Classifying the Number One

Date: 11/27/98 at 11:53:22
From: Liz Lim
Subject: Number theory

Dear Dr. Math,

My students have learned about prime and composite numbers. We are 
wondering what classification name fits the number 1. We know that it 
is neither prime nor composite. Some textbooks just say that 1 is a 
"special number." Can you help us?

Sincerely,
Liz Lim

Date: 11/27/98 at 12:54:37
From: Doctor Rob
Subject: Re: Number theory

The number 1 is a "unit"; that is, an integer whose reciprocal is also 
an integer. In the integers, only 1 and -1 are units, and you are 
restricting your attention to positive integers, so -1 is excluded.

- Doctor Rob, The Math Forum
  http://mathforum.org/dr.math/