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, and its factorization is:

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.