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91 (ninety-one) is the natural number following 90 and preceding 92.

In mathematics

91 is:

• the twenty-seventh distinct semiprime and the second of the form (7.q), where q is a higher prime.
• the aliquot sum of 91 is 21; 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).
• the 13th triangular number.
• a hexagonal number, one of the few such numbers to also be a centered hexagonal number.
• a centered nonagonal number.
• a centered cube number.
• a square pyramidal number, being the sum of the squares of the first six integers.
• 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):
  91 = 6 + (−5) = 4 + 3. (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 = 1 + 2 + 3 + 4 + 5 + 6.
• The only other ways to write 91 as a sum of distinct squares are:
  91 = 1 + 4 + 5 + 7 and
• 91 = 1 + 3 + 9.
• the smallest pseudoprime satisfying the congruence 3 ≡ 3 mod n.
• a repdigit in base 9 (111₉).
• palindromic in bases 3 (10101₃), 9 (111₉), and 12 (77₁₂).
• a Riordan number.
• the smallest number that looks prime but isn't, proven using the Rotten Theorem by John Conway.

The decimal equivalent of the fraction 1⁄91 can be obtained by using powers of 9.

In science

• 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

In other fields

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

References

1. Sloane, N. J. A. (ed.). "Sequence A001358". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
2. "A000217 - OEIS". oeis.org. Retrieved 2024-11-28.
3. "Sloane's A000384 : Hexagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
4. "Sloane's A003215 : Hex (or centered hexagonal) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
5. "Sloane's A060544 : Centered 9-gonal (also known as nonagonal or enneagonal) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
6. "Sloane's A005898 : Centered cube numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
7. "Sloane's A000330 : Square pyramidal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
8. Sloane, N. J. A. (ed.). "Sequence A047696 (Smallest positive number that can be written in n ways as a sum of two (not necessarily positive) cubes.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
9. Friedman, Erich. What's Special About This Number? Archived 2018-02-23 at the Wayback Machine
10. "Sloane's A005043 : Riordan numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
11. "John Conway proves that 91 is the smallest number which looks prime but isn't". Ryan Andersen. 31 December 2020. Retrieved 2024-05-09.
12. "Prime Numbers". 9 May 2024.

• Integers