# Definition:Proper Subset

## Definition

If a set $S$ is a subset of another set $T$, that is, $S \subseteq T$, and also:

$S \ne T$
$S \ne \O$

then $S$ is referred to as a proper subset of $T$.

The set $T$ properly contains, or strictly contains, the set $S$.

If $S \subseteq T$ and $S \ne T$, then the notation $S \subsetneqq T$ is used.

If we wish to refer to a set which we specifically require not to be empty, we can denote it like this:

$\O \subsetneqq S$

and one which we want to specify as possibly being empty, we write:

$\O \subseteq S$

Thus for $S$ to be a proper subset of $T$, we can write it as $\O \subsetneqq S \subsetneqq T$.

### Proper Superset

If $S$ is a proper subset of $T$, then $T$ is a proper superset of $S$.

This can be expressed by the notation $T \supsetneqq S$.

This can be interpreted as $T$ properly contains $S$.

## Improper Subset

$S$ is an improper subset of $T$ if and only if $S$ is a subset of $T$ but specifically not a proper subset of $T$.

That is, either:

$S = T$

or:

$S = \O$

## Also known as

$S \subsetneqq T$ can also be read as:

$S$ is properly included in $T$, or $T$ properly includes $S$
$S$ is strictly included in $T$, or $T$ strictly includes $S$

The following usage can also be seen for $S \subsetneqq T$:

$S$ is properly contained in $T$, or $T$ properly contains $S$
$S$ is strictly contained in $T$, or $T$ strictly contains $S$

However, beware of the usage of contains: $S$ contains $T$ can also be interpreted as $S$ is an element of $T$.

Hence the use of contains is not supported on $\mathsf{Pr} \infty \mathsf{fWiki}$.

## Notation

Notation in the literature for the concept of a subset can be confusing.

On $\mathsf{Pr} \infty \mathsf{fWiki}$, the convention is as follows:

$\subseteq$ is used for the general subset
$\subsetneq$ or $\subsetneqq$ is used for the concept of a proper subset.

The notation $\subset$ is deprecated on $\mathsf{Pr} \infty \mathsf{fWiki}$, on the grounds that it can mean either.

## Also defined as

Some authors do not require that $S \ne \O$ for $S$ to be a proper subset of $T$.