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The representation of functions (or objects which are in some generalize the notion of function) as constant linear combinations of sines and cosines at integer multiples of a given frequency, as Fourier transforms or as Fourier integrals.

4 votes
1 answer
2k views

Characterizations of a linear subspace associated with Fourier series

Let $c_0$ be the Banach space of doubly infinite sequences $$\lbrace a_n: -\infty\lt n\lt \infty, \lim_{|n|\to \infty} a_n=0 \rbrace.$$ Let $T$ be the space of $2\pi$ periodic functions integrable …
TCL's user avatar
  • 744
3 votes
7 answers
1k views

Continuous or analytic functions with this property of sinc function

This question is motivated by my previous post in SE (math.stackexchange.com). Prove or disprove that $\frac{\sin x}{x}$ is the only nonzero entire (i.e. analytic everywhere), or continuous, function …
TCL's user avatar
  • 744
2 votes

Continuous or analytic functions with this property of sinc function

In fact, the identity is true for the function $f(x)=\frac{\sin ax}{ax}$ for each $0\lt a\le \pi$. For the integral part, just apply substitution. For the series part, use the fact that $\sum_{n=1}^ …
TCL's user avatar
  • 744
2 votes
Accepted

Characterizations of a linear subspace associated with Fourier series

This is to summarize what were discussed in the comments, so the title will not be listed as unanswered. The linear subspace $S$ of $c_0(\mathbb{Z})$ is equal to the convolution product of two copies …
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