I attended lectures from Steve Vickers on Mathematical Structures in Computer Science, which is to date the hardest course or study I have ever done. I mean, I have studied advanced logic, pure maths, electronic circuit design and those were all a walk in the park compared to this. If you want to get a taste of how difficult and abstract the topic is, have a look at some of his papers, eg: https://www.cambridge.org/core/journals/journal-of-symbolic-... or https://scholar.google.com/scholar_lookup?title=Topical+cate...

Sadly I didn't get him to sign my original copy of the ZX81 Manual :-(