When Sauniere's body is found in the Louvre, he has a series of numbers written around his body called the Fibonacci sequence. This is related to the Golden Ratio. The Golden Ratio is a subject that Langdon lectured about at Harvard.
Known by the Greek letter phi (Φ), it is an irrational number with some very interesting properties. The value is commonly expressed as 1.618,033,989 and they never repeat. The way it is related to the Fibonacci numbers is that if you divide two consecutive numbers in the Fibonacci sequence, you always get the approximation of phi. Pretty awesome, right?
Golden Ratio has a few different names including: the Divine Proportion, the Golden Mean, or the Golden Section. The ratio is found with surprising frequency in natural structures as well as man-made art and architecture. The ratio of length to width of 1.618 is seen as visual pleasing.
Interestingly, the ancient Greeks believed that the understanding of the ratio would help one get closer to God because God was "in" the number. The reason is because it is seen as divine in the makeup and infinite in the meanings.
In the human body, the ventricles of the human heart reset themselves at the Golden Ratio point of the heart's rhythmic cycle. If you divide the pitch of a DNA spiral by its diameter, you get the Golden Ratio.
Try it for yourself. Go to this Fibonacci link and divide two of the numbers, greater than 3, in the series at the top of the page. See what you get.