We have unraveled two neat and powerful algorithms for calculating certain multiples of nine. These discussions might make for an interesting introduction to a number theory course, or a supplemental project in calculus or advanced algebra. The mathematics involved is within a student’s grasp, and the results are quite startling.
Hurley, Kevin, "Some Interesting Multiples of Nine: Use Your Digits to Get the Digits!" (2004). Undergraduate Mathematics Day: Proceedings and Other Materials. 2.