Questions on Fourier series, the expansion of a function in terms of basis functions that satisfy an orthogonality relation. Usually, complex exponentials or sines and cosines are used.

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2
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18 views

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 ...
1
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1answer
33 views

Does the Fourier series converge at $x=0$?

Let $f(x)$, a $2\pi$ periodic funciton such that $f(0) = 1$ and for every $0\ne x\in[-\pi,\pi]$: $f(x) = 1 + \sin \frac{\pi^2}{x}$. Is the Fourier series of $f(x)$ converges at $x=0$? If so, what ...
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1answer
72 views

Sum of trigonometric infinite series

I am trying to prove that for any $x\geq 1$ we have: $$ \sum_{m=1}^{\infty} \frac{\sin\frac{(2m-1)\pi}{x}}{\left(\frac{(2m-1)\pi}{x}\right)^3} = \frac{x}{8}(x-1). $$ Could I have some help please? I ...
2
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0answers
40 views

Orthonormal Basis of $L^2$

Theorem: ' ' The Orthonormal family $e_n(x)=e^{2\pi i n x},\ n\in\mathbb{N}$ is a basis for $\mathcal{L}^2([0,1])$.`` In this case, $\{e_n(x)\}_{n\in\mathbb{N}}$ being a basis would mean that any ...
2
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1answer
38 views

Intuitive reason for Fourier Series Convergence

I read that Fourier Series Converges to average of left side and right side limits at Jump Discontinuities. What is the intuitive explanation for it? Is it something regarding Energy minimization?
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1answer
30 views

Find the Fourier coefficients of $g(x)$

Let $f:\mathbb{R}\to\mathbb{C}$, $2\pi$ periodic function and $f\in C^1$, such that the n-th Fourier coefficient is: $\hat{f}(n) = 3^{-n^2}$. Find the Fourier coefficients of $g(x) = \pi ...
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3answers
39 views

Are the coefficients on the Fourier transform arbitrary?

I was just wondering the other day about a convention I'd always taken for granted. I've seen the Fourier transform written a lot of ways. The first way (which, for reference, I'll call Scheme 1) is: ...
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0answers
42 views

How do you find the Fourier series of $\max(0, \sqrt{1 - \cos{\theta}})$?

I was trying to express the following periodic function: $$ f(x) = \max \left( 0, \sqrt{1 - \cos{x}} - \frac{\sqrt{2}}{2} \right)$$ as a summation of cosines and sine waves $f(x) \approx a_0 + ...
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1answer
32 views

Prove that the following function is $C^\infty$ [duplicate]

Prove that the following function is $C^\infty$ (and in the point $ξ=0$) : $$f:\Bbb R\to\Bbb C:\xi\mapsto {e^{i\cdot\xi\cdot λ}-1\over i\cdot\xi}-λ$$ for whichever $$λ>0$$ I am trying to find a ...
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3answers
94 views

Prove that the following function is $C^{\infty}$ [duplicate]

Prove that the following function: $$r:x \mapsto \begin{cases} e^{-{1\over (1-x^2)}}, & \text{if $|x|<1$} \\ 0, & \text{if $|x| \ge 1$} \end{cases}$$ is $C^{\infty}$ I found this problem ...
2
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2answers
37 views

Find the fourier series of the function

Find the fourier series of the function $g(x) = \sum\limits_{n=1}^\infty \frac{sin(nx)}{6^n sin(x)}$ for $x \not= k\pi$, and $g(k\pi) = \lim_{x\to k\pi} g(x)$, $(k \in \mathbb{Z})$
2
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1answer
63 views

What does $\Bbb R/2\pi$ for a set mean?

I simply cannot figure out what this means. I read this on an article about the scalar product of $2\pi$ periodic functions. it says that < f,g > goes from $\Bbb R/2\pi \to \Bbb C$ (complex) Do ...
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0answers
20 views

Let $A,B:V\to V$ positive definite operators in complex linear space with inner product $V$, $dimV<\infty$

Let $$A,B:V\to V$$ positive definite operators in complex linear space with inner product $$V$$, $$dimV<\infty$$ Show that $$log det(A\cdot B^{-1})=-\int_{0}^\infty tr(e^{-t\cdot A}-e^{-t\cdot ...
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4answers
117 views
+100

Proving a function is continuous and periodic

Suppose we are given a function $$g\left ( x \right )= \sum_{n=1}^{\infty}\frac{\sin \left ( nx \right )}{10^{n}\sin \left ( x \right )},x\neq k\pi , k\in\mathbb{Z}$$ and $$g\left ( k\pi \right ...
4
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2answers
82 views

Fourier series for $\sec(x)$

Expand in Fourier series the function $$f(x)=\sec(x) \quad x\in(-\pi/4,\pi/4).$$ Hint: Deduce a relation between the coefficients $a_n$ and $a_{n-2}$ Since this function is even, $b_n=0$ and ...
2
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2answers
58 views

Replicating Kolmogorov's Counterexample for Fourier Series in Context of Fourier Transforms

It is a famous result of Kolmogorov that there exists a (Lebesgue) integrable function on the torus such that the partial sums of Fourier series of $f$ diverge almost everywhere (a.e.). More ...
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0answers
22 views

The decay of the Fourier coefficients of the disjoint union of arcs

Given $N$ disjoint arcs $\{I_{\alpha}\}_{\alpha=1}^{N}\subset\mathbb{T} $,set $f=\displaystyle\sum_{\alpha=1}^{N}\chi_{I_{\alpha}}$ show that $$\sum_{|v|>k}|\hat{f}(v)|^2\le\dfrac{CN}{k}$$ This ...
0
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1answer
6 views

Increasing order of fourier coefficients on the boolean cube

Given a function $f:\{0,1\}^n\rightarrow \{0,1\}$, is it true that for any $S,T\subseteq[n]$, such that $S\cap T =\phi$, then $\hat{f}(S\cup T)\leq \hat{f}(S)$? It seems so to me cause, if if you just ...
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1answer
32 views

Function Integrable in an improper sense that does not satisfy Riemann's Theorem

I need some help over the subject of Fourier series... Do you know if there's a function $g(t)$ integrable in a improper sense over an interval $[a,b]$ and such that $\lim\limits_{p\rightarrow ...
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0answers
23 views

Solve one dimensional wave equation using fourier transform

I'm trying solve this wave equation using fourier method, but I am stuck... $${ u }_{ tt } ={ c }^{ 2 }{ u }_{ xx } - \alpha{ u } =0, \ 0<x\le L, t >0 $$ $${ u }( 0,t) = { u }( L,t) = 0$$ $${ ...
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2answers
41 views

Damped Wave Problem Analysis [closed]

I'm honestly not sure where to begin with this problem. Any help would be greatly appreciated!1
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1answer
43 views

Solving wave equation by fourier method

I'm trying solve this wave equation using fourier method, but I am stuck... $${ u }_{ tt } ={ c }^{ 2 }{ u }_{ xx } - \alpha{ u } =0, \ 0<x\le L, t >0 $$ $${ u }( 0,t) = { u }( L,t) = 0$$ $${ ...
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0answers
43 views

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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1answer
26 views

When is it appropriate to neglect all terms after the first non-zero term of a Taylor expansion series?

Suppose I am interested in the Taylor expansion series of a Cosine function at the neighbourhood of a=0. In computing the series from n=0 to n = infinity, when would it be appropriate to neglect all ...
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0answers
23 views

Fourier series for $x$ in $(0,\pi)$ for the complete family $\{ cos(kx) \}_{k \geq 0}$

So I have to obtain the fourier coefficients $C_k$ so that: $x= \sum_{k=0}^{\infty}C_k\cos(kx) $ $x \in{[0,\pi]}$ and for $k \in \mathbb{N_{0}} . $ I have used ...
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1answer
18 views

Cropping off the Taylor Series

We know that the Taylor series is for expansion of any function, but for digitization we need to crop off some parts? How can we determine upto which derivative should we consider.. I am mainly ...
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0answers
11 views

Find a derivative of equation that contains Fourier series

I need to find a derivative of follow equation $$ \left(r_{0} + \sum [a_{i}\cos(i\phi) + b_{i}\sin(i\phi)] \right)({\sin\phi-k\cos\phi}) - b = 0 $$ I know the derivative of $\left(r_{0} + \sum ...
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1answer
37 views

An equality about Fourier transform

I have read an equality about Fourier transforms which I can not proof. It is as following: Let $u\in C_0(\mathbb{R}^n)$ and \begin{equation} g(x_1,x_2,...,x_{n-1}):=u(x_1,x_2,...,x_{n-1},0). ...
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1answer
47 views

What if the Fourier series of a periodic function also has periodic coefficients $a_k$

If given that $x(t)$ is a periodic continuous time signal, with periodic $T$. It can be expressed by the Fourier series, i.e. $x(t)=\sum\limits_{k=-\infty}^{+\infty}\,a_k\cdot e^{j k \frac{2 ...
2
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0answers
23 views

Fourier Transform by hand

For an exam we have to calculate Fourier Transform by hand in complex space and in $\mathbb{Z}_{32}$ space ($\mathbb{Z}$ mod 32). I am familiar with recursive algorithm in a complex space (example in ...
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1answer
40 views

Finding the eigenvalues and eigenfunction (tricky)

I'm given $$X"- vX' +X \lambda=0$$ (v is a constant) I have worked x' to be: X'(x) = $$\frac{1}{2} B v e^{\frac{v x}{2}} \sin \left(\frac{1}{2} x \sqrt{v^2-4 \beta ^2}\right)+\frac{1}{2} B ...
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0answers
43 views

How to motivate those expansions?

I've been reading a paper where the author needs to solve the biharmonic equation on the plane. In truth, the function being saught is a function $v$ such that $v = \nabla \times U$ and $\nabla^4 U = ...
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0answers
28 views

A sufficient condition for a series of functions to be $\mathcal{C}^i$ (Differentiable)?

Suppose we have the Fourier Series : f(x)=$\sum_{k=1}^{\infty} C_k f_k(x)$=$\sum_{k=1}^{\infty} C_k \sin(kx)$ defined in $(a,b) \in \mathbb{R}$ Using Dirichlet criterion I have shown the sum is ...
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1answer
19 views

Bessel equation of half-order (asymptotic)

Not really optimistic about getting a reply for a question tagged under "Bessel function" but here goes, I have $$J_{\frac{1}{2}} = (a_1 \cos(z) + a_2 \sin(z))Z^{-\frac{1}{2}} $$ and ...
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1answer
67 views

What is $\lim_{n \to \infty} n^3 a_n$? [duplicate]

$a_n$ is the Fourier coefficient of $$f(x) = \left(1 - \frac{|x|}{\pi}\right)^4$$ The answer is infinity, but can someone give an answer that doesn't require explicit computation of the $a_n$? I'm ...
2
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1answer
41 views

A Hölder continuous function whose Fourier coefficients do not decay very fast

At Stein's book of Fourier analysis (Chapter 3, page 91, exercise 15) I was trying to solve the following problem I have to prove that the result ...
3
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0answers
43 views

Using a Fourier Series to Solve Differential Equation

The problem states to use the fourier series of the function f(t) defined as follows: $f(t)= t+1 , -1<t<0 $ $f(t)=1-t , 0<t<1$ to solve the differential equation: x''+4x=f(t), x(0)=1, ...
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1answer
62 views

“Counterexample” for a weaker version of Riemann–Lebesgue lemma

My teacher gave us this version of Riemann–Lebesgue lemma in class: Let $g(t)$ be an absolutely integrable function on $[a,b]$, then $$\lim_{p\to\infty} \int_a^b g(t)\sin(pt)dt=0$$ Similarly for ...
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0answers
12 views

The bond between Fourier Transform and Epicycle theory

Can someone help me understanding the bond between the Fourier Transform and the epicycle theory? I have searched in many places such as: http://math.stackexchange.com/a/72479/185138 ...
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0answers
6 views

A theorem regarding epicycles

Can somebody help me understanding the theorem on the last page of that article about Fourier Series and Epicyles? ...
2
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1answer
30 views

Find the half range cosine fourier series expansion for $f(x)=(x-1)^2,\quad 0<x<1$.

Find the half range cosine fourier series expansion for $$f(x)=(x-1)^2,\quad 0<x<1$$ and hence deduce that $$\pi^2=8\left(\frac 1 {1^2}+\frac 1 {3^2}+\frac 1 {5^2}+\ldots\right)\tag{1}$$ My ...
1
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1answer
50 views

Finding the limit that involves Fourier coefficients,

Given the function $f(x) = 1 - \dfrac{|x|}{\pi}$, I had computed its Fourier coefficients, using integration by parts and got: $$ a_n = \begin{cases} 0, & \text{for $n$ even}, \\[6pt] ...
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1answer
32 views

Fourier series and transform (epicycles)

Let $\gamma:[a,b]\to\mathbb{C}$ be a continuous curve. 1) Is it true that one can find a sequence of numbers $(r_n)_{n\in\mathbb{N}}\subset (0,\infty)$ and some function $\varphi:\mathbb{R}\times ...
3
votes
2answers
98 views

Why is $\sigma_1(0)$ not $-\frac{1}{12}$?

The Eisenstein series $\mathbb{G}_2$ is given by $$\mathbb{G}_2(z) = -\frac{1}{24} + \sum_{n=1}^\infty \sigma_1(n) q^n$$ with $q=e^{2\pi i z}$ and $$\sigma_1(n):=\sum_{d\mid n} d$$ for $n\in\mathbb ...
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0answers
15 views

FFT differential equations

Given a generical differential equation what is the procedure to solve it using fft command. Can anyone explain me how to do it? For example: $$\frac{d^2y}{dt^2}+10\cdot \frac{d\:y}{dt}=-5\cdot ...
2
votes
1answer
41 views

Coefficient in the Fourier expansion of the cusp form

Ideal of cusp for $\Gamma_{0}(4)$ is principal and generated by $f(z)=η(2z)^{12}=q+\sum a(n)q^n $, this is discussed here. How one can compute the coefficient $a(n)$ when $n$ is rather large ? for ...
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1answer
28 views

Fourier transform of PDE on finite and infinite bound simultaneously.

Consider $$u_{xx} + u_{yy} = 0 $$ on the bounds: $$o < x < L$$ and $$-\infty<y<\infty$$ The initial condition is: $$u(0,y) = f(y)$$ and $$u(L,y)=g(y)$$ I've tried performing fourier ...
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0answers
23 views

relation between $\zeta(2)$ and the fourier transform of $x^2$

I have problem with see the relation between the transform of $x^2$ in $[-\pi,\pi]$ and the function $\zeta$ de Riemann in the point 2, this say that using the transform fourier of $x^2$ prove that ...
0
votes
1answer
32 views

Applying Fourier transform to heat equation with source

I haven't had any experience with applying of FT to heat equation with source. But this popped up in an exercise. Any help in the right direction would be great. Consider: $$\frac{\partial ...
-1
votes
0answers
27 views

How do I show that the fourier transform of the dirac delta function is given by this equation?

I must show that the Fourier transform of the dirac delta function is $$\delta (x-x_0)=\frac{1}{2\pi}\int_{-\infty}^{\infty}e^{-i \omega(x-x_{0}) }d\omega$$ Here's my attempt: To avoid confusion ...