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

Terms

a(0) =5a(1) =12a(2) =26a(3) =54a(4) =110a(5) =222a(6) =446a(7) =894a(8) =1790a(9) =3582a(10) =7166a(11) =14334a(12) =28670a(13) =57342a(14) =114686a(15) =229374a(16) =458750a(17) =917502a(18) =1835006a(19) =3670014a(20) =7340030a(21) =14680062a(22) =29360126a(23) =58720254a(24) =117440510a(25) =234881022a(26) =469762046a(27) =939524094a(28) =1879048190a(29) =3758096382