@robjohn For example, one of the citations looks like this, up to some formatting lost:
"K. Mahler and J. Popken, On a maximum problem in arithmetic. (Dutch), Nieuw Arch.
Wiskunde"
Everything follows a name, title, journal, volume, issue, page format, but the bibtex entry given from the source I linked does not include a journal, but a publisher instead. In the plain bibliography style publishers are omitted so the citation just goes author, title, volume, issue, page. I am concerned that it looks like a glaring mistake in the bibliography.