Prove the series is absolutely convergent

limitsreal-analysissequences-and-series

How to go about proving that the series
$$\sum_{n=1}^{\infty} \frac{c^n}{n! + n}$$
is absolutely convergent for every real number c.

I originally thought of proving the sequence is a null sequence by multiplying through by $\frac{1}{n!}$. However, this would only hold for $|c| <1$ if I’m not mistaken. Also, how to show it is absolutely convergent.

Best Answer

At first consider series $$\sum\limits_{n = 1}^\infty \frac{|c|^n}{n!}$$ It converges: apply ratio test for $c \ne 0$ (if $c = 0$ the convergence is obvious) $$\frac{|c|^{n+1}}{(n+1)!} \frac{n!}{|c|^n} = \frac{|c|}{n} \rightarrow 0$$

Then your series $$\sum\limits_{n = 1}^\infty \frac{c^n}{n! + n}$$ is absolutely convergent by comparison test: $$\frac{|c|^n}{n!} > \left|\frac{c^n}{n! + n}\right|$$

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