Dirichlet g.f.: Product_{k>=2} 1 / (1 - k^(-s))^(4^(k-1)).

Terms

a(0) =1a(1) =4a(2) =16a(3) =74a(4) =256a(5) =1088a(6) =4096a(7) =16660a(8) =65672a(9) =263168a(10) =1048576a(11) =4199584a(12) =16777216a(13) =67125248a(14) =268439552a(15) =1073810115a(16) =4294967296a(17) =17180148256a(18) =68719476736a(19) =274878974464