A group of space explorers attempt to protect the secret that they are no longer aging in this well written SF novel. Although these (essentially) immortal characters are not especially mathematical, along the way they meet various characters who are.
 An retired elderly actuary is contacted by someone seeking to learn the life expectancy of a person who can die of disease or accident but not from aging. A very nice description is given of how this can be worked out using a geometric series.
 A young number theorist seeks the help of a "haldane" (a sort of psychologist). Since something seems fishy in is description of his past, she inquires about his research to see whether he might be lying to her. However, his account of the math that interests him is sufficiently detailed to convince the haldane that he is sincere and also enough to convince me that the author knows what he is talking about as well. (Not too surprising since he has a PhD in mathematics and published some real research papers on mathematical physics back around 1970.)
 Towards the end, a discussion with the rich operator of casinos (who in a strange way also happens to be the same person as the two previously mentioned characters) repeats the old stereotype that ``mathematics is a young man's game'' .
 The haldane's younger brother is a genius who participates in the ``puzzle network''. The puzzles he posts for his friends are very mathematical, involving higher dimensional topology.
