Bisection of A000043.

Terms

a(0) =3a(1) =7a(2) =17a(3) =31a(4) =89a(5) =127a(6) =607a(7) =2203a(8) =3217a(9) =4423a(10) =9941a(11) =19937a(12) =23209a(13) =86243a(14) =132049a(15) =756839a(16) =1257787a(17) =2976221a(18) =6972593a(19) =20996011a(20) =25964951a(21) =32582657a(22) =42643801a(23) =57885161