a(n) = 7*4^n+1.

Terms

a(0) =8a(1) =29a(2) =113a(3) =449a(4) =1793a(5) =7169a(6) =28673a(7) =114689a(8) =458753a(9) =1835009a(10) =7340033a(11) =29360129a(12) =117440513a(13) =469762049a(14) =1879048193a(15) =7516192769a(16) =30064771073a(17) =120259084289a(18) =481036337153