# A simple question from mathematical analysis (assumption changed) [closed]

Let $\forall n=0,1,2,\dots$, $\alpha_{n}(x)$ are POLYNOMIALS in $x$. Next, let for all $x\neq0$ the power series $$\sum_{n=0}^{\infty}\alpha_{n}(x)t^{n}$$ has positive radius of convergence. Can one say the radius of convergence of the series $$\sum_{n=0}^{\infty}\alpha_{n}(0)t^{n}$$ is also positive?

My quess is NO, but I have no example.

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## closed as off topic by Benoît Kloeckner, Andreas Blass, Bill Johnson, Pietro Majer, Felipe VolochApr 19 '13 at 3:35

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As you suspect, there are easy counterexamples. Take any $\alpha_n$ with $\alpha_n(0) = n!$ and $\alpha_n(x) = 0$ for $|x| \geq 1/n$. Then for non-zero $x$ the series is a polynomial, and $\sum_{n=1}^\infty n!t^n$ diverges except at 0. – Ben Barber Apr 17 '13 at 11:51
No. Consider series $\sum_{n=0}^\infty n!e^{-n^2x^2}t^n$. – Andrew Apr 17 '13 at 11:51
Very nice example, thx! – Twi Apr 17 '13 at 18:05
Let's make it a little bit more difficult. Suppose, in addition, $\alpha_{n}(x)$ is a POLYNOMIAL in $x$. – Twi Apr 17 '13 at 18:07

Still no: $$\sum_{n=1}^{+\infty}n!\left(1-\frac{x^2}n\right)^{n^3}t^n.$$
I was wondering if there are still counter-examples when the power series is assumed to be convergent for all complex values of $x\neq 0$. Using Cauchy's formula it appears at first that some control exists on $\alpha_n(0)$, although there is a swap in quantifiers that needs some looking carefully, which I don't have that much time to do right now... – Loïc Teyssier Apr 19 '13 at 7:31