G.f.: Product_{i>=1, j>=1} (1 + x^(i*j))^(i*j).

Terms

a(0) =1a(1) =1a(2) =4a(3) =10a(4) =24a(5) =52a(6) =125a(7) =253a(8) =549a(9) =1126a(10) =2290a(11) =4525a(12) =8987a(13) =17259a(14) =33174a(15) =62669a(16) =117425a(17) =217295a(18) =399904a(19) =726984a(20) =1314257a(21) =2354807a(22) =4191671a(23) =7405590a(24) =13009916a(25) =22696115a(26) =39384232a(27) =67937488a(28) =116584833a(29) =199001304