Convergence of a Power series

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Consider the power series $\sum^{\infty}_{n=0} a_nx^n$. It is fairly easy to impose conditions on the value of $x$, so as to make the series convergent. However, I was wondering if it is possible to impose conditions on the sequence $\{a_n\}$, so as to ensure convergence of the series, for all $x \in \mathbb R$. Is this possible?

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Sure. If $|a_n| \leq \frac{c}{n!}$, where $c$ is a fixed constant, then the series will always converge. The series expansions for $e^x, \sin x$ and $\cos x$ are good examples.