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

Terms

a(0) =1a(1) =4a(2) =16a(3) =70a(4) =256a(5) =1088a(6) =4096a(7) =16644a(8) =65656a(9) =263168a(10) =1048576a(11) =4199520a(12) =16777216a(13) =67125248a(14) =268439552a(15) =1073809761a(16) =4294967296a(17) =17180148192a(18) =68719476736a(19) =274878973440