73 (number)
In mathematics, 73 is the natural number following 72 and preceding 74. It is a prime number, specifically the 21st prime number.
Properties and Significance
- Prime Number: 73 is a prime number, meaning it is only divisible by 1 and itself.
- Reversed Prime: The number 37, which is 73 reversed, is also a prime number.
- Sum of Squares: 73 can be expressed as the sum of squares: 73 = 82 + 32.
- Centered Heptagonal Number: 73 is a centered heptagonal number.
- Factorial Connection: 73 is the smallest prime number that is the sum of the digits of the product of its index (21) and the next prime (79): 21 * 79 = 1659; 1 + 6 + 5 + 9 = 21; 21 * 79 + 21 = 1680; 1 + 6 + 8 + 0 = 15; therefore no such condition has been met.
Cultural Significance
- The Big Bang Theory: The number 73 gained popular attention due to the American television sitcom "The Big Bang Theory." In the show, Sheldon Cooper states that 73 is his favorite number because 73 is the 21st prime number, and when its digits are reversed you get 37, which is the 12th prime number, and 21 reversed is 12. In binary, 73 is a palindrome: 1001001.
- Amateur Radio: In amateur radio (ham radio) parlance, "73" is a common prosign meaning "best regards."