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91 (ninety-one) is the natural number following 90 and preceding 92.
Look up ninety-one in Wiktionary, the free dictionary.
91 is:
- the twenty-seventh distinct semiprime[1] and the second of the form (7.q), where q is a higher prime.
- the aliquot sum of 91 is 21 33; itself a semiprime, within an aliquot sequence of two composite numbers (91,21,11, 1,0) to the prime in the 11-aliquot tree. 91 is the fourth composite number in the 11-aliquot tree.(91,51,21,18).
- a triangular number.
- a hexagonal number,[2] one of the few such numbers to also be a centered hexagonal number.[3]
- a centered nonagonal number.[4]
- a centered cube number.[5]
- a square pyramidal number, being the sum of the squares of the first six integers.[6]
- the smallest positive integer expressible as a sum of two cubes in two different ways if negative roots are allowed (alternatively the sum of two cubes and the difference of two cubes):[7]
91 = 63 + (−5)3 = 43 + 33. (See 1729 for more details).
This implies that 91 is the second cabtaxi number. - the smallest positive integer expressible as a sum of six distinct squares:
91 = 12 + 22 + 32 + 42 + 52 + 62. - The only other ways to write 91 as a sum of distinct squares are:
91 = 12 + 42 + 52 + 72 and - 91 = 12 + 32 + 92.
- the smallest pseudoprime satisfying the congruence 3n ≡ 3 mod n.[8]
- a repdigit in base 9 (1119).
- palindromic in bases 3 (101013), 9 (1119), and 12 (7712).
- a Riordan number.[9]
- the smallest number that looks prime but isn't, proven using the Rotten Theorem by John Conway.[10][11]
The decimal equivalent of the fraction 1⁄91 can be obtained by using powers of 9.
- 91 is the atomic number of protactinium, an actinide.
- McCarthy 91 function, a recursive function in discrete mathematics
- Messier object M91, a magnitude 11.5 spiral galaxy in the constellation Coma Berenices
- The New General Catalogue object NGC 91, a single star in the constellation Andromeda
Ninety-one is also:
- The code for international direct dial phone calls to India
- In cents of a U.S. dollar, the amount of money one has if one has one each of the coins of denominations less than a dollar (penny, nickel, dime, quarter and half dollar)
- The ISBN Group Identifier for books published in Sweden.
- Psalm 91 is known as the Psalm of Protection.
- STS-91 Space Shuttle Discovery mission to the International Space Station, June 2, 1998
- Swedish comic strip 91:an
- The 91st Missile Wing (91 SW) is a Minuteman (missile) III unit of the United States Air Force, based at Minot Air Force Base, North Dakota
- The number of the French department Essonne
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