a(n) = (5*2^n + 7*(-1)^n)/3.

Terms

a(0) =4a(1) =1a(2) =9a(3) =11a(4) =29a(5) =51a(6) =109a(7) =211a(8) =429a(9) =851a(10) =1709a(11) =3411a(12) =6829a(13) =13651a(14) =27309a(15) =54611a(16) =109229a(17) =218451a(18) =436909a(19) =873811a(20) =1747629a(21) =3495251a(22) =6990509a(23) =13981011a(24) =27962029a(25) =55924051a(26) =111848109a(27) =223696211a(28) =447392429a(29) =894784851