G.f.: Product_{k>=1, j>=1} ((1 + x^(k*j)) / (1 - x^(k*j)))^2.

Terms

a(0) =1a(1) =4a(2) =16a(3) =52a(4) =156a(5) =428a(6) =1120a(7) =2772a(8) =6616a(9) =15224a(10) =34032a(11) =74020a(12) =157340a(13) =327244a(14) =667824a(15) =1338828a(16) =2641332a(17) =5133372a(18) =9840432a(19) =18621476a(20) =34818852a(21) =64374564a(22) =117768176a(23) =213306948a(24) =382733816a(25) =680630120a(26) =1200198784a(27) =2099417544a(28) =3644332860a(29) =6280017740