Definition:Proper Subset

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


 * $S \ne T$
 * $S \ne \varnothing$

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:
 * $\varnothing \subsetneqq S$

... and one which we want to specify as possibly being null, we write:
 * $\varnothing \subseteq S$

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

Proper Superset
In a similar vein to the concept of a superset, $T \supsetneqq S$ means $T$ is a proper superset of $S$. This can be interpreted as $T$ properly contains $S$.

Also defined as
Some authors do not require that $S \ne \varnothing$ for $S$ to be a proper subset of $T$.

The symbol $\subsetneq$ is an alternative to $\subsetneqq$, but the advantage of the latter is that it is easier to distinguish from $\subseteq$.

The literature can be confusing. Many authors use $\subset$ for what we have defined $\subseteq$ to be. Others use $\subset$ to mean $\subsetneqq$.

Because of this confusion, this website does not endorse the use of $\subset$, however neater it is than the unwieldy and clumsy-looking $\subsetneqq$.