Ninety-one

The ninety-one (91) is the natural number that follows ninety and precedes ninety-two.

Math

• It is a composite number, which has the following factors: 1, 7, and 13. As the sum of its factors is 21 ≤ 91, it is a defective number.
• It is the least pseudoprimo that satisfies congruence 3ⁿ = 3 mod n.
• It is a triangular and hexagonal number, and it is also one of the few hexagonal numbers that is also hexagonal centered.
• Square pyramidal number.
• McCarthy's 91 function is a recursive function.

Science

• 91 is the atomic number of protactinio (Pa).
• Object of Messier M91 is a coiled spiral galaxy in the constellation Coma Berenices.
• (91) Aegine is an asteroid that forms part of the asteroid belt.