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# Contents

## Definition

Given a function $f: X \to Y$ and a subset $S$ of $Y$, the preimage (sometimes also called the inverse image, though that may mean something different) of $S$ under $f$ is a subset of $X$, consisting of those arguments whose values belong to $S$.

That is,

$f^*(S) = \{ a: X \;|\; f(a) \in S \} .$

A traditional notation for $f^*$ is $f^{-1}$, but this can conflict with the notation for an inverse function of $f$ (which indeed might not even exist). In fact $f^\ast$ is borrowed from a notation for pullbacks; indeed, a preimage is an example of a pullback:

$\array{ f^\ast(S) & \hookrightarrow & X \\ \downarrow & (pb) & \downarrow \mathrlap{f} \\ S & \hookrightarrow & Y }$

Notice also that $f^\ast$ may be regarded as an operator $P(Y) \to P(X)$ between power sets. Power sets $P(X)$ are exponential objects $2^X$ in the topos $Set$; under this identification the pre-image operator $f^\ast$ is thereby identified with the map $2^f: 2^Y \to 2^X$ (variously called “pulling back along $f$ or substituting along $f$) obtained by currying the composite map

$2^Y \times X \stackrel{1 \times f}{\to} 2^Y \times Y \stackrel{eval}{\to} 2.$

The appearance of the asterisk as a superscript in $f^\ast$ serves as a reminder of the contravariance of the map $f \mapsto f^\ast = 2^f$. Similarly, one uses a subscript notation such as $f_*$ (or sometimes $f_!$) for the direct image, considered as an operator $f_\ast: 2^X \to 2^Y$ in the covariant direction.

Naturally all of this generalizes to the context of toposes, where the set $2$ is replaced by the subobject classifier $\Omega$ and $f^\ast = \Omega^f$, with a pullback description similar to the above.

## Properties

• pre-images preserve unions and intersections (a general reason for this being that unions are colimits, intersections are limits, and $f^\ast$ is simultaneously a left- and a right-adjoint: $f^\ast$ is right-adjoint to the existential quantifier $\exists_f$ and left-adjoint to the universal quantifier $\forall_f$)

As emphasized by Lawvere, the quantifiers $\exists_f, \forall_f$ are vastly generalized by the concept of enriched Kan extensions which provide left and right adjoints to pulling-back operators $V^f: V^D \to V^C$ for $V$-enriched functors $f: C \to D$.

For a generalisation to sheaves, see inverse image.

## References

Last revised on November 3, 2020 at 15:31:05. See the history of this page for a list of all contributions to it.