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# 31 (number)

31 is the natural number following 30 and preceding 32.

 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 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.

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:

03-10-2013 05:06:04