Expansion of 1 / (1 - Sum_{i>=1, j>=1} x^(i*j*(j + 1)/2)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =10a(5) =21a(6) =47a(7) =99a(8) =211a(9) =455a(10) =973a(11) =2081a(12) =4464a(13) =9558a(14) =20466a(15) =43848a(16) =93914a(17) =201140a(18) =430844a(19) =922818a(20) =1976553a(21) =4233613a(22) =9067960a(23) =19422576a(24) =41601229a(25) =89105550a(26) =190854784a(27) =408791400a(28) =875589076a(29) =1875421302