Students are told that well-known numbers such as pi and e are irrational numbers, but rarely does a student see a proof that a specific number is irrational, other than the square root of 2, or perhaps the square root of any non-perfect-square number. We present a straightforward proof (from What is Mathematics? by Courant & Robbins, 1941) that the number e (the base of the natural logarithms) is an irrational number.
