91 (Number)
From Halbeeg, the open encyclopedia · Af-Soomaali
74 languages
91 is a natural number that comes after 90 and before 92. It is a composite number, meaning it has divisors other than 1 and itself: 91 = 7 × 13. Its divisors are 1, 7, 13, and 91.
In base 10, 91 is a two-digit number. In binary it is written as 1011011, while in Roman numerals it is XCI.
Mathematical Properties
91 is a triangular number: it is the sum of the integers from 1 to 13, calculated as 13 × 14 ÷ 2 = 91. It is also a square pyramidal number, as it equals the sum of the first six perfect squares: 1 + 4 + 9 + 16 + 25 + 36 = 91. It is also among the hexagonal numbers.
91 is notable as an example of a composite number that is easily mistaken for a prime. It is not divisible by 2, 3, or 5, so on quick tests it may appear prime, even though 7 divides it. It is also an example used in testing primality by Fermat's theorem.
Other Uses
Beyond mathematics, this number is used as a symbol in many fields. For example, in the periodic table of chemical elements, atomic number 91 stands for protactinium. In the international telephone numbering system, 91 is the country code for India.