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Let $f$ be an entire function. Assume that $\mid f(1/n)\mid\le e^{-n}$ for all $n\in \mathbb{N}$. Show that $f$ vanishes identically.

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    Oh, yes, sorry for mistake, I have corrected it. Thanks for hint, I will try it.2012-10-08

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Taking the limit as $n\to\infty$, you will find that $f(0)=0$. If $f$is not identically zero, you can write $f(z)=\sum_{k=n}^\infty a_kz^k$ with $n\ge1$ and $a_n\ne0$ (I trust you know why). Now consider $\lim_{z\to0}\frac{f(z)}{z^n}$ in general, and compare with the special case $z=1/n$ as $n\to\infty$.

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    Thanks, got it $ lim$ goes to zero, from other side it should be equal to $a_{n}$ which is equal to $f^{n}(0)/n!$2012-10-08