Can a (stapler) PhD thesis comprise multiple unrelated papers? Traditionally, a thesis is assimilated to a monograph and comprises a unified body of work. However, with "stapler" theses becoming more common, I'd like to know if a thesis is allowed to comprise multiple unrelated projects.



To make the question more regional, assume that the country is Germany and that the subject is mathematics.

Indeed for the first few years, I have been working on several projects that had as outputs papers that are completely unrelated (except for the fact that they fall within the same broad subarea of mathematics -- really, one of those listed in the arxiv math section: https://arxiv.org/).
This is dependent on your supervisor, the comittee and the referees (in the systems I know, almost exclusively the supervisor). Moreover, I would expect from any sane supervisor that they know the projects you are working on and at least warn you if you work too much on topics which can not be put into your thesis.

Most of the time (if your supervisor agrees) and you write an introduction outlining connections between the topics, you are fine. I have seen theses where the introduction was "We present some.papers in the area X, here is an introduction to X" and everything was great. It's just a formality.