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- Seventy-three is the 21st prime number. The previous is seventy-one, with which it comprises the 8th twin prime. It is also a permutable prime with thirty-seven. 73 is a star number.
- 73 is 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.
- 73 is the smallest prime congruent to 1 modulo 24.
- 73 is an emirp, meaning that the reverse of 73, that is, 37, is also a prime number. 73 is also the 21st prime number while 37 is the 12th prime number.
- 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.
- Every positive integer can be written as the sum of 73 or fewer sixth powers (see Waring's problem).
- In base 5, the smallest prime with a composite sum of digits is 73.
- 73 is the only prime repunit in base 8 (1118).
- 73 is the smallest factor of the first composite generalized Fermat number in base 10 (104 + 1 = 10,001 = 73 × 137).
- The Saros number of the solar eclipse series which began on 717 BC July and ended on 582 September. The duration of Saros series 73 was 1298.1 years, and it contained 73 solar eclipses.
- 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 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 TV Torpedo Patrol (PT) boat in the TV show PT-73
- 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 51 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 Philadelphia 76ers lost in the 1972–73 season (9–73), the most losses in NBA history.
- NFL: New England Patriots John Hannah's retired No. 73
- NFL: New York Jets Joe Klecko's retired No. 73
- NFL: San Francisco 49'ers Leo Nomellini retired No. 73
- 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).
- The rhythmic clapping at sporting events that precedes "Let's Go!" is Morse code for "73", an abbreviation meaning "best regards". 73 was put at the end of 19th century telegraph conversations, as well as at the end of certain Ham radio exchanges. Clap-<pause>-clap-<pause>-clap-clap-clap<pause>clap clap clap <pause> equates to --- --- ...(7) ...--- ---(3).