a(n) = (5*2^n - 2*(-1)^n - 9)/3.

Terms

a(0) =1a(1) =3a(2) =11a(3) =23a(4) =51a(5) =103a(6) =211a(7) =423a(8) =851a(9) =1703a(10) =3411a(11) =6823a(12) =13651a(13) =27303a(14) =54611a(15) =109223a(16) =218451a(17) =436903a(18) =873811a(19) =1747623a(20) =3495251a(21) =6990503a(22) =13981011a(23) =27962023a(24) =55924051a(25) =111848103a(26) =223696211a(27) =447392423a(28) =894784851a(29) =1789569703