91 (number)

In mathematics

• the twenty-seventh distinct semiprime and the second of the form.
• a 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 :.. This implies that 91 is the second cabtaxi number.
• the smallest positive integer expressible as a sum of six distinct squares:. The only other ways to write 91 as a sum of distinct squares are: and.
• the smallest pseudoprime satisfying the congruence.
• a repdigit in base 9.
• palindromic in bases 3, 9, and 12.
• the fourth composite number in the 11-aliquot tree. The aliquot sum of 91 is 21 within the aliquot sequence.

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 NGC 91, a single star in the constellation Andromeda

  In other fields

• 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
• The ISBN Group Identifier for books published in Sweden.
• Psalm 91 is known as the Psalm of Protection. It is also the title of a book by Peggy Joyce Ruth
• 91 is a solitaire card game where the object is to move cards, so the top cards total 91.
• STS-91 Space Shuttle Discovery mission to the International Space Station, June 2, 1998
• Swedish comic strip
• The 91st Space Wing is a Minuteman III unit of the United States Air Force, based at Minot Air Force Base, North Dakota
• The number of the French department Essonne
• Ninety One or 91 is a Kazakhstan Q-pop boy band formed by JUZ Entertainment in 2014