Uniform convergence
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Uniform convergence
Examine if there exists a sequence \(\displaystyle{\left(p_n(z)\right)_{n\in\mathbb{N}}}\) of
complex polynomials such that \(\displaystyle{p_n(z)\to \dfrac{1}{z}}\) uniformly to
\(\displaystyle{C_{r}=\left\{z\in\mathbb{C}: |z|=r\right\}}\) (\(\displaystyle{r>0}\)).
complex polynomials such that \(\displaystyle{p_n(z)\to \dfrac{1}{z}}\) uniformly to
\(\displaystyle{C_{r}=\left\{z\in\mathbb{C}: |z|=r\right\}}\) (\(\displaystyle{r>0}\)).
Re: Uniform convergence
Suppose that this is not the case. If such sequence existed then the convergence at the compact set $\left| z \right| = r$ would be uniform. However,
$$2 \pi i = \oint \limits_{\mathcal{C}_r} \frac{{\rm d}z}{z} = \oint \limits_{\mathcal{C}_r} \lim_{n \rightarrow +\infty} p_n (z) \, {\rm d}z = \lim_{n \rightarrow +\infty} \oint \limits_{\mathcal{C}_r} p_n (z) \, {\rm d}z = \lim_{n \rightarrow +\infty} 0 = 0 $$
since the polynomials have no poles inside the circle thus the contour integral is zero. The last relation is an obscurity of course. Thus, no complex polynomials exist. This completes our arguement.
$$2 \pi i = \oint \limits_{\mathcal{C}_r} \frac{{\rm d}z}{z} = \oint \limits_{\mathcal{C}_r} \lim_{n \rightarrow +\infty} p_n (z) \, {\rm d}z = \lim_{n \rightarrow +\infty} \oint \limits_{\mathcal{C}_r} p_n (z) \, {\rm d}z = \lim_{n \rightarrow +\infty} 0 = 0 $$
since the polynomials have no poles inside the circle thus the contour integral is zero. The last relation is an obscurity of course. Thus, no complex polynomials exist. This completes our arguement.
$\displaystyle \sum_{n=1}^{\infty}\frac{1}{n^s}= \prod_{p \; \text{prime}}\frac{1}{1-p^{-s}}$
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