Inverse of Surjection is Relation both Left-Total and Right-Total
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Theorem
Let $f$ be an surjective mapping.
Then its inverse $f^{-1}$ is a relation which is both left-total and right-total.
Proof
We are given that $f$ is a surjective mapping.
By Inverse of Mapping is Right-Total Relation, $f^{-1}$ is a right-total relation.
By definition of surjection, $f$ is itself a right-total relation.
From Inverse of Right-Total Relation is Left-Total, $f^{-1}$ is a left-total relation.
Hence the result.
$\blacksquare$