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15 (fifteen) is the natural number following 14 and preceding 16.
In mathematics
Fifteen is a triangular number and a hexagonal number, and the 4th Bell number. Fifteen is the double factorial of 5. It is a composite number; its proper divisors being 1, 3 and 5. With only two exceptions, all prime quadruplets enclose a multiple of 15, with 15 itself being enclosed by the quadruplet (11, 13, 17, 19).
In hexadecimal, as well as all higher bases, fifteen is represented as F.
Fifteen is the magic constant of a 3 by 3 magic square:
<math>
\begin{bmatrix}
8 & 1 & 6 \\
3 & 5 & 7 \\
4 & 9 & 2 \\
\end{bmatrix}
<math>
23 is the smallest number that can be factorized using Shor's quantum algorithm
In science
In astronomy,
- Messier object M15, a magnitude 7.5 globular cluster in the constellation Pegasus
- The New General Catalogue object (http://www.ngcic.org/) NGC 15, a spiral galaxy in the constellation Pegasus
- The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -2557 July 1 and ended on -1223 September 8. The duration of Saros series 15 was 1334.2 years, and it contained 75 solar eclipses.
- The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -2219 April 30 and ended on -921 June 19. The duration of Saros series 15 was 1298.1 years, and it contained 73 lunar eclipses.
In other fields
Age 15 is:
- The age of a quinceañera, a Hispanic girl celebrating her fifteenth birthday.
- The age for obtaining a driver's (or learner's) permit in jurisdictions where the age for a driver's license is sixteen.
Fifteen is:
- The number of days in each of the 24 cycles of the Chinese calendar.
- The number of guns in a gun salute to Army, Air Force and Marine Corps Lieutenant Generals, and Navy and Coast Guard Vice Admirals.
- The designation of Interstate 15, a freeway that runs from California to Montana.
- The number of players on a rugby union team.
- The number of checkers each side has in the beginning of a backgammon game.
- Fifteen, a television program.
- A restaurant in London, built by celebrity chef Jamie Oliver
- In the United States, the customary percentage to tip a waitress in a restaurant by one or two guests
- The number of balls in the eight ball variant of billiards
- The number of function keys on most Mac keyboards
- Historical years: 15 A.D., 15 B.C., or 1915
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