a(n) = (5*2^n + (-1)^n - 3)/3.

Terms

a(0) =1a(1) =2a(2) =6a(3) =12a(4) =26a(5) =52a(6) =106a(7) =212a(8) =426a(9) =852a(10) =1706a(11) =3412a(12) =6826a(13) =13652a(14) =27306a(15) =54612a(16) =109226a(17) =218452a(18) =436906a(19) =873812a(20) =1747626a(21) =3495252a(22) =6990506a(23) =13981012a(24) =27962026a(25) =55924052a(26) =111848106a(27) =223696212a(28) =447392426a(29) =894784852