Suspension of mathematical disbelief is hard to sustain seriously or at length. Accordingly, the best mathematical SF/fantasy I can think of is in short storie

Suspension of mathematical disbelief is hard to sustain seriously or at length. Accordingly, the best mathematical SF/fantasy I can think of is in short stories or novellas which are meant to be humorous. In particular, those criteria characterize: -- Most of the stories in Fantasia Mathematica and its quasi-sequel anthology Mathematical Magpie. -- Flatland, which is novella length. -- The Dragons of Probability, or anything else in Lem's Cyberiad.