Summary: At the Louvre, Sophie Neveu tells Fache that she believes the numbers Saunière scrawled on the floor are a scrambled Fibonacci sequence: “a progression [of numbers] in which each term is equal to the sum of the two preceding terms.” She suggests that Saunière intended nothing more than a numerical joke, a suggestion Fache utterly rejects. After Neveu leaves, Langdon hangs up on his emergency phone call, telling Fache that a friend at home has been in an accident. Shaken, Langdon heads for the men’s restroom. Fache consults with Collet in Saunière’s office, instructing the other agent to continue to keep close tabs on Langdon via the GPS beacon.
Analysis: The cryptological challenge at the heart of this brief chapter, the Fibonacci sequence, was, as Neveu tells Fache, created in the 13th century by Leonardo Fibonacci, also known as Leonardo of Pisa. In addition, Fibonacci introduced the decimal system into Europe (using Arabic numerals and the symbol for zero) (http:www.mcs.surrey.ac.uk/Personal/R.Knott/