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- the 21st prime number. The previous is 71, with which it composes the 8th twin prime.
- a permutable prime with 37.
- a star number.
- the largest minimal Primitive root in the first 100,000 primes. In other words, if p is one of the first 100,000 primes, then at least one of the primes 3, 5, 7, 11, 13, 17, ..., 73 is a primitive root modulo p.
- the smallest prime congruent to 1 modulo 24.
- an emirp, meaning that the reverse of 73, that is, 37, is also a prime number. 73 is the 21st prime number while 37 is the 12th prime number.
- the smallest prime with a composite sum of digits, in base 5.
- palindromic in bases 2 (10010012) and 8 (1118).
- the only prime repunit in base 8 (1118).
- the smallest factor of the first composite generalized Fermat number in base 10 (104 + 1 = 10,001 = 73 × 137).
- The number 21 has prime factors 7 and 3. The number 21 in binary is 10101; 7 in binary is 111, 3 in binary is 11, and seventy-three in binary is 1001001. All of these are palindromes. In addition, of the 7 binary digits representing 73, there are 3 ones. Also, 37 + 12 = 49 (seven squared) and 73 + 21 = 94 = 47 × 2, 47 + 2 also being equal to seven squared. Additionally, both 73 and its mirror, 37, are sexy primes twice over, as 31, 43, 67 and 79 are all prime numbers.
- In addition to having prime factors 7 and 3, the number 21 represents the ternary (base-3) equivalent of the decimal numeral 7, or 213 = 710.
- 73 = 343, a palindrome. Replacing the first two digits of 343 with their sum yields 73.
- Every positive integer can be written as the sum of 73 or fewer sixth powers (see Waring's problem).
- Messier object M73, a magnitude 9.0 apparent open cluster in the constellation Aquarius
- The New General Catalogue object NGC 73, a barred spiral galaxy in the constellation Cetus
- The number of seconds it took for the Space Shuttle Challenger OV-099 shuttle to explode after launch.
- 73 is the number of rows in the 1,679-bit Arecibo message, sent to space in search for extraterrestrial intelligence.
In other fields
73 is also:
- The number of days in 1/5 of a non-leap year.
- The year AD 73, 73 BC, or 1973.
- The number of books in the Catholic Bible.
- Amateur radio operators and other morse code users commonly use the number 73 as a "92 Code" abbreviation for "best regards", typically when ending a QSO (a conversation with another operator). These codes also facilitate communication between operators who may not be native English speakers.  In Morse code, 73 is an easily recognized palindrome - ( - - · · · · · · - - ).
- 73 (also known as 73 Amateur Radio Today), was an amateur radio magazine published from 1960 to 2003.
- 73 was the number on the Torpedo Patrol (PT) boat in the TV show McHale's Navy
- The registry of the U.S. Navy's nuclear aircraft carrier USS George Washington (CVN-73), named after U.S. President George Washington.
- No. 73 was the name of a 1980s children's television programme in the United Kingdom. It ran from 1982–1988 and starred Sandi Toksvig
- Pizza 73 is a Canadian pizza chain with 52 stores
- Game show Match Game '73 in 1973
- Fender Rhodes Stage 73 Piano
- Sonnet 73 by William Shakespeare
- The number of the French department Savoie
- On a CB radio, 10-73 means "speed trap at..."
- In international curling competitions, each side is given 73 minutes to complete all of its throws.
- In baseball, the single-season home run record set by Barry Bonds in 2001.
- In basketball, the number of games the Golden State Warriors won in the 2015-16 season (73-9), the most wins in NBA history.
- NFL: In the 1940 NFL championship game, the Bears beat the Redskins 73–0, the largest score ever in an NFL game. (The Redskins won their previous regular season game, 7–3).
- "Sloane's A003154 : Centered 12-gonal numbers. Also star numbers: 6*n*(n-1) + 1". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
- "Sloane's A006567 : Emirps". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
- "Catholic Bible 101". Catholic Bible 101. Retrieved 16 September 2018.