G.f.: exp( Sum_{n>=1} sigma(n*2^(n-1))*x^n/n ).

Terms

a(0) =1a(1) =1a(2) =4a(3) =13a(4) =33a(5) =97a(6) =273a(7) =709a(8) =1866a(9) =5120a(10) =13036a(11) =33582a(12) =87333a(13) =219269a(14) =555681a(15) =1413270a(16) =3488571a(17) =8687949a(18) =21624643a(19) =52958400a(20) =130132799a(21) =319007916a(22) =773803200a(23) =1877699413a(24) =4549855097a(25) =10950145994a(26) =26310132088a(27) =63155284600