31 (number)
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31 is the natural number following 30 and preceding 32.
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| Cardinal | thirty-one | ||
| Ordinal | 31st (thirty-first) | ||
| Factorization | prime | ||
| Roman numeral | XXXI | ||
| Binary | 11111 | ||
| Hexadecimal | 1F | ||
In mathematics
Thirty-one is the third Mersenne prime ( 25 - 1 ) as well as the fourth primorial prime, and together with twenty-nine, another primorial prime, it comprises a twin prime. As a Mersenne prime, 31 is related to the perfect number 496, since 496 = 25 - 1 ( 25 - 1). 31 is the eighth Mersenne prime exponent. 31 is also the fourth lucky prime.
31 is a centered triangular number, a centered pentagonal number and centered decagonal number.
At 31, the Mertens function sets a new low of -4, a value which is not subceded until 110.
No integer added up to its base 10 digits results in 31, making 31 a self number.
In science
- The atomic number of gallium
In astronomy,
- Messier object M31, a magnitude 4.5 galaxy in the constellation Andromeda. It is also known as the Andromeda Galaxy, and is readily visible to the naked eye in a modestly dark sky.
- The New General Catalogue object (http://www.ngcic.org/) NGC 31, a spiral galaxy in the constellation Phoenix
- The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -1805 January 31 and ended on -489 March 31. The duration of Saros series 31 was 1316.2 years, and it contained 74 solar eclipses.
- The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -1774 May 30 and ended on -476 July 17. The duration of Saros series 31 was 1298.1 years, and it contained 73 lunar eclipses.
In other fields
Thirty-one is also:
- The number of days in the months January, March, May, July, August, October and December.
- The code for international direct dial phone calls to the Netherlands.
- A type of card game. See Thirty-one (game).
- A type of game played on a backgammon board.
- The number of flavors of Baskin Robbins.
- In Star Trek, the name of Section 31, a secret police shadow organization of the Federation.
- ISO 31 is the ISO's standard for quantities and units
- In Family Guy, the Griffin house number on Spooner Street
- Historical years: 31 A.D., 31 B.C., or 1931
da:31 (tal)
es:Treinta y uno eo:Tridek unu fr:31 (nombre) ko:31 ia:31 it:Trentuno nl:31 (getal) ja:31 no:31 (tall) pt:Trinta e um sl:31 (število) sv:31 (tal) zh:31