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Ninety-one

Ninety-one (91, XCI) is the natural number following ninety and preceding ninety-two. It is a triangular number, the sum of the squares of the first six integers, as well as a composite number, with its divisors being 7 and 13.

It is the smallest pseudoprime satisfying the congruence 3ⁿ = 3 mod n.

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).

This article is about the number. For the year AD 91, see 91.

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ninety-one".