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

Terms

a(0) =8a(1) =13a(2) =29a(3) =55a(4) =113a(5) =223a(6) =449a(7) =895a(8) =1793a(9) =3583a(10) =7169a(11) =14335a(12) =28673a(13) =57343a(14) =114689a(15) =229375a(16) =458753a(17) =917503a(18) =1835009a(19) =3670015a(20) =7340033a(21) =14680063a(22) =29360129a(23) =58720255a(24) =117440513a(25) =234881023a(26) =469762049a(27) =939524095a(28) =1879048193a(29) =3758096383