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31 (Number)31 is the natural number following 30 and preceding 32. In mathematics Thirty-one is the third Mersenne prime ( 25 - 1 ) as well as a 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 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 NGC 31, a spiral galaxy in the constellation Phoenix
- The Saros number 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 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:
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