The number one (1), also called "unity," is the first positive integer. It is an odd number. Although the number
1 used to be considered a prime number, it requires
special treatment in so many definitions and applications involving primes greater
than or equal to 2 that it is usually placed into a class of its own (Wells 1986,
p. 31). The number 1 is sometimes also called "unity,"
so the th roots of 1 are often called the th roots of
unity. Fractions having 1 as a numerator
are called unit fractions. If only one root, solution,
etc., exists to a given problem, the solution is called unique.