Note that Fermat's Last Theorem (the statement that there are no positive, whole number solutions to the equation xⁿ+yⁿ=zⁿ when n>2) is famous for being a mathematical statement that is very simple to make but notoriously difficult to prove or disprove. In fact, it was an open problem for hundreds of years until Andrew Wiles finally completed a proof in 1994. It is a common popular belief that there must be some elementary proofs of the theorem out there (either waiting to be discovered or known in secret as the story suggests). However, Wiles' proof is quite complicated (depending on advanced mathematical knowledge of elliptic curves and modular functions which are not available to a casual puzzle solver) and it seems likely that the statement cannot be proved in a much more elementary way at all.

The author makes a few disappointing mistakes in his description of FLT. He does not seem to realize, for instance, that the theorem states that there is no solution to the famous equation. Also, he claims that Wiles used "the world's most advanced computer program" in his famous resolution of the problem in the 1990's. (So far as I know, there was no computer assistance involved in Wiles' proof at all.)

In the end, Salander seems to realize that Fermat's solution was a sort of joke rather than anything mathematical at all, even suggesting that it would have been better solved by a philosopher than a mathematician. Although it is not described in detail, it seems that she comes up with some solution to the equation x³+y³=z³ that was more of a pun than an actual solution and so was not something that mathematicians (who are apparently too humorless) would ever think of. Since Fermat originally claimed (and Wiles proved) that there are no integer solutions, this doesn't make any sense to me at all. I mean, I think Larsson was trying to make a joke, but that he didn't understand the math enough and so his joke falls flat for anyone who even understands what FLT says.

This book seems to please readers as a thriller, gaining enormous popularity around the world after the author's untimely death, but as mathematical fiction I'm afraid I cannot recommend it.

Jessica writes with an interpretation based on the use of "Zzzzz..." to represent snoring:

This joke does not quite work for me, in part because it seems that Fermat did somehow recognize that there were no positive integer solutions to xⁿ+yⁿ=zⁿ when n>2 and that he apparently thought this was interesting (not sleep inducing), and also because (as Jessica herself questions) it is not certain that the "Zzzzz..."/sleep correspondence would have been recognized in 17th Century France. Furthermore, I don't think Fermat actually had any z's in his formulation of it. (As far as I know, all he wrote on the subject was "To divide a cube into two other cubes, a fourth power or in general any power whatever into two powers of the same denomination above the second is impossible, and I have assuredly found an admirable proof of this, but the margin is too narrow to contain it.") However, none of these objection particularly matter in this case because it could well be that this is what Larsson intended...or that other readers get a kick out of Jessica's interpretation even if Larsson had no particular idea in mind.