# Distinct (mathematics)

In mathematics, two things are called distinct if they are not equal. In physics two things are distinct if they cannot be mapped to each other.[1]

## Example

A quadratic equation over the complex numbers has two roots.

The equation

${\displaystyle x^{2}-3x+2=0}$

factors as

${\displaystyle (x-1)(x-2)=0}$

and thus has as roots x = 1 and x = 2. Since 1 and 2 are not equal, these roots are distinct.

In contrast, the equation:

${\displaystyle x^{2}-2x+1=0}$

factors as

${\displaystyle (x-1)(x-1)=0}$

and thus has as roots x = 1 and x = 1. Since 1 and 1 are (of course) equal, the roots are not distinct; they coincide.

In other words, the first equation has distinct roots, while the second does not. (In the general theory, the discriminant is introduced to explain this.)

## Proving distinctness

In order to prove that two things x and y are distinct, it often helps to find some property that one has but not the other. For a simple example, if for some reason we had any doubt that the roots 1 and 2 in the above example were distinct, then we might prove this by noting that 1 is an odd number while 2 is even. This would prove that 1 and 2 are distinct.

Along the same lines, one can prove that x and y are distinct by finding some function f and proving that f(x) and f(y) are distinct. This may seem like a simple idea, and it is, but many deep results in mathematics concern when you can prove distinctness by particular methods. For example,

## Notes

1. ^ Martin, Keye (2010). "Chapter 9: Domain Theory and Measurement: 9.6 Forms of Process Evolution". In Coecke, Bob. New Structures for Physics. Volume 813 of Lecture Notes in Physics. Heidelberg, Germany: Springer Verlag. pp. 579–580. ISBN 978-3-642-12820-2.