Let $(a_k)$, with $0<\liminf |a_k|\le\limsup |a_k|<\infty$. Determine the radius of convergence of $\sum a_kX^k$.
I want to check if my reasoning is correct. Because $0<\liminf |a_k|\le\limsup |a_k|<\infty$ then the sequence $(|a_k|)$ is bounded and eventually different than zero.
And we have that for any fixed $\ell$, $\limsup\sqrt[k]{|\ell|}=1$, then exists some $0<m\le|a_k|$ and some $M\ge|a_k|$ for all $k\ge N$, for enough large $N$. Then
$$1=\limsup\sqrt[k]{m}\le\limsup\sqrt[k]{|a_k|}\le\limsup\sqrt[k]{M}=1$$
Then the radius of convergence is
$$\rho_a=\frac1{\limsup\sqrt[k]{|a_k|}}=1$$
Eventually, $(|a_n|)$ is bounded below by some $\epsilon >0$ (otherwise $0$ is a cluster point, so $\liminf \le 0$) and above by some $M$ (otherwise $\limsup = \infty$). So we have:
$$\epsilon \sum |x|^n \le \sum |a_n x^n| \le M \sum |x|^n$$
Which shows that the radius of convergence is $1$.