a(n) = 7*2^n - 3.

Terms

a(0) =4a(1) =11a(2) =25a(3) =53a(4) =109a(5) =221a(6) =445a(7) =893a(8) =1789a(9) =3581a(10) =7165a(11) =14333a(12) =28669a(13) =57341a(14) =114685a(15) =229373a(16) =458749a(17) =917501a(18) =1835005a(19) =3670013a(20) =7340029a(21) =14680061a(22) =29360125a(23) =58720253a(24) =117440509a(25) =234881021a(26) =469762045a(27) =939524093a(28) =1879048189a(29) =3758096381