# Preimage of Set Difference under Mapping

## Theorem

Let $f: S \to T$ be a mapping.

Let $T_1$ and $T_2$ be subsets of $T$.

Then:

$f^{-1} \sqbrk {T_1 \setminus T_2} = f^{-1} \sqbrk {T_1} \setminus f^{-1} \sqbrk {T_2}$

where:

$\setminus$ denotes set difference
$f^{-1} \sqbrk {T_1}$ denotes preimage.

### Corollary 1

Let $f: S \to T$ be a mapping.

Let $T_1 \subseteq T_2 \subseteq T$.

Then:

$\relcomp {f^{-1} \sqbrk {T_2} } {f^{-1} \sqbrk {T_1} } = f^{-1} \sqbrk {\relcomp {T_2} {T_1} }$

where:

$\complement$ (in this context) denotes relative complement
$f^{-1} \sqbrk {T_1}$ denotes preimage.

### Corollary 2

Let $f: S \to T$ be a mapping.

Let $T_1$ be a subset of $T$.

Then:

$\relcomp S {f^{-1} \sqbrk {T_1} } = f^{-1} \sqbrk {\relcomp T {T_1} }$

where:

$\complement_S$ (in this context) denotes relative complement
$f^{-1} \sqbrk {T_1}$ denotes preimage.

## Proof

From Inverse of Mapping is One-to-Many Relation, we have that $f^{-1}: T \to S$ is one-to-many.

Thus we can apply One-to-Many Image of Set Difference:

$\RR \sqbrk {T_1 \setminus T_2} = \RR \sqbrk {T_1} \setminus \RR \sqbrk {T_2}$

where in this context $\RR = f^{-1}$.

$\blacksquare$