a(n) = (1/n) * Sum_{ d divides n } mu(n/d) * (2^d - 1).

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =6a(5) =9a(6) =18a(7) =30a(8) =56a(9) =99a(10) =186a(11) =335a(12) =630a(13) =1161a(14) =2182a(15) =4080a(16) =7710a(17) =14532a(18) =27594a(19) =52377a(20) =99858a(21) =190557a(22) =364722a(23) =698870a(24) =1342176a(25) =2580795a(26) =4971008a(27) =9586395a(28) =18512790a(29) =35790267