|A 17 year-old girl from Los Angeles finds herself in a sexual/romantic relationship with a not-quite-human time-traveller in this book which continues the author's "Skolian saga".
The story is actually much more complicated than that summary suggests. For one thing, the time traveller realizes too late that he is moving not only through time but also between different universes. (He comes from a universe where the city of Los Angeles is in a country called the "United States of America", but in the universe where he meets the girl it is the "Federated States of America".) Moreover, it turns out that they have common ancestry in ancient Central America: she is of Mayan descent and he is descended from an ancient Native American culture whose people were taken aboard a space ship and over time evolved to live in another solar system.
But, what about math? Asaro likes writing about the use of complex-valued time variables in special relativity (a subject on which she also wrote a non-fictional article in the American Journal of Physics) and about the branch cuts between different sheets in a Riemann surface. Here, these ideas are used to explain both how he travels through time and how he ended up in a "different universe". Moreover, his mother, it turns out, was a mathematician (before she became a pharaoh) and developed a mathematical transform between physical space and a sort of psychic space (called "Kyle space" in the book).
This work was originally published as a single book by Tor but in 2022 the author tells me a modified version will soon be re-released by Open Road in two volumes under the title "Lightning Strikes". [Note: It was the two volume "Lightning Strikes" version which I have read, and I am not positive whether any of the things I mention above were different in the original.]