Partial sums of A000166.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =13a(5) =57a(6) =322a(7) =2176a(8) =17009a(9) =150505a(10) =1485466a(11) =16170036a(12) =192384877a(13) =2483177809a(14) =34554278858a(15) =515620794592