Can the Fejér Kernel Be Approximated by Polynomials?

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SUMMARY

The Fejér kernel can be approximated by polynomials through methods involving the sum of exponentials. By utilizing the series expansion of exponentials, one can effectively approximate the Fejér kernel. Additionally, the Stone-Weierstrass theorem provides a foundational result that supports this approximation process. These methods are well-established in functional analysis and provide a clear pathway for approximating the Fejér kernel.

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  • Familiarity with exponential functions and their series expansions
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TL;DR
Methods to approximate Fejér kernel
Hello, I'm currently studying the Fejér kernel, which has the form of
F_{n}(x)={\frac  {1}{n}}\left({\frac  {\sin {\frac  {nx}{2}}}{\sin {\frac  {x}{2}}}}\right)^{2}={\frac  {1}{n}}\left({\frac  {1-\cos(nx)}{1-\cos x}}\right)
. I want to know whether there are some methods to approximate this function into polynomials.

Thanks a lot for the help!
 
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Take a look here:

https://en.wikipedia.org/wiki/Fejér_kernel
We can rewrite the kernel as a sum of exponentials, and exponentials are easily approximated by polynomials (take some terms of their series expansion). Thus we can approximate the kernel as well.

Alternatively, you can use results like Stone-Weierstrass.
 

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