a(n) = 2*a(n-2) + 1.

Terms

a(0) =4a(1) =6a(2) =9a(3) =13a(4) =19a(5) =27a(6) =39a(7) =55a(8) =79a(9) =111a(10) =159a(11) =223a(12) =319a(13) =447a(14) =639a(15) =895a(16) =1279a(17) =1791a(18) =2559a(19) =3583a(20) =5119a(21) =7167a(22) =10239a(23) =14335a(24) =20479a(25) =28671a(26) =40959a(27) =57343a(28) =81919a(29) =114687