Content Warning: This section of the guide includes discussion of physical abuse.

Fermat’s last theorem is a mathematical assertion made by Pierre de Fermat, a French mathematician, in 1637. Fermat proposed that xn + yn = zn cannot be completed where n is any natural number greater than 2, which would become the Pythagorean theorem: x2 + y2 = z2. In Fermat’s copy of Arithmetica by Diophantus of Alexandria, he wrote that he had a proof for his theorem, but it would not fit in the margin. Andrew Wiles and his student, Richard Tayler, created a proof for the theorem in 1995, but the length of time that passed without a proof suggests that Fermat himself had not created one either.

In the context of The Girl Who Played With Fire, Fermat’s last theorem is a puzzle that Lisbeth becomes obsessed with solving. She has developed an interest in mathematics during her recent travels, and Fermat’s last theorem becomes a symbol of her desire to find herself. Though she is seeking a proof for the theorem, this abstract search is paired with her more concrete decisions to get breast implants and to travel, each of which is framed as a pursuit of identity.