73 (number)
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73 is a natural number that lies between 72 and 74. It is a prime number, meaning it is divisible only by 1 and 73 with no remainder.
The number 73 has several notable mathematical properties discussed in number theory, particularly its relationship to the number 37 and its form in different numeral systems.
Prime properties
73 is the 21st prime number, beginning from 2. It is also an emirp: when its digits are reversed it becomes 37, which is also prime. 73 and 79 are sexy primes (primes differing by six); 67 and 73 are also sexy primes.
Other numerical forms
In binary, 73 is 1001001—a palindromic number that reads the same forwards and backwards. In octal it is 111. It can also be expressed as the sum of two squares: 73 = 64 + 9, or 8² + 3².
Geometric type
73 is classified as a star number—a number arising from a pattern shaped like a six-pointed star with points arranged geometrically. It is also used in examples for teaching the properties of prime numbers.