Kernel (set theory)

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

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

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

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

Like any binary relation, the kernel of a function may be thought of as a subset of the Cartesian product ${\displaystyle X\times X.}$ In this guise, the kernel may be denoted ${\displaystyle \operatorname {ker} f}$ (or a variation) and may be defined symbolically as[2]

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

The study of the properties of this subset can shed light on ${\displaystyle f.}$

If ${\displaystyle X}$ and ${\displaystyle Y}$ are algebraic structures of some fixed type (such as groups, rings, or vector spaces), and if the function ${\displaystyle f:X\to Y}$ is a homomorphism, then ${\displaystyle \operatorname {ker} f}$ is a congruence relation (that is an equivalence relation that is compatible with the algebraic structure), and the coimage of ${\displaystyle f}$ is a quotient of ${\displaystyle X.}$[2] The bijection between the coimage and the image of ${\displaystyle f}$ is an isomorphism in the algebraic sense; this is the most general form of the first isomorphism theorem.

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

• Filters in topology – Use of filters to describe and characterize all basic topological notions and results.

• Awodey, Steve (2010) [2006]. Category Theory. Oxford Logic Guides. Vol. 49 (2nd ed.). Oxford University Press. ISBN 978-0-19-923718-0.
• Dolecki, Szymon; Mynard, Frederic (2016). Convergence Foundations Of Topology. New Jersey: World Scientific Publishing Company. ISBN 978-981-4571-52-4. OCLC 945169917.