Kernel (set theory)

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In set theory, the kernel of a function ${\displaystyle f}$ (or equivalence kernel^[1]) 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 ${\displaystyle f}$ can tell",^[2] or
• the corresponding partition of the domain.

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

This definition is used in the theory of filters to classify them as being free or principal.

Definition[]

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]

Quotients[]

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

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 ${\displaystyle \operatorname {im} f}$).

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 ${\displaystyle X\times X.}$ In this guise, the kernel may be denoted ${\displaystyle \operatorname {ker} f}$ (or a variation) and may be defined symbolically as^[2]

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

In algebraic structures[]

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.

In topological spaces[]

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.

See also[]

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

References[]

Bibliography[]

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