Note that kernel pairs and difference kernels (aka binary equalisers) sometimes go by the name "kernel"; while related, these aren't quite the same thing and are not discussed in this article.
Table of contents |
2 First properties 3 Examples 4 Relation to other categorical concepts 5 Relationship to algebraic kernels |
Let C be a category. In order to define a kernel in the general category-theoretical sense, C needs to have zero morphisms. In that case, if f: A → B is an arbitrary morphism in C, then a kernel of f is an equaliser of f and the zero morphism from A to B. In symbols:
Note that in many concrete contexts, one would refer to the object K as the "kernel", rather than the morphism k. In those situations, K would be a subset of A, and that would be sufficient to reconstruct k as an inclusion map; in the nonconcrete case, in contrast, we need the morphism k to describe how K is to be interpreted as a subobject of A. One may prefer to think of the kernel as the pair (K,k) rather than as simply K or k alone.
Not every morphism needs to have a kernel, but if it does, then all its kernels are isomorphic in a strong sense: if k : K → A and l : L → A are kernels of f : A → B, then there exists a unique isomorphism φ : K → L such that l o φ = k.
Every kernel is a monomorphism.
Kernels are familiar in many categories from abstract algebra, such as the category of groupss or the category of (left) modules over a fixed ring (including vector spaces over a fixed field). To be explicit, if f: A → B is a homomorphism in one of these categories, and K is its kernel in the usual algebraic sense, then K is a subalgebra of A and the inclusion homomorphism from K to A is a kernel in the categorical sense.
Note that in the category of monoids, category-theoretic kernels exist just as for groups, but these kernels don't carry sufficient information for algebraic purposes. Therefore, the notion of kernel studied in monoid theory is slightly different. Conversely, in the category of ringss, there are no kernels in the category-theoretic sense; indeed, this category does not even have zero morphisms. Nevertheless, there is still a notion of kernel studied in ring theory. See Relationship to algebraic kernels below for the resolution of this paradox.
We have plenty of algebraic examples; now we should give examples of kernels in categories from topology and functional analysis.
The dual concept to that of kernel is that of cokernel. That is, the kernel of a morphism is its cokernel in the opposite category, and vice versa.
As mentioned above, a kernel is a type of binary equaliser, or difference kernel. Conversely, in a preadditive category, every binary equaliser can be constructed as a kernel. To be specific, the equaliser of the morphisms f and g is the kernel of the difference g − f. In symbols:
Every kernel, like any other equaliser, is a monomorphism. Conversely, a monomorphism is called normal if it is the kernel of some morphism. A category is called normal if every monomorphism is normal.
Abelian categories, in particular, are always normal. In this situation, the kernel of the cokernel of any morphism (which always exists in an abelian category) turns out to be the image of that morphism; in symbols:
Universal algebra defines a notion of kernel for homomorphisms between two algebraic structures of the same kind. This concept of kernel measures how far the given homomorphism is from being injective. There is some overlap between this algebraic notion and the categorical notion of kernel since both generalize the situation of groups and modules mentioned above. In general, however, the universal-algebraic notion of kernel is more like the category-theoretic concept of kernel pair. In particular, kernel pairs can be used to interpret kernels in monoid theory or ring theory in category-theoretic terms.