =27962027a(25) =55924054a(26) =111848107a(27) =223696214a(28) =447392427a(29) =894784854