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91 (number)

91 is the natural number following 90 and preceding 92.

List of numbers — Integers << 90 91 92 93 94 95 96 97 98 99 >>
Cardinal	ninety-one
Ordinal	91st (ninety-first)
Factorization	<math>7 \cdot 13<math>
Roman numeral	LXXVIII
Binary	1011011
Hexadecimal	4E

In mathematics

Ninety-one is a triangular number and a hexagonal number, one of the few such numbers to also be a centered hexagonal number, and it is also a centered cube number. It is the sum of the squares of the first six integers.

It is also the lowest natural number which looks prime but isn't, because neither of its factors 7 and 13 have simple divisibility tests.

It is also the smallest pseudoprime satisfying the congruence <math>3^n = 3 mod n<math>.

In other fields

Ninety-one is also:

• The atomic number of protactinium, an actinide.
• The designation of American Interstate 91, a freeway that runs from Connecticut to Vermont.
• 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 year AD 91, 91 BC, or 1991.