Someone already mentioned Permutation City by Egan, but look at his website for other math ones; he is considered one of the hardest sci-fi authors out there (?

Someone already mentioned Permutation City by Egan, but look at his website for other math ones; he is considered one of the hardest sci-fi authors out there (?). I second "The Three Body Problem" series (writing is meh but I think the ideas are excellent. Disclaimer: first half of book one is a slog to get through but the second half onwards is fantastic, in my opinion. I keep thinking about the book many years later on a near biweekly basis). "The Gods Themselves" by Asimov isn't a book mentioned here yet—it has nice math-physics energy ideas.