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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/).