a(n) = 14 * 4^n - 1.

Terms

a(0) =13a(1) =55a(2) =223a(3) =895a(4) =3583a(5) =14335a(6) =57343a(7) =229375a(8) =917503a(9) =3670015a(10) =14680063a(11) =58720255a(12) =234881023a(13) =939524095a(14) =3758096383a(15) =15032385535a(16) =60129542143a(17) =240518168575a(18) =962072674303