"She spent breakfast scribbling figures on a napkin and pondering Pierre de Fermat’s (x3+y3=z3)."
Pierre de Fermat
Public DomainPierre de Fermat
Pierre de Fermat 17 August 1601 or 1607/8 – 12 January 1665)

Fermat was a French lawyer at the Parlement of Toulouse. As an amateur mathematician he laid the foundations that eventually led to the development of modern calculus. This 'puzzle' that Salander dwells on throughout the book is called Fermat's Last Theorem. The ambiguity of an equation with no solution is an ingenious way of provoking Salander; who always has a sharp sense of right and wrong.