73 is Sheldon Cooper's favorite number in The Big Bang Theory. He first expresses his love for it in "The Alien Parasite Hypothesis".
Subsequently he often wears a t-shirt with the number 73 on it.
- Sheldon: "The best number is 73. Why? 73 is the 21st prime number. Its mirror, 37, is the 12th and its mirror, 21, is the product of multiplying 7 and 3... and in binary 73 is a palindrome, 1001001, which backwards is 1001001."
- Sheldon: "Chuck Norris wishes... all Chuck Norris backwards gets you is Sirron Kcuhc!"'