[Math] $f$ an entire function, $|f(1/n)|\leq n^{-n}$ for $n\in \mathbb{N}$. Show that $f$ is constant.

complex-analysis

I'm working on old qualifying exam problems and this one came up.

Suppose $f$ is an entire function on $\mathbb{C}$, and that $|f(\frac{1}{n})| \leq n^{-n}$ for all $n\in \mathbb{Z}_{>0}$. Prove that $f$ is constant.

My approaches: One obvious thing to do is to try and show that $f$ is bounded and appeal to Liouville's theorem. However, our bounds are for small values of $z$, so this did not yield much success.

One can see that $f(0)=0$ from this condition. Also,
$$
f'(0) = \lim_{n\to \infty} \frac{f(\frac{1}{n}) – f(0)}{\frac{1}{n}} = \lim_{n\to \infty} n f\left(\frac{1}{n}\right).
$$
Note that
$$
\left|nf\left(\frac{1}{n}\right)\right| \leq n^{-n+1} \to 0 \text{ as } n\to \infty.
$$
Thus, $f'(0)=0$.

I would like to keep trying to show that $f^{(n)}(0)=0$ and use the fact that $f$ is entire to get that $f=0$ identically on $\mathbb{C}$. But I get stuck because I don't have an estimate for $f'(1/n)$.

I think what makes this problem harder is that we only have an estimate on $f$ at a countable set, instead of the usual problem where we know some kind of bound on $f$ everywhere.

Any hints are appreciated.

Best Answer

If $f$ is not identically zero then $f$ has a zero of multiplicity $k \ge 0$ at $z=0$, i.e. $$ f(z) = z^k g(z) $$ where $g$ is an entire function with $g(0) \ne 0$.

Then $$ \bigl|g(\frac 1n)\bigr| = n^k \, \bigl|f(\frac 1n)\bigr| \le \frac{n^k}{n^n} $$ for all positive integers $n$. The right-hand side tends to zero for $n \to \infty$ and it follows that $g(0) = 0$, which is a contradiction.