a(n) = 7*2^(n-1) - 2*n - 5.

Terms

a(0) =0a(1) =5a(2) =17a(3) =43a(4) =97a(5) =207a(6) =429a(7) =875a(8) =1769a(9) =3559a(10) =7141a(11) =14307a(12) =28641a(13) =57311a(14) =114653a(15) =229339a(16) =458713a(17) =917463a(18) =1834965a(19) =3669971a(20) =7339985a(21) =14680015a(22) =29360077a(23) =58720203a(24) =117440457a(25) =234880967a(26) =469761989a(27) =939524035a(28) =1879048129a(29) =3758096319