a(n) = 7*2^n - 1.

Terms

a(0) =6a(1) =13a(2) =27a(3) =55a(4) =111a(5) =223a(6) =447a(7) =895a(8) =1791a(9) =3583a(10) =7167a(11) =14335a(12) =28671a(13) =57343a(14) =114687a(15) =229375a(16) =458751a(17) =917503a(18) =1835007a(19) =3670015a(20) =7340031a(21) =14680063a(22) =29360127a(23) =58720255a(24) =117440511a(25) =234881023a(26) =469762047a(27) =939524095a(28) =1879048191a(29) =3758096383