In the first issue of Fundamenta Mathematicae, 1920, Wacław Sierpiński has 13 single-authored papers and 1 joint with Stefan Mazurkiewicz. In the same issue Mazurkiewicz has 3 addtional single-authored papers. Kazimierz (Casimir) Kuratowski has 3 single-authored papers and 1 joint with Zygmunt Janiszewski (a posthumous publication for the latter). FundamentaThe papers are not parts of bigger ones. Sierpiński also contributed several open problems to the first issue (one jointly with his student Tadeusz Felsztyn); Mazurkiewicz (alone) and Kuratowski (jointly with Bronisław Knaster) contributed 1 problem each.
Sierpiński and Mazurkiewicz also were editors-in-chief of the journal at the time, taking over after the death of Janiszewski, who got the idea of the journal and prepared the first issue for print. Fundamenta were supposed to concentrate on set theory and topology, the specialties of the still-new Polish school of mathematics, so it is natural that the specialized journal featured so many contributions by then-active top Polish mathematicians. Especially the first issue, which was also meant as a kind of an introduction of the Polish school to the broader mathematical community. But even in later pre-war issues there are often multiple papers by Sierpiński and/or other authors. The
The papers can be viewed and downloaded for free at https://www.impan.pl/en/publishing-house/journals-and-series/fundamenta-mathematicae/all/1