# Definition:Analytic Function/Real Numbers

## Definition

Let $f$ be a real function which is smooth on the open interval $\openint a b$.

Let $\xi \in \openint a b$.

Let $\openint c d \subseteq \openint a b$ be an open interval such that:

$(1): \quad \xi \in \openint c d$
$(2): \quad \ds \forall x \in \openint c d: \map f x = \sum_{n \mathop = 0}^\infty \frac {\paren {x - \xi}^n} {n!} \map {f^{\paren n} } x$

Then $f$ is described as being analytic at the point $\xi$.

That is, a function is analytic at a point if and only if it equals its Taylor series expansion in some interval containing that point.

## Also known as

This is also known as a real analytic function.