a(n) = 28*2^n - 15.

Terms

a(0) =13a(1) =41a(2) =97a(3) =209a(4) =433a(5) =881a(6) =1777a(7) =3569a(8) =7153a(9) =14321a(10) =28657a(11) =57329a(12) =114673a(13) =229361a(14) =458737a(15) =917489a(16) =1834993a(17) =3670001a(18) =7340017a(19) =14680049a(20) =29360113a(21) =58720241a(22) =117440497a(23) =234881009a(24) =469762033a(25) =939524081a(26) =1879048177a(27) =3758096369a(28) =7516192753a(29) =15032385521