What is Sheldon's favorite number?

Sheldon Cooper’s favorite number is 73.

Sheldon explains his choice in “The Alien Parasite Hypothesis,” an episode from season four of The Big Bang Theory. He calls 73 the 21st prime number, while its reversed digits, 37, form the 12th prime number. Reversing 21 gives 12, creating the numerical pattern Sheldon finds especially elegant.

The number also has a second connection: 7 multiplied by 3 equals 21, the prime-number position of 73. In binary, 73 is written as 1001001, which reads the same forward and backward, making it a binary palindrome. Sheldon’s shirt bearing 73 reinforces the joke throughout the series.

The answer is sometimes confused with 42, the famous “answer to life, the universe, and everything” from The Hitchhiker’s Guide to the Galaxy. Sheldon’s number is not simply a pop-culture reference, however; it is built around a chain of prime-number and digit-reversal properties. Mathematicians later dubbed the related pattern the “Sheldon prime.”