Sheldon discovers the paradox of zero that was the subject of the popular book Zero: The Biography of a Dangerous Idea by Charles Seife, published on Feb 7, 2000.
Zero. Zero is simply a placeholder. It doesn't exist except as a placeholder. Both of the professors and Sheldon would be very well aware of this. The concept of zero has been around in a number of ancient civilizations from Babylon to India to Mesopotamia to Mayans.