# Definition:Characteristic Function (Set Theory)/Relation

This page is about Characteristic Function in the context of Relation Theory. For other uses, see Characteristic Function.

## Definition

The concept of a characteristic function of a subset carries over directly to relations.

Let $\RR \subseteq S \times T$ be a relation.

The characteristic function of $\RR$ is the function $\chi_\RR: S \times T \to \set {0, 1}$ defined as:

$\map {\chi_\RR} {x, y} = \begin {cases} 1 & : \tuple {x, y} \in \RR \\ 0 & : \tuple {x, y} \notin \RR \end{cases}$

It can be expressed in Iverson bracket notation as:

$\map {\chi_\RR} {x, y} = \sqbrk {\tuple {x, y} \in \RR}$

More generally, let $\displaystyle \mathbb S = \prod_{i \mathop = 1}^n S_i = S_1 \times S_2 \times \ldots \times S_n$ be the cartesian product of $n$ sets $S_1, S_2, \ldots, S_n$.

Let $\RR \subseteq \mathbb S$ be an $n$-ary relation on $\mathbb S$.

The characteristic function of $\RR$ is the function $\chi_\RR: \mathbb S \to \set {0, 1}$ defined as:

$\map {\chi_\RR} {s_1, s_2, \ldots, s_n} = \begin {cases} 1 & : \tuple {s_1, s_2, \ldots, s_n} \in \RR \\ 0 & : \tuple {s_1, s_2, \ldots, s_n} \notin \RR \end {cases}$

It can be expressed in Iverson bracket notation as:

$\map {\chi_\RR} {s_1, s_2, \ldots, s_n} = \sqbrk {\tuple {s_1, s_2, \ldots, s_n} \in \RR}$