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

Terms

a(0) =6a(1) =27a(2) =111a(3) =447a(4) =1791a(5) =7167a(6) =28671a(7) =114687a(8) =458751a(9) =1835007a(10) =7340031a(11) =29360127a(12) =117440511a(13) =469762047a(14) =1879048191a(15) =7516192767a(16) =30064771071a(17) =120259084287a(18) =481036337151