Condition for Trivial Relation to be Mapping

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Let $S$ and $T$ be sets.

Let $\RR = S \times T$ be the trivial relation in $S$ to $T$.

Then $\RR$ is a mapping if and only if either:

$(2): \card S = 0$


$(1): \card T = 1$

where $\card {\, \cdot \,}$ denotes cardinality.


By definition, the trivial relation in $S$ to $T$ is the set:

$\RR = \set {\tuple {s, t}: s \in S, t \in T}$

Proof by Cases:

$(1)$: Let $\card S = 0$.

That is, $S = \O$.

By definition of the trivial relation, in this case $\RR = \O$.

This is the empty mapping.

From Empty Mapping is Mapping, this is a mapping.

$(2)$: Let $\card S \ne 0$.

That is, $S \ne \O$.

$(2) \, \text (a)$: Let $\card T = 0$.

That is, $T = \O$.

From Relation to Empty Set is Mapping iff Domain is Empty, $\RR$ cannot be a mapping for $S \ne \O$.

$(2) \, \text (b)$: Let $\card T = 1$.

So, let $T = \set t$.

Then the trivial relation in $S$ to $T$ is:

$\RR = \set {\tuple {s, t}: s \in S}$

We have that each element of $S$ is associated with exactly one element of $T$.

Hence $\RR$ is a mapping by definition.

$(2) \, \text (c)$: Let $\card T > 1$.


$\exists t_1, t_2 \in T: t_1 \ne t_2$

We have that $S \ne \O$, so $\exists s \in S$.

Thus by definition of trivial relation:

$\tuple {s, t_1} \in \RR$


$\tuple {s, t_2} \in \RR$

but $t_1 \ne t_2$.

Thus $\RR$ is not a mapping.

Hence the result.