G.f.: exp( Sum_{n>=1} (x^n/n) / Product_{d|n} (1 - n*x^d/d) ).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =21a(6) =54a(7) =148a(8) =442a(9) =1433a(10) =5061a(11) =19394a(12) =80308a(13) =357241a(14) =1697870a(15) =8577240a(16) =45845235a(17) =258198133a(18) =1526631800a(19) =9445795717a(20) =60988643813a(21) =409933740177