G.f.: Sum_{n>=0} x^n / Product_{d|n} (1-d*x).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =37a(6) =114a(7) =384a(8) =1405a(9) =5542a(10) =23395a(11) =105140a(12) =501157a(13) =2525266a(14) =13403833a(15) =74657894a(16) =434731997a(17) =2637999706a(18) =16641634913a(19) =108969490492a(20) =739936135562