One

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The concept of one has many synonyms

Unity
Unique
Monad
...

The number one

One has sometimes not been considered a number in some cultures in the past (because it does not involve plurality.) The number one is represented by the numeral 1. 1 is the first positive integer and the first positive odd number.

Although 1 has been considered prime until the beginning of the 20^th century (former definition: no divisors apart from 1 and itself, itself not necessarily distinct from 1) the unit (i.e. multiplicatively invertible element) 1 is now widely known as the empty product (defined as the multiplicative identity, i.e. 1) of primes, where an integer $\scriptstyle n \,$ is now considered prime if and only if it has exactly two divisors, a unit and a nonunit, where associates of $\scriptstyle k \,$ (i.e. product of some unit $\scriptstyle u \,$ with $\scriptstyle k \,$ ) are not considered distinct divisors. This is the definition (resulting from the development of abstract algebra at the turn of the 20^th century) now accepted by most mathematicians.