single-valued relation

let $\text{[math]}$ , we say $\text{[math]}$ is an single-valued relation if for every $\text{[math]}$ there exists at most a single $\text{[math]}$ such that $\text{[math]}$

$\text{[math]}$ $\text{[math]}$ this relation isnt single-valued because some $\text{[math]}$ have more than 1 corresponding $\text{[math]}$

$\text{[math]}$ this relation is single-valued