Kernel (set theory)

In set theory, the kernel of a function f may be taken to be either In set theory, the kernel of a function f may be taken to be either For the formal definition, let X and Y be sets and let f be a function from X to Y.Elements x1 and x2 of X are equivalent if f(x1) and f(x2) are equal, i.e. are the same element of Y.The kernel of f is the equivalence relation thus defined. 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 X /=f is called the coimage of the function f, and denoted coim f (or a variation).The coimage is naturally isomorphic (in the set-theoretic sense of a bijection) to the image, im f; specifically, the equivalence class of x in X (which is an element of coim f) corresponds to f(x) in Y (which is an element of im f). Like any binary relation, the kernel of a function may be thought of as a subset of the Cartesian product X × X.In this guise, the kernel may be denoted ker f (or a variation) and may be defined symbolically as The study of the properties of this subset can shed light on f. If X and Y are algebraic structures of some fixed type (such as groups, rings, or vector spaces), and if the function f from 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 coinage and the image of f is an isomorphism in the algebraic sense; this is the most general form of the first isomorphism theorem. See also Kernel (algebra). If X and Y are topological spaces and f is a continuous function between them, 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.