73

73 is a base-2 palindrome: $$1001001_2$$. Write it backwards and you have $$1001001_2$$.

73 is a base-8 palindrome: $$111_8$$. Write it backwards and you have $$111_8$$. This is its base-2 representation with the zeros removed.

73 is the 21st prime number. Its mirror, 37, is the 12th, and its mirror, 21, is the product of $$7 × 3$$. (See The Best Number below.)

In English, it is the smallest natural number with twelve letters in its spelled out name: seventy-three.

The Best Number

From the The Big Bang Theory (TV Series) episode “The Alien Parasite Hypothesis” (December 9, 2010).1

Sheldon Cooper: What is the best number? By the way, there’s only one correct answer.

Raj Koothrappali: Five million, three hundred eighteen thousand and eight?

Sheldon Cooper: Wrong. The best number is 73. You’re probably wondering why.

Howard Wolowitz: No.

Raj Koothrappali: We’re good.

Sheldon Cooper: 73 is the 21st prime number. Its mirror, 37, is the 12th, and its mirror, 21, is the product of multiplying—hang on to your hats—seven and three. Heh? Heh? Did I lie?

Leonard Hofstadter: We get it. 73 is the Chuck Norris of numbers.

Sheldon Cooper: Chuck Norris wishes. In binary, 73 is a palindrome: 1001001, which backwards is 1001001. Exactly the same. All Chuck Norris backwards gets you is “sirron kcuhc”.

Raj Koothrappali: Just for the record, when you enter 5,318,008 in a calculator, upside down, it spells “boobies”.

Footnotes

1. “Quotes” from episode “The Alien Parasite Hypothesis”, The Big Bang Theory (TV Series). IMDb.com. <https://www.imdb.com/title/tt1632225/quotes/>