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Please how to find order of $ f_k(z) = \prod\limits_{n=1}^{\infty} \left(1-\frac{z}{n^k}\right) .$

Let $M(r) = \max \{|f_k(z)|:|z| = r\}.$ Then order of $f_k(z)$ is defined as : $\lambda = \limsup_{r\to \infty} \frac{\log \log M(r)}{\log r}.$

Can someone help to solve this. Thank you.

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    @Hurkyl: I fairly new to this...can u help me start.2012-03-17

1 Answers 1

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The order of this function is $\frac{1}{k}$, and we must have $k>1$.

Proof: First, note that $\prod_{n=1}^{\infty}\left(1-\frac{z}{n^{k}}\right)$ will attain its maximum modulus on the circle of radius $r$ at $z=-r$. Taking the logarithm of this, we need to examine $h(r)=\log\prod_{n=1}^{\infty}\left(1+\frac{r}{n^{k}}\right)=\sum_{n=1}^{\infty}\log\left(1+\frac{r}{n^{k}}\right).$ If we can prove that this is bounded below and above by something close to $r^\frac{1}{k}$, then we will have shown that $\lambda=\frac{1}{k}$ since $\lambda = \lim_{r\rightarrow \infty} \frac{\log h(r)}{\log r}.$

Lower Bound: We have that $\sum_{n=1}^{\infty}\log\left(1+\frac{r}{n^{k}}\right)\geq\sum_{n\leq r^{\frac{\epsilon}{k}}}\log\left(1+\frac{r}{n^{k}}\right)\geq r^{\frac{\epsilon}{k}}\log\left(1+r^{1-\epsilon}\right)$ for any $0<\epsilon<1.$ Taking $\epsilon=1-\frac{1}{\log r},$ we see that $r^{1-\epsilon}=e,$ and hence the above is $\geq e^{-\frac{1}{k}}r^{\frac{1}{k}}\log\left(1+e\right)\geq e^{-\frac{1}{k}}r^{\frac{1}{k}}.$ This then implies that $\lambda\geq\frac{1}{k}.$

Upper Bound: We split the sum based on the size of $n$. If $n>r^{\frac{1}{k}},$ by using the bound $\log\left(1+\frac{r}{n^{k}}\right)\leq\frac{r}{n^{k}},$ we have $\sum_{n>r^{\frac{1}{k}}}^{\infty}\log\left(1+\frac{r}{n^{k}}\right)\leq r\sum_{n>r^{\frac{1}{k}}}^{\infty}\frac{1}{n^{k}}\ll\frac{r}{r^{\frac{1}{k}k-1}}=r^{\frac{1}{k}}.$ Next, for $n^{k}\leq r,$ we have that $\log\left(1+\frac{r}{n^{k}}\right)\leq\log\left(r+1\right),$ which implies that $\sum_{n\leq r^{\frac{1}{k}}}^{\infty}\log\left(1+\frac{r}{n^{k}}\right)\leq r^{\frac{1}{k}}\log\left(r+1\right).$ Thus $\sum_{n=1}^{\infty}\log\left(1+\frac{r}{n^{k}}\right)\ll r^{\frac{1}{k}}\log r,$ which implies that $\lambda\leq\frac{1}{k}.$

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    @Yuri: Writing $f(x)\ll g(x)$ is just another notation for $f(x)=O(g(x))$. See [Big-O notation](http://en.wikipedia.org/wiki/Big_O_notation). This just means there exists a constant $M$ such that $|f(x)|\leq M|g(x)|$ for all large $x$.2012-03-18