Fourier analysis, also known as spectral analysis, encompasses all sorts of Fourier expansions, including Fourier series, Fourier transform and the discrete Fourier transform (and relatives). The non-commutative analog is (representation-theory).

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Simplifying an expression with Fourier transform

Can anyone simplify the following expression? I guess something from Fourier transform can help: $$f(\omega) = \lim_\limits{R \to \infty} \frac{1}{R^2} \int_{r=0}^{R}{re^{i \omega r^{-\gamma}}} ...
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Problem in computing complex integrals for fourier transform

This is from a problem set of open course 8.02 by MIT OCW. I am not able to understand how the integral was solved. I have basic knowledge of Fourier transformation, and the Dirac delta function ...
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How to interchange sum and integral?

We fix the point $\xi_{0}\in \mathbb R.$ Choose sequence $\{f_{n}\}_{n\in \mathbb N}\subset L^{1}(\mathbb R)$ with the following property : (1) $\|f_{n}\|_{L^{1}(\mathbb R)} \leq 1, $ for $n\in ...
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show that $\int_{0}^{\infty} \frac {\sin^3(x)}{x^3}dx=\frac{3\pi}{8}$

show that $$\int_{0}^{\infty} \frac {\sin^3(x)}{x^3}dx=\frac{3\pi}{8}$$ using different ways thanks for all
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An identity involving Gauss sums and convolution

For a Dirichlet character $\chi$ modulo $N$, the Gauss sum attached to $\chi$ is given by $$G_\chi(m) = \sum_{k \in \mathbb{Z}_N} \chi(k) e^{2\pi i mk/N}.$$ Suppose one has an $N$-periodic function ...
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How is the study of fractals related to Fourier/spectral/harmonic analysis?

In chap. 3 of "Fractal Geometry of Nature" Mandelbrot mentions that "part of the study of fractals is the geometric face of harmonic analysis" (spectral or Fourier, he specifies), but to my dismay, ...
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Detrending sine waves accurately

I am doing some data analysis where I look at electricity demand over the course of a day, but need to separate the intra-day (constant and periodic) components from daily changes (assumed linear). At ...
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How does this conjecture correspond to Carleson's theorem for the case of $d=1$?

From this source, on page 36 (bottom) there is a conjecture stated and it was said that the case of $d = 1$ corresponds to Carleson's theorem. A picture included here : But when I look at wiki ...
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Linear independence of primitive Dirichlet characters and convolution

This is not an exercise but merely a question I have. Fix $N \in \mathbb{N}$ and suppose there exist some values $a_k \in \mathbb{C}$, for $k \in \mathbb{Z}_N$, such that $$ \sum_{k \in \mathbb{Z}_N} ...
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36 views

Dirichlet energy and Fourier transform

Is there a direct relationship between the Dirichlet energy of a function: $$E(f)=\int_{\Omega}\lvert\nabla f(\mathbf{x})\rvert^2\mathrm{d}V$$ and its Fourier transform ...
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Fourier transform with $\sin(t^2)$

This exercise gave me nightmares this night. I have $$ x(t)=\sin(t^2)e^{-2|t-2|} $$ to Fourier transform. First I though about solving the integral. (should I divide the signal in $2$, first for ...
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An intuitive definition of the frequency spectrum of a function.

In a PDE book I'm reading, the author introduces the Fourier transform by first introducing the Fourier series, and then the Fourier integral representation of a function. The Fourier integral ...
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Relation Fourier/Laplace Transform

I have a question about the relation between Fourier and Laplace transforms. I have seen in some places that the transfer functions in the Laplace space are represented as $G(s)$ where $s$ is the ...
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Fourier transforms of $f(t)=\frac{\sin{at}}{t}$

I want to derive the following pair of Fourier transforms: First: $$f(t)=\dfrac{\sin{at}}{t}$$ $$F(\lambda)= \begin{cases} \sqrt{\dfrac{\pi}{2}}, & \text{if } |\lambda|<a \\ 0, & ...
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Fourier transform of Bessel functions

I'm curious as to how the Fourier transform of the various types of Bessel functions would be calculated. The Wikipedia page on the Fourier transform gives the transform of $J_o(x)$ as being ...
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Do they have a mistake in this heat equation?

I need to know if there is a mistake in these notes: In the second page we have a representation of a function $f(x)$ as a $\sin$ series. Dont we need to have $f(0)=0=f'(l)$ for such a ...
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How to use Fourier transform to solve Fisher-Kolmogorov?

How can I use Fourier Transform to solve Fisher-Kolmogorov Equation in 1D? \begin{equation} u_t(x,t) = u_{xx}(t) + u(1-u) \end{equation} \begin{equation} u(0,x) = \phi(x) \end{equation} with ...
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Changing integration limits in a Fourier transform

I've been reading these notes about the Fourier transform of Heaviside function, but I don't fully understand the derivation right after formula (5) - why does it hold only for $t>0$? The author ...
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Fourier Transform of spherical harmonics

I am trying seeking for definition (or some source) of the Fourier Transform of Spherical Harmonics (see https://en.wikipedia.org/wiki/Spherical_harmonics). Any help will be really appreciated. ...
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Discrete Fourier Transform - proof that columns of matrix are orthogonal

The DFT matrix of size $n$ has entries $M_{ij} = \omega^{ij}$ where $\omega$ is the $n$th root of 1. My textbook states that the columns of this matrix are orthogonal because their dot product is 0. ...
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Fourier transform of periodic function

Is it possible to Fourier transform a periodic function f(x) = f(x+L) with period L, numerically, only over the range x = 0 to L and use periodic boundary conditions to enforce the periodicity of the ...
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Infinite sum of elements in a finite field

This is a bit of a curiosity that intrigues me. Let $p$ be a prime and consider the sum of reciprocals of squares divisible by $p$. This is just $$ \dfrac{1}{p^2}\sum_{n=1}^\infty \dfrac{1}{n^2} = ...
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Which Fourier series are “legal”?

Let's consider a continuous function $f(x)$ and real numbers $\lambda_n=(\alpha+\beta n)\pi$ where both $\alpha$ and $\beta$ are integers. In any interval $I$, is it true that $$ f(x)=\sum_{n\geq ...
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$\int x J_0(k x)e^{-x^2/2}dx$ Bessel function decomposition of a gaussian

$$\int ^\infty _0 x J_0(k x)e^{-x^2/2}dx$$ The integral above corresponds to fourier transform in radial coordinates. The fourier transform of a 2D gaussian is still a 2D gaussian. So the integral ...
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Fourier Transform of Positive and Negative Parts of Functions

Suppose I have a function of the form: $G(t,x) = \alpha\left(P(t,x) - \Theta(t,x) \right)^+ + \beta \left( P(t,x) - \Theta(t,x) \right)^-$. Here, $P(t,x)$ and $\Theta(t,x)$ have compact support and ...
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Confusion about a simple Fourier Transform

I was looking at a table of Fourier transform pairs, and one entry is really confusing me. There's one on the second page that states $$ \mathcal{F}(\cos(\omega_0t))(\omega) = \pi(\delta(\omega - ...
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352 views

Discrete Fourier Transform: Understand Negative Frequencies

I am trying to learn DFT on my own. I have been struggling for a while now around understanding the concept of negative frequencies and notably what happens when $k$ is greater than $N/2$ in the ...
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70 views

Is this operator a Fourier multiplier operator?

I want to study the Fourier transform of $$L_{\alpha}(t) = \frac{e^{i\alpha t}}{t^2} - i\frac{\alpha}{t}$$ Basically i am trying to get a grip on, given a $f$, what is $f(t)\ast L_{\alpha}(t)$ and am ...
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Possibly use a Fourier Transform to perform deconvolution

I need to determine the function g(B) in order to determine its prefactors $a_n$. Here's what I have: h(f,B) is a Gaussian function g(B) is the unknown function i(f) is an inhomogeneous function and a ...
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Discrete Fourier Transform of $\omega^{n(n-1)/2}$

For the sequence $x_0$, $x_1$, $\ldots$,$x_{N-1}$, let $\omega=e^{2\pi i/N}$ and define the discrete Fourier transform as $$X_k = \frac{1}{\sqrt{N}}\sum_{n=0}^{N-1}x_n\omega^{nk}\,.$$ I'm interested ...
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Mapping properties of a pseudo-differential operator of negative order

Let $H^s$ denote the Sobolev space on $\mathbb{R}^d$. Let $P$ be a pseudo-differential operator of negative order $-m$ where $m > 0$. Let $P^*$ denote its $H^0$-adjoint. Is $P^* P : H^s \to ...
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How to extract power of top two frequencies of a spectrum without using an FFT?

What I'm trying to do is see if a particular frequency component becomes dominant (and I don't really know what the dominant frequency is). Therefore, I figured that I can get the top two peaks of ...
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How does one define the Fourier transform of a probability distribution?

Say $p_X$ and $p_Y$ are two probability distributions on a $m$ element set. Then I see an equality written as, $$\sqrt{m} \vert \vert p_X - p_Y \vert \vert _2 = \sqrt{ \sum_{k=0}^{m-1} \vert ...
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Operator $T$ commutes with all translations $\Leftrightarrow$ $T$ is multiplication operator

Let $T: L^2(\mathbb R) \to L^2(\mathbb R)$ be an operator that commutes with all translations $f(x) \mapsto f(x-y)$. Why does it follow that the Fourier transform of $Tf$ is then given by ...
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Fourier transform with initial limit starting from 0

For a function $f(p)$, the Fourier transform is $\hat{f}(p)=\int_{-\infty} ^{\infty}e^{-i xp}f(x)dx$. What are the conditions that i write it as; $\hat{f}(p)=2\int_{0} ^{\infty}e^{-ixp}f(x)dx$? I ...
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How to Prove Plancherel's Formula?

I have difficulty in proving Plancherel's formula in Fourier transform. Here is what I have thought: In this question, I denote complex conjugation by an overline and (inverse) Fourier transform is ...
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Fourier transform of the Heaviside function

As you can see from the title I want to calculate the Fourier transform of the Heaviside function $u(t)$. Proven the the Heaviside function is a tempered distribution I must evaluate: $$ \langle ...
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$\{f\in L^{1} : \hat{f} \in L^{p} \}$ closed under convolution?

Let $f, g\in L^{1}(\mathbb R),$ we may define the convolution of $f$ and $g$ as follows: $f\ast g(x)= \int_{\mathbb R} f(x-y)g(y) dy, (x\in \mathbb R).$ We note that $L^{1}(\mathbb R) \ast ...
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Sets of Divergence for Fourier Partial Integals

It is a consequence of Carleson's theorem together with a transference argument that (see Section 4.3.5 in L Grafakos, Classical Fourier Analysis for proof) that the Fourier partial integrals of a ...
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Heat equation with fourier transformation

I want to understand a solution from an exercise where we should find a solution of the heat equation: $$\frac{\partial u(x,t)}{\partial t}=\sum_{j=1}^{n}\frac{\partial^2 u(x,t)}{\partial x_j^2} $$ ...
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Characterization of $H^k$ by Fourier transform

Let $u\in H^k(\mathbb{R}^n)$ a function with complex-valued. Question 1: If $u\in H^k(\mathbb{R}^n)$ and $D^\alpha u\in L_2(\mathbb{R}^n)$, by definition for each $|\alpha|\le k,$ then we have ...
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What can we say about the transform of a function on a truncated domain, with respect to the transform on the full domain?

Let $f$ be a function on R and $\hat f$ its Fourier transform. Consider a truncated version of $f$ called $\bar f$ whose value outside an interval is $0$. Formally, $\bar f(x) = f(x) * 1_{x \in I}$ ...
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Question about the limits of definite integrals

Let me take an example that I've come across while studying Fourier series, We all know that $$\int_{-a}^{a} \sin \left( \frac{n\pi x}{a} \right) dx = 2 \int_{0}^{a} \sin \left(\frac{n \pi x}{a} ...
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Role of i in Fourier transform

I've seen several derivations of the Fourier transform, but most don't cover the conversion to the form $$ S(f) = \int_{\infty}^{-\infty} s(t)e^{-i2\pi ft} \;\mathrm{d}t $$ What is the role of ...
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Show there exists $n\in \Bbb{Z}$ such that $\lambda =({2\pi n\over T })^4$.

Let $f:\Bbb{R}\to \Bbb{C}$ be an infinitely differentiable function with period T. Also, $f^{(4)}=\lambda f$. Show there exists $n\in \Bbb{Z}$ such that $\lambda =({2\pi n\over T })$. Attempt: I did ...
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The question about the support of Fourier transform of $|f|^p$

Suppose $f$ is a smooth function with $\mathbb{supp}{(\mathcal{F}{f})} \subset B(0,1)$. In addition, assume $f$ is non-negative. We can observe that the function $|f|^2$ has a nice property : ...
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Integration of a generic radial function in polar coordinates

I need to perform the following integral $\int{P(k) e^{i \vec{k}\cdot \vec{\Delta r}} \frac{d^2k}{(2 \pi) ^2}}$ using polar coordinates. I think the result should depend on some Bessel function, but ...
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Positiveness of partial sums of type $ \psi * D_N $

In his paper about Extremal Functions for the Fourier Transform (see, for example, here? https://projecteuclid.org/download/pdf_1/euclid.bams/1183552525), Jeffrey Vaaler, while trying to build ...
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Confused by certain interpretation of expected value…

I read the following in Stein / Shakarchi's Fourier Analysis book, where they discussed the notion of expectation of a probility density. "Consider the simpler (idealized) situation where we are ...
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To fast invert a real symmetric positive definite matrix that is almost similar to Toeplitz [duplicate]

I have asked this question on mathoverflow also. (my question, I wasn't sure if its ok ask at another similar forum, on stack exchange, but I hope it would reach more people). It is well known how to ...