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In mathematics 
Forty-three is the 14th smallest prime number. The previous is forty-one, with which it comprises a twin prime, and the next is forty-seven. 43 is the smallest prime that is not a Chen prime. It is also the third Wagstaff prime.
43 is the fourth term of Sylvester's sequence, one more than the product of the previous terms (2 × 3 × 7).
43 is a centered heptagonal number.
Let a(0) = a(1) = 1, and thenceforth a(n) = (a(0)2 + a(1)2 + ... + a(n-1)2) / (n-1). This sequence continues 1 1 2 3 5 10 28 154... (sequence A003504 in OEIS). Amazingly, a(43) is the first term of this sequence that is not an integer.
43 is a Heegner number.
43 is a repdigit in base 6 (111).
43 is the largest natural number that is not an (original) McNugget number.
This is the smallest prime number expressible as the sum of 2, 3, 4, or 5 different primes:
- 43 = 41 + 2
- 43 = 11 + 13 + 19
- 43 = 2 + 11 + 13 + 17
- 43 = 3 + 5 + 7 + 11 + 17.
In science 
- The chemical element with the atomic number 43 is technetium. It has the lowest atomic number of any element that does not possess stable isotopes.
- Messier object M43, a magnitude 7.0 H II region in the constellation of Orion, a part of the Orion Nebula, and also sometimes known as de Mairan's Nebula
- The New General Catalogue object NGC 43, a barred spiral galaxy in the constellation Andromeda
- The Saros number of the solar eclipse series which began on April 29, 1513 BC and ended on June 5, 233 BC . The duration of Saros series 43 was 1280.1 years, and it contained 72 solar eclipses.
- The Saros number of the lunar eclipse series which began on August 27, 1482 BC and ended on March 15, 70. The duration of Saros series 43 was 1550.5 years, and it contained 87 lunar eclipses.
In sports 
In auto racing:
- The number for Richard Petty's race car when he won his seven Winston Cup Championships. He also won 200 races in his career, 95% of them in the famous #43.
- The maximum number of cars participating in a NASCAR race in the Cup Series, and, through the 2012 season, the Nationwide Series.
- Legendary driver Dale Earnhardt, Sr. died in a last-lap crash during the 43rd running of the Daytona 500 in 2001.
- The number is also used by Ken Block on his rally cars.
- Brad Daugherty, ESPN NASCAR analyst and retired American basketball player. His #43 jersey, a number he picked as a tribute to NASCAR legend Richard Petty (whom Daugherty lists as his favorite sportsman), was retired by the Cavaliers on March 1, 1997.
In Ice Hockey:
- Patrice Brisebois (Montreal Canadiens) from 1991 to 2004
- Jan Alston (ZSC Lions, Zurich, Switzerland) from 2001 to 2010
- Sri Lanka's lowest ODI score ever made chasing 302 to win against South Africa (January 11, 2012)
In a film by Pixar called Cars, the champion racecar in the film, Strip "The King" Wheathers is racecar number 43 based on Richard Petty's car. Petty also provides the voice of "The King" in the film.
In season 6 episode 8 of The Big Bang Theory 43 is a number discovered on a black board in a room where Sheldon Cooper spends 20 minutes a day to "rest" from various activities occupying his mind. The room was discovered by Raj and Howard when they were following Sheldon to figure out what he did within the 20 minute empty span in his schedule. Raj and Howard thought of many scientific explanations for the number when in reality the number only represented Sheldon's Hacky Sacking high score.
In other fields 
- The designation of Interstate 43, a freeway in Wisconsin.
- +43, the code for direct dial international phone calls to Austria.
- Bush 43, George W. Bush, 43rd President of the United States.
- "43", a song by Level 42 on the album Level 42.
- "43," a song by Mushroomhead on the album "Mushroomhead".
- U.S.S. Coral Sea CV-43 (Aircraft Carrier).
- The name of a popular Spanish liqueur, Cuarenta y tres, which is distilled with 43 different herbs and spices.
- Kellogg, William O. (2010). Barron's AP United States History (9th ed.). Barron's Educational Series. p. 364. ISBN 9780764141843. "George H. W. Bush (Republican) [Bush 41—i.e., the first president Bush, George H. W. Bush was the forty-first President of the United States, and so some have referred to him in this way since the election of his son, George W. Bush or Bush 43—the forty-third president of the United States.]"
- Lehmer, Derrick, List of prime numbers from 1 to 10,006,721, Carnegie Institution of Washington, 1914
- Wells, David, Prime Numbers: The Most Mysterious Figures in Math, Wiley, 2005, ISBN 0-471-46234-9
- Crandall, Richard and Pomerance, Carl, Prime Numbers: A Computational Perspective, Springer, 2005, ISBN 0-387-25282-7