The graph of a function $f : X \to Y$ is the subset that $f$ “carves out” of the cartesian product $X \times Y$.
The traditional standard definition of a graph of a function is this:
The graph $graph(f)$ of a function $f: X \to Y$ is that subset $graph(f) \hookrightarrow X \times Y$ of the cartesian product $X \times Y$ defined by the property that $(a,b) \in X \times Y$ belongs to the graph of $f$ if and only if $f(a) = b$:
This can be understood as a special case of a graph of a functor by the following observation
For $f : X \to Y$ a function, define a function
by regarding a set as a 0-category and a 0-category as a (-1)-category-enriched category and then setting
Then $\chi_f$ is the characteristic function of $graph(f)$ in that the diagram
is a pullback diagram.
In other words this means that in the context of (-1)-category-enriched category theory the graph of a function $f$, regarded as an enriched functor is the category of elements of the corresponding profunctor. More on this at graph of a functor.
It is easy to identify the properties of those subsets of $X \times Y$ that are the graphs of functions, and $f = g$ if they have the same graph (given $X$ and $Y$). Consequently, it is common, especially (but not only) in material set theory, to define a function from $X$ to $Y$ as such a subset, that is to identify a function with its graph. On the other hand, from a more categorial foundation, as discussed above, it's common to define a subset to be a characteristic function!
More generally, we can say that the graph of a binary relation from $X$ to $Y$ is a subset of $X \times Y$; $(a,b)$ belongs to the graph if and only if $a$ is related to $b$. (Note that every subset of $X \times Y$ defines a unique relation; such a subset is the graph of a function if and only if the relation is both functional and entire.)
Notice that with a function $f : X \to Y$ regarded as a profunctor $X \times Y \to (-1)Cat$ as described above, a relation $R \subset X \times Y$ corresponds to a general such profunctor. More precisely we have a pullback square
where
$R \subset X \times Y$ is the relation $R$ regarded as a subset of $X \times Y$ in the traditional sense;
$\chi_R : X \times Y \to \{0,1\}$ is the characteristic function of this subset.
So in this sense the ordinary notion of relation as a subset does really define the graph of the relation, while the relation itself is more naturally understood as the corresponding 0-profunctor/characteristic function $\chi_f$.
The graph of a binary relation from $X$ to $X$ is related to the notion of graph from graph theory; more precisely, such relations correspond to directed loop graphs (in the sense defined at graph) with vertex set $X$, and either can be defined as a subset of $X^2$. In a similar way, spans from $X$ to $X$ correspond to directed pseudographs with vertex set $X$.
For the case of a relation from $X$ to $Y$ without $X = Y$, see under the cograph below.
The graph of a relation of arbitrary arity is similarly a subset of an arbitrary cartesian product; see relation theory for more on this.
Bill Lawvere has also considered the cograph of a function, which is dually a quotient set of the disjoint union $X \uplus Y$; $a$ is identified with $b$ if $f(a) = b$ (and additional identifications may follow). However it may make more sense to define the cograph to be a quotient poset of (the discrete poset) $X \uplus Y$; we declare $a \lt b$ if $f(a) = b$ (and no additional relationships follow). By regarding again a set as a 0-category, the latter notion of cograph is a special case of the notion of cograph of a functor, as follows:
A function $f : X \to Y$ determines a functor $\bar f : I \to Set$ from the interval category $I = \mathbf{2} = \{a \to b\}$ to Set by setting $\bar f(a) = X$, $\bar f(b) = Y$ and $\bar f(a \to b) = f$.
Then let $cograph(f)$ be the corresponding category of elements, given by the 2-pullback
which is computed by the strict pullback
The cograph of $f$ in the sense of Lawvere is the set of connected components of this category, i.e. $\pi_0(cograph(f))$.
The notion of cograph of a function may be even more related to the sense of graph in graph theory; although the identifications are not done there, the cograph draws a picture in which any relation (or multispan) of any arity becomes a directed graph (or directed multigraph) whose vertex set is the disjoint union of the relation's domains. When the vertex set is broken up into a disjoint union in this way, graph theorists study this as multipartite graphs; in particular, directed bipartite graphs with vertex set broken up as $X + Y$ correspond precisely to binary relations from $X$ to $Y$.
The notion of graph of a function is a special case of the notion graph of a functor obtained for functors between 0-categories.
Accordingly, the notion of cograph of a function is a special case of the notion of cograph of a functor.
Last revised on June 4, 2022 at 06:35:35. See the history of this page for a list of all contributions to it.