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## Summary

In set theory, the kernel of a function $f$ (or equivalence kernel) may be taken to be either

• the equivalence relation on the function's domain that roughly expresses the idea of "equivalent as far as the function $f$ can tell", or
• the corresponding partition of the domain.

An unrelated notion is that of the kernel of a non-empty family of sets ${\mathcal {B}},$ which by definition is the intersection of all its elements:

$\ker {\mathcal {B}}~=~\bigcap _{B\in {\mathcal {B}}}\,B.$ This definition is used in the theory of filters to classify them as being free or principal.

## Definition

Kernel of a function

For the formal definition, let $f:X\to Y$  be a function between two sets. Elements $x_{1},x_{2}\in X$  are equivalent if $f\left(x_{1}\right)$  and $f\left(x_{2}\right)$  are equal, that is, are the same element of $Y.$  The kernel of $f$  is the equivalence relation thus defined.

Kernel of a family of sets

The kernel of a family ${\mathcal {B}}\neq \varnothing$  of sets is

$\ker {\mathcal {B}}~:=~\bigcap _{B\in {\mathcal {B}}}B.$

The kernel of ${\mathcal {B}}$  is also sometimes denoted by $\cap {\mathcal {B}}.$  The kernel of the empty set, $\ker \varnothing ,$  is typically left undefined. A family is called fixed and is said to have non-empty intersection if its kernel is not empty. A family is said to be free if it is not fixed; that is, if its kernel is the empty set.

## Quotients

Like any equivalence relation, the kernel can be modded out to form a quotient set, and the quotient set is the partition:

$\left\{\,\{w\in X:f(x)=f(w)\}~:~x\in X\,\right\}~=~\left\{f^{-1}(y)~:~y\in f(X)\right\}.$

This quotient set $X/=_{f}$  is called the coimage of the function $f,$  and denoted $\operatorname {coim} f$  (or a variation). The coimage is naturally isomorphic (in the set-theoretic sense of a bijection) to the image, $\operatorname {im} f;$  specifically, the equivalence class of $x$  in $X$  (which is an element of $\operatorname {coim} f$ ) corresponds to $f(x)$  in $Y$  (which is an element of $\operatorname {im} f$ ).

## As a subset of the square

Like any binary relation, the kernel of a function may be thought of as a subset of the Cartesian product $X\times X.$  In this guise, the kernel may be denoted $\ker f$  (or a variation) and may be defined symbolically as

$\ker f:=\{(x,x'):f(x)=f(x')\}.$

The study of the properties of this subset can shed light on $f.$

## Algebraic structures

If $X$  and $Y$  are algebraic structures of some fixed type (such as groups, rings, or vector spaces), and if the function $f:X\to Y$  is a homomorphism, then $\ker f$  is a congruence relation (that is an equivalence relation that is compatible with the algebraic structure), and the coimage of $f$  is a quotient of $X.$  The bijection between the coimage and the image of $f$  is an isomorphism in the algebraic sense; this is the most general form of the first isomorphism theorem.

## In topology

If $f:X\to Y$  is a continuous function between two topological spaces then the topological properties of $\ker f$  can shed light on the spaces $X$  and $Y.$  For example, if $Y$  is a Hausdorff space then $\ker f$  must be a closed set. Conversely, if $X$  is a Hausdorff space and $\ker f$  is a closed set, then the coimage of $f,$  if given the quotient space topology, must also be a Hausdorff space.

A space is compact if and only if the kernel of every family of closed subsets having the finite intersection property (FIP) is non-empty; said differently, a space is compact if and only if every family of closed subsets with F.I.P. is fixed.