a(n) = 8*3^n + 1.

Terms

a(0) =9a(1) =25a(2) =73a(3) =217a(4) =649a(5) =1945a(6) =5833a(7) =17497a(8) =52489a(9) =157465a(10) =472393a(11) =1417177a(12) =4251529a(13) =12754585a(14) =38263753a(15) =114791257a(16) =344373769a(17) =1033121305a(18) =3099363913a(19) =9298091737a(20) =27894275209a(21) =83682825625a(22) =251048476873a(23) =753145430617