a(n) = phi(2^n - 1)/2.

Terms

a(0) =1a(1) =3a(2) =4a(3) =15a(4) =18a(5) =63a(6) =64a(7) =216a(8) =300a(9) =968a(10) =864a(11) =4095a(12) =5292a(13) =13500a(14) =16384a(15) =65535a(16) =69984a(17) =262143a(18) =240000a(19) =889056a(20) =1320352a(21) =4105040a(22) =3317760a(23) =16200000a(24) =22358700a(25) =56733696a(26) =66382848a(27) =266913216a(28) =267300000a(29) =1073741823