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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =3a(5) =4a(6) =8a(7) =9a(8) =11a(9) =19a(10) =23a(11) =28a(12) =42a(13) =51a(14) =62a(15) =89a(16) =108a(17) =130a(18) =178a(19) =215a(20) =260a(21) =344a(22) =413a(23) =496a(24) =639a(25) =766a(26) =916a(27) =1155a(28) =1380a(29) =1641