Is $f(z)$ a constant in $\Bbb{C}$? Suppose $f:\Bbb{C}\rightarrow \Bbb{C}$ is holomorphic, $f(z+1)=f(z)$ for all $z \in \Bbb{C}$ and $$|f(z)|\leq \exp\left(\frac{2}{\pi}|z|\right).$$ Is $f(z)$ a constant in $\Bbb{C}$?
 A: By periodicity of $f$ the following function is well defined and holomorphic on $\mathbb{C}^{\ast}$:
$$
h(z) = f\left(\frac{\log(z)}{2\pi i}\right)
$$
Considering all branches of $\log$ the modulus of $h$ can be estimated by
$$
|h(z)| = \left|f\left(\frac{\log(z)}{2\pi i}\right) \right| \leq \min_{k \in \mathbb{Z}} \exp \left( \left| \frac{\log(z)}{ \pi^2 i} + \frac{2 k}{\pi} \right| \right) \leq 
\exp \left( \frac{1}{\pi} + \frac{|\log|z| \,|}{ \pi^2} \right).
$$
For $|z| \leq 1$ this implies
$$
| z \, h(z) | \leq e^{1/\pi} |z|^{1-1/\pi^2}
$$
which shows that $z\,h(z)$ is bounded near $z=0$ and hence extends to an entire function. Since $z\,h(z)$ vanishes at $z=0$ it follows that $h(z)$ is itself entire.
Similarly for $|z| \geq 1$ we get
$$
|z \, h(z^{-1})| \leq e^{1/\pi} |z|^{1-1/\pi^2}
$$
and therefore $h(z^{-1})$ is also entire.  Then $h(z)$ is a bounded entire function so it must be constant.  then $f$ itself is constant as well.
A: I think you can use Liouville theroem. Since $f$ is an entire function, then $g=f\exp(2z/\pi)$ is also an entire function. Easily we can see that $g$ is a constant in $\Bbb C.$ So we can get $f(z)=c\exp(2z/\pi)$, however, we have $f(z)=f(z+1)$, so $c=0,$ and as a result $f(z)=0$ for all $z\in \mathbb{C}$
