E.g.f.: Product_{i>=1, j>=1} (1 + x^(i*j) / (i*j)!).

Terms

a(0) =1a(1) =1a(2) =2a(3) =8a(4) =17a(5) =87a(6) =366a(7) =1514a(8) =8770a(9) =45585a(10) =267586a(11) =1612624a(12) =11914416a(13) =73215391a(14) =522906754a(15) =4364545708a(16) =33150679697a(17) =263662491935