The Last Theorem by Arthur C. Clarke and Frederik Pohl
This is one of the most congenial SciFi novels I've read in a long time. Meaning it's just a pleasant read (for the most part, anyway, with only a couple of minor exceptions, such as the period where poor Ranjit, our hero, is mistakenly locked up and “interrogated” as a suspected terrorist), entertaining, and intriguing all at once. Not to mention that it's the first, last, and only collaboration between these two science fiction greats, grandmasters both. This alone makes it essential reading for all serious science fiction buffs, such as myself.
Much, even most of the story takes place on the island of Sri Lanka, Arthur C. Clarke's chosen home for many decades. Unfortunately, he had passed on by the time this book made it into print.
Our primary protagonist is a brilliant young mathematician, who lives an eventful, but satisfying life. Among his accomplishments is finally solving Fermat's Last Theorem, one of the perennially classic mathematical challenges. In fact, he actually comes up with with the proof while in prison, suffering from those currently infamous “enhanced” interrogation techniques.
That wouldn't be enough on its own for a good SciFi story, though. What makes the novel more interesting is the fact that we (the human race, that is) have inadvertently and unknowingly come to the attention of the galactic civilization, such as it is, and they have decided that we are likely to be dangerous, and should be quickly snuffed out. There are interesting scientific developments going on here on earth, as well, such as the construction of the first “space elevator,” an actual cable stretching into earth orbit territory, which makes it possible to lift large masses of material into space much less expensively than with rockets.
How these various elements come together into a satisfyingly happy ending keeps you guessing all along the way. Definitely recommended essential reading for SciFi aficionados.