# Characteristic function

In mathematics, the term "characteristic function" can refer to any of several distinct concepts:

$\mathbf {1} _{A}\colon X\to \{0,1\},$ which for a given subset A of X, has value 1 at points of A and 0 at points of X − A.
• There is an indicator function for affine varieties over a finite field: given a finite set of functions $f_{\alpha }\in \mathbb {F} _{q}[x_{1},\ldots ,x_{n}]$ let $V=\left\{x\in \mathbb {F} _{q}^{n}:f_{\alpha }(x)=0\right\}$ be their vanishing locus. Then, the function ${\textstyle P(x)=\prod \left(1-f_{\alpha }(x)^{q-1}\right)}$ acts as an indicator function for $V$ . If $x\in V$ then $P(x)=1$ , otherwise, for some $f_{\alpha }$ , we have $f_{\alpha }(x)\neq 0$ , which implies that $f_{\alpha }(x)^{q-1}=1$ , hence $P(x)=0$ .
• The characteristic function in convex analysis, closely related to the indicator function of a set:
$\chi _{A}(x):={\begin{cases}0,&x\in A;\\+\infty ,&x\not \in A.\end{cases}}$ • In probability theory, the characteristic function of any probability distribution on the real line is given by the following formula, where X is any random variable with the distribution in question:
$\varphi _{X}(t)=\operatorname {E} \left(e^{itX}\right),$ where $\operatorname {E}$ denotes expected value. For multivariate distributions, the product tX is replaced by a scalar product of vectors.
• The characteristic function of a cooperative game in game theory.
• The characteristic polynomial in linear algebra.
• The characteristic state function in statistical mechanics.
• The Euler characteristic, a topological invariant.
• The receiver operating characteristic in statistical decision theory.
• The point characteristic function in statistics.