# Radius of Convergence of Power Series over Factorial/Real Case

## Theorem

Let $\xi \in \R$ be a real number.

Let $\ds \map f x = \sum_{n \mathop = 0}^\infty \frac {\paren {x - \xi}^n} {n!}$.

Then $\map f x$ converges for all $x \in \R$.

That is, the interval of convergence of the power series $\ds \sum_{n \mathop = 0}^\infty \frac {\paren {x - \xi}^n} {n!}$ is $\R$.

## Proof

This is a power series in the form $\ds \sum_{n \mathop= 0}^\infty a_n \paren {x - \xi}^n$ where $\sequence {a_n} = \sequence {\dfrac 1 {n!} }$.

Applying Radius of Convergence from Limit of Sequence, we find that:

 $\ds \lim_{n \mathop \to \infty} \size {\dfrac {a_{n + 1} } {a_n} }$ $=$ $\ds \lim_{n \mathop \to \infty} \size {\dfrac {\dfrac 1 {\paren {n + 1}!} } {\dfrac 1 {n!} } }$ $\ds$ $=$ $\ds \lim_{n \mathop \to \infty} \size {\dfrac {n!} {\paren {n + 1}!} }$ $\ds$ $=$ $\ds \lim_{n \mathop \to \infty} \size {\dfrac 1 {n + 1} }$ $\ds$ $=$ $\ds 0$ Sequence of Powers of Reciprocals is Null Sequence

Hence the result.

$\blacksquare$