Definition:Preimage/Mapping/Element

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

Let $f^{-1} \subseteq T \times S$ be the inverse of $f$, defined as:


 * $f^{-1} = \left\{{\left({t, s}\right): f \left({s}\right) = t}\right\}$

Every $s \in S$ such that $f \left({s}\right) = t$ is called a preimage of $t$.

The preimage of an element $t \in T$ is defined as:


 * $f^{-1} \left ({t}\right) := \left\{{s \in S: f \left({s}\right) = t}\right\}$

This can also be written:
 * $f^{-1} \left ({t}\right) := \left\{{s \in \operatorname{Im} \left({f^{-1}}\right): \left({t, s}\right) \in f^{-1}}\right\}$

That is, the preimage of $t$ under $f$ is the image of $t$ under $f^{-1}$.

Also known as
The preimage of an element is also known as its inverse image.

As well as using the notation $\operatorname{Im}^{-1} \left ({f}\right)$ to denote the preimage of an entire mapping, the symbol $\operatorname{Im}^{-1}$ can also be used as follows:

For $t \in \operatorname{Im} \left({f}\right)$:
 * $\operatorname{Im}^{-1}_f \left ({t}\right) := f^{-1} \left ({t}\right)$

but this notation is clumsy and generally not preferred.

Also see

 * Domain
 * Codomain
 * Range


 * Image


 * Preimage of a Relation


 * From Preimages All Exist iff Surjection $f^{-1} \left({t}\right)$ is guaranteed not to be empty iff $f$ is a surjection.


 * From Preimages All Unique iff Injection, if $f^{-1} \left({t}\right)$ is not empty, then it is guaranteed to be a singleton iff $f$ is an injection.

Thus, while $f^{-1}$ is always a relation, it is not actually a mapping unless $f$ is a bijection.