Kernel (algebra)

In the various branches of mathematics that fall under the heading of abstract algebra, the kernel of a homomorphism measures the degree to which the homomorphism fails to be injective.
The definition of kernel takes various forms in various contexts. But in all of them, the kernel of a homomorphism is trivial (in a sense relevant to that context) if and only if the homomorphism is injective. The fundamental theorem on homomorphisms (or first isomorphism theorem) is a theorem, again taking various forms, that applies to the quotient algebra defined by the kernel.
In this article, we first survey kernels for some important types of algebraic structures; then we give general definitions from universal algebra for generic algebraic structures.
Contents 
Survey of examples
Linear operators
Let V and W be vector spaces and let T be a linear transformation from V to W. If 0_{W} is the zero vector of W, then the kernel of T is the preimage of the singleton set {0_{W}}; that is, the subset of V consisting of all those elements of V that are mapped by T to the element 0_{W}. The kernel is usually denoted "ker T" (or a variation). In symbols:
 <math> \mathop{\mathrm{ker}}\, T := \{\mathbf{v} \in V : T\mathbf{v} = \mathbf{0}_{W}\}\mbox{.} \! <math>
Since a linear transformation preserves zero vectors, the zero vector 0_{V} of V must belong to the kernel. The transformation T is injective if and only if its kernel is only the singleton set {0_{V}}.
It turns out that ker T is always a subspace of V. Thus, it makes sense to speak of the quotient space V/(ker T). The first isomorphism theorem for vector spaces states that this quotient space is naturally isomorphic to the image of T (which is a subspace of W). As a consequence, the dimension of V equals the dimension of the kernel plus the dimension of the image.
If V and W are finitedimensional and bases have been chosen, then T can be described by a matrix M, and the kernel can be computed by solving the homogenous system of linear equations Mv = 0. In this representation, the kernel corresponds to the nullspace of M. The dimension of the nullspace, called the nullity of M, is given by the number of columns of M minus the rank of M, as a consequence of the ranknullity theorem.
Solving homogeneous differential equations often amounts to computing the kernel of certain differential operators. For instance, in order to find all twicedifferentiable functions f from the real line to itself such that
 xf''(x) + 3f'(x) = f(x),
let V be the space of all twice differentiable functions, let W be the space of all functions, and define a linear operator T from V to W by
 (Tf)(x) = xf''(x) + 3f'(x)  f(x)
for f in V and x an arbitrary real number. Then all solutions to the differential equation are in ker T.
One can define kernels for homomorphisms between modules over a ring in an analogous manner. This includes kernels for homomorphisms between abelian groups as a special case. This example captures the essence of kernels in general abelian categories; see Kernel (category theory).
Group homomorphisms
Let G and H be groups and let f be a group homomorphism from G to H. If e_{H} is the identity element of H, then the kernel of f is the preimage of the singleton set {e_{H}}; that is, the subset of G consisting of all those elements of G that are mapped by f to the element e_{H}. The kernel is usually denoted "ker f" (or a variation). In symbols:
 <math> \mathop{\mathrm{ker}} f := \{g \in G : f(g) = e_{H}\}\mbox{.} \! <math>
Since a group homomorphism preserves identity elements, the identity element e_{G} of G must belong to the kernel. The homomorphism f is injective if and only if its kernel is only the singleton set {e_{G}}.
It turns out that ker f is not only a subgroup of G but in fact a normal subgroup. Thus, it makes sense to speak of the quotient group G/(ker f). The first isomorphism theorem for groups states that this quotient group is naturally isomorphic to the image of f (which is a subgroup of H).
In the special case of abelian groups, this works in exactly the same way as in the previous section.
Ring homomorphisms
Let R and S be rings and let f be a ring homomorphism from R to S. If 0_{S} is the zero element of S, then the kernel of f is the preimage of the singleton set {0_{S}}; that is, the subset of R consisting of all those elements of R that are mapped by f to the element 0_{S}. The kernel is usually denoted "ker f" (or a variation). In symbols:
 <math> \mathop{\mathrm{ker}} f := \{r \in R : f(r) = 0_{S}\}\mbox{.} \! <math>
Since a ring homomorphism preserves zero elements, the zero element 0_{R} of R must belong to the kernel. The homomorphism f is injective if and only if its kernel is only the singleton set {0_{R}}.
It turns out that, although ker f is generally not a subring of R since it may not contain the multiplicative identity, it is nevertheless a twosided ideal of R. Thus, it makes sense to speak of the quotient ring R/(ker f). The first isomorphism theorem for rings states that this quotient ring is naturally isomorphic to the image of f (which is a subring of S).
To some extent, this can be thought of as a special case of the situation for modules, since these are all bimodules over a ring R:
 R itself;
 any twosided ideal of R (such as ker f);
 any quotient ring of R (such as R/(ker f)); and
 the codomain of any ring homomorphism whose domain is R (such as S, the codomain of f).
However, the isomorphism theorem gives a stronger result, because ring isomorphisms preserve multiplication while module isomorphisms (even between rings) in general do not.
This example captures the essence of kernels in general Mal'cev algebras.
Monoid homomorphisms
Let M and N be monoids and let f be a monoid homomorphism from M to N. Then the kernel of f is the subset of the direct product M × M consisting of all those ordered pairs of elements of M whose components are both mapped by f to the same element in N. The kernel is usually denoted "ker f" (or a variation). In symbols:
 <math> \mathop{\mathrm{ker}} f := \{(m,m') \in M : f(m) = f(m')\}\mbox{.} \! <math>
Since f is a function, the elements of the form (m,m) must belong to the kernel. The homomorphism f is injective if and only if its kernel is only the diagonal set {(m,m) : m in M}.
It turns out that ker f is an equivalence relation on M, and in fact a congruence relation. Thus, it makes sense to speak of the quotient monoid M/(ker f). The first isomorphism theorem for monoids states that this quotient monoid is naturally isomorphic to the image of f (which is a submonoid of N).
This is very different in flavour from the above examples. In particular, the preimage of the identity element of N is not enough to determine the kernel of f. This is because monoids are not Mal'cev algebras.
Universal algebra
All the above cases may be unified and generalized in universal algebra.
General case
Let A and B be algebraic structures of a given type and let f be a homomorphism of that type from A to B. Then the kernel of f is the subset of the direct product A × A consisting of all those ordered pairs of elements of A whose components are both mapped by f to the same element in B. The kernel is usually denoted "ker f" (or a variation). In symbols:
 <math> \mathop{\mathrm{ker}} f := \{(a,a') \in A : f(a) = f(a')\}\mbox{.} \! <math>
Since f is a function, the elements of the form (a,a) must belong to the kernel. The homomorphism f is injective if and only if its kernel is only the diagonal set {(a,a) : a in A}.
It turns out that ker f is an equivalence relation on A, and in fact a congruence relation. Thus, it makes sense to speak of the quotient algebra A/(ker f). The first isomorphism theorem in general universal algebra states that this quotient algebra is naturally isomorphic to the image of f (which is a subalgebra of B).
Note that the definition of kernel here (as in the monoid example) doesn't depend on the algebraic structure; it is a purely settheoretic concept. For more on this general concept, outside of abstract algebra, see kernel of a function.
Mal'cev algebras
In the case of Mal'cev algebras, this construction can be simplified. Every Mal'cev algebra has a special neutral element (the zero vector in the case of vector spaces, the identity element in the case of groups, and the zero element in the case of rings or modules). The characteristic feature of a Mal'cev algebra is that we can recover the entire equivalence relation ker f from the equivalence class of the neutral element.
To be specific, let A and B be Mal'cev algebraic structures of a given type and let f be a homomorphism of that type from A to B. If e_{B} is the neutral element of B, then the kernel of f is the preimage of the singleton set {e_{B}}; that is, the subset of A consisting of all those elements of A that are mapped by f to the element e_{B}. The kernel is usually denoted "ker f" (or a variation). In symbols:
 <math> \mathop{\mathrm{ker}} f := \{a \in A : f(a) = e_{B}\}\mbox{.} \! <math>
Since a Mal'cev algebra homomorphism preserves neutral elements, the identity element e_{A} of A must belong to the kernel. The homomorphism f is injective if and only if its kernel is only the singleton set {e_{A}}.
The notion of ideal generalises to any Mal'cev algebra (as subspace in the case of vector spaces, normal subgroup in the case of groups, twosided ring ideal in the case of rings, and submodule in the case of modules). It turns out that although ker f may not be a subalgebra of A, it is nevertheless an ideal. Then it makes sense to speak of the quotient algebra G/(ker f). The first isomorphism theorem for Mal'cev algebras states that this quotient algebra is naturally isomorphic to the image of f (which is a subalgebra of B).
The connection between this and the congruence relation is for more general types of algebras is as follows. First, the kernelasanideal is the equivalence class of the neutral element e_{A} under the kernelasacongruence. For the converse direction, we need the notion of quotient in the Mal'cev algebra (which is division on either side for groups and subtraction for vector spaces, modules, and rings). Using this, elements a and a' of A are equivalent under the kernelasacongruence if and only if their quotient a/a' is an element of the kernelasanideal.
Abelian algebras
I need to look up more stuff on universal algebra.
Algebras with nonalgebraic structure
Sometimes algebras are equipped with a nonalgebraic structure in addition to their algebraic operations. For example, one may consider topological groups or topological vector spaces, with are equipped with a topology. In this case, we would expect the homomorphism f to preserve this additional structure; in the topological examples, we would want f to be a continuous map. The process may run into a snag with the quotient algebras, which may not be wellbehaved. In the topological examples, we can avoid problems by requiring that topological algebraic structures be Hausdorff (as is usually done); then the kernel (however it is constructed) will be a closed set and the quotient space will work fine (and also be Hausdorff).
Kernels in category theory
The notion of kernel in category theory is a generalisation of the kernels of abelian algebras; see Kernel (category theory). The categorical generalisation of the kernel as a congruence relation is the kernel pair. (There is also the notion of difference kernel, or binary equaliser.)de:Kern (Mathematik) fr:Noyau (algèbre) ja:核 (数学) pl:Jądro przekształcenia liniowego