What is Fourier coefficients: Definition and 71 Discussions

In mathematics, a Fourier series () is a periodic function composed of harmonically related sinusoids, combined by a weighted summation. With appropriate weights, one cycle (or period) of the summation can be made to approximate an arbitrary function in that interval (or the entire function if it too is periodic). As such, the summation is a synthesis of another function. The discrete-time Fourier transform is an example of Fourier series. The process of deriving weights that describe a given function is a form of Fourier analysis. For functions on unbounded intervals, the analysis and synthesis analogies are Fourier transform and inverse transform.

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  1. Z

    How to write sin(n*pi/2) as an expression (-1)^f(n)?

    I could summarize this as ##\frac{1}{n\pi}## for ##n=1, 5, 9, \ldots## ##\frac{-1}{n\pi}## for ##n=3, 7, 11, \ldots## and ##0## for all other ##n##. How would I go about writing this in a single expression, with ##(-1)^{f(n)}## where ##f(n)## summarizes both cases above?
  2. Skaiserollz89

    Relationship between Fourier coefficients and power spectral density

    Here, ##\Phi(f_{x_n},f_{y_m})=|\mathscr{F(\phi(x,y))}|^2 ## is the Power Spectral Density of ##\phi(x,y)## and ##\mathscr{F}## is the Fourier transform operator. Parseval's Theorem relates the phase ##\phi(x,y)## to the power spectral density ##\Phi(f_{x_n},f_{y_m})## by...
  3. S

    Comp Sci Plot periodic function with Fourier coefficients

    I have plotted the function for ##T=15## and ##\tau=T/30## below with the following code in Python: import numpy as np import matplotlib.pyplot as plt def p(t,T,tau): n=np.floor(t/T) t=t-n*T if t<(2*np.pi*tau): p=np.sin(t/tau) else: p=0 return p...
  4. S

    Characterize Fourier coefficients

    I would try to determine whether ##p(t)## is even or odd. This would be so much easier if the values of ##\tau## and ##T## would be specified, but maybe it's possible to do without it, which I'd prefer. If for example ##\tau=1/2## and ##T=2\pi##, then ##p(t)=\sin{(2t)}## for ##0\leq t <\pi ##...
  5. C

    A Calculation of Fourier coefficients using SAMBA methodology

    Hello everyone. I have 4 samples of 50 elements from 4 unknown random variables obtained from a Karhunen-Loève decomposition using Matlab's pca (each one is a column of size 50 from the coefficient matrix). I am following the article SAMBA: Sparse Approximation of Moment-Based Arbitrary...
  6. docnet

    Finding the Fourier Coefficients of a Function

    Consider the function ##f:[0,1]\rightarrow \mathbb{R}## given by $$f(x)=x^2$$ (1) The Fourier coefficients of ##f## are given by $$\hat{f}(0)=\int^1_0x^2dx=\Big[\frac{x^3}{3}\Big]^1_0=\frac{1}{3}$$ $$\hat{f}(k)=\int^1_0x^2e^{-2\pi i k x}dx$$ Can this second integral be evaluated?
  7. S

    MATLAB Turning Fourier coefficients into an interpolated freq domain function

    Hi, I am interested in understanding the relationships between Fourier series and Fourier transform better. My goal is 1) Start with a set of ordered numbers representing Fourier coefficients. I chose to create 70 coefficients and set the first 30 to the value 1 and the remaining to zero. 2)...
  8. A

    Energy gaps for quasi-free electrons in a 2D lattice

    Hi! Situation: quasi-free electron in a 2D lattice, considering atomic potential V(r) = exp{-|r|/b} (r is the distance from the atom) I'm trying to compute the first five energy gaps at point (10), firstly I don't understand the meaning of calculated 5 energy gaps at one point and usually we...
  9. M

    MHB Calculate the integral using the Fourier coefficients

    Hey! :o A real periodic signal with period $T_0=2$ has the Fourier coefficients $$X_k=\left [2/3, \ 1/3e^{j\pi/4}, \ 1/3e^{-i\pi/3}, \ 1/4e^{j\pi/12}, \ e^{-j\pi/8}\right ]$$ for $k=0,1,2,3,4$. I want to calculate $\int_0^{T_0}x^2(t)\, dt$. I have done the following: It holds that...
  10. Jason-Li

    Comp Sci Fourier analysis & determination of Fourier Series

    ANY AND ALL HELP IS GREATLY APPRECIATED :smile: I have found old posts for this question however after reading through them several times I am having a hard time knowing where to start. I am happy with the sketch that the function is correctly drawn and is neither odd nor even. It's title is...
  11. M

    Engineering Reconstruct a signal by determining the N Fourier Coefficients

    %My code: %Type of signal: square T = 40; %Period of the signal [s] F=1/T; % fr D = 23; % length of signal(duration) dt=(D/T)*100; N = 50; %Number of coefficientsw0 = 2*pi/T; %signal pulset1= 0:0.002:T; % original signal sampling x1 = square((2*pi*F)*(t1),dt);%initial square signal t2=...
  12. N

    I Separation of variables - Getting the Fourier coefficients

    Hey there! I am current taking an introductory course on PDE's, and our professor hasn't really emphasized last part of solutions from separation of variables. Now its not strictly going to be on the exam, however I remember doing this with ease a few years back, but for some reason now I...
  13. Ineedhelp0

    I Parseval's theorem and Fourier Transform proof

    Given a function F(t) $$ F(t) = \int_{-\infty}^{\infty} C(\omega)cos(\omega t) d \omega + \int_{-\infty}^{\infty} S(\omega)sin(\omega t) d \omega $$ I am looking for a proof of the following: $$ \int_{-\infty}^{\infty} F^{2}(t) dt= 2\pi\int_{-\infty}^{\infty} (C^{2}(\omega) + S^{2}(\omega)) d...
  14. M

    Understanding Fourier Coefficients in the Fourier Transform of a Function

    Hi PF! Unsure how to begin. Fourier transform of ##f## I've given as an equation. I'm not sure what is meant by Fourier coefficients. Fourier coefficients of what?
  15. Morbidly_Green

    Finding the Sine Representation of an Odd Function Using Fourier Series

    I am attempting to find the sine representation of cos 2x by letting $$f(x) = \cos2x, x>0$$ and $$-\cos2x, x<0$$ Which is an odd function. Hence using $$b_n = \dfrac{2}{l} \int^\pi _0 f(x) \sin(\dfrac{n\pi x}{l})dx$$ I obtain $$b_n = \dfrac{2n}{\pi} \left( \dfrac{(-1)^n - 1}{4-n^2} \right)$$...
  16. K

    How to Find Fourier Coefficients for a Given Function

    Hello, I need help with question #2 c) from the following link (already LateX-formatted so I save some time...): https://wiki.math.ntnu.no/_media/tma4135/2017h/tma4135_exo1_us29ngb.pdf I do understand that the a0 for both expressions must be the same, but what about an and bn? I don't...
  17. Matt Chu

    How Do You Prove the Fourier Transform Definition Using Integral Evaluation?

    Homework Statement Given a continuous non-periodic function, its Fourier transform is defined as: $$f(x) = \int_{-\infty}^\infty c(k) e^{ikx} dk, \ \ \ \ \ \ \ \ \ \ \ \ \ c(k) = \frac{1}{2\pi} \int_{-\infty}^\infty f(x) e^{-ikx} dx$$ The problem is proving this is true by evaluating the...
  18. D

    Finding the fourier spectrum of a function

    Homework Statement Find the Fourier spectrum ##C_k## of the following function and draw it's graph: Homework Equations 3. The Attempt at a Solution [/B] I know that the complex Fourier coefficient of a rectangular impulse ##U## on an interval ##[-\frac{\tau}{2}, \frac{\tau}{2}]## is ##C_k =...
  19. binbagsss

    Hecke Operators and Eigenfunctions, Fourier coefficients

    Homework Statement Consider the action of ##T_2## acting on ##M_k(\Gamma_{0}(N)) ##, and show that ##\theta^4(n)+16F ## and ##F(t)## are both eigenfunctions. Functions are given by: Homework Equations For the Hecke Operators ##T_p## acting on ##M_k(\Gamma_{0}(N)) ##, the Hecke conguence...
  20. bananabandana

    I Negative and Positive energy modes of KG equation

    If we have the normal KG scalar field expansion: $$ \hat{\phi}(x^{\mu}) = \int \frac{d^{3}p}{(2\pi)^{3}\omega(\mathbf{p})} \big( \hat{a}(p)e^{-ip_{\mu}x^{\mu}}+\hat{a}^{\dagger}(p)e^{ip_{\mu}x^{\mu}} \big) $$ With ## \omega(\mathbf{p}) = \sqrt{|\mathbf{p}^{2}|+m^{2}}## Then why do we associate...
  21. G

    Find Fourier coefficients - M. Chester text

    Homework Statement I am self studying an introductory quantum physics text by Marvin Chester Primer of Quantum Mechanics. I am stumped at a problem (1.10) on page 11. We are given f(x) = \sqrt{ \frac{8}{3L} } cos^2 \left ( \frac {\pi}{L} x \right ) and asked to find its Fourier...
  22. binbagsss

    Relationships between Fourier coefficients

    Homework Statement I have ## f(t) = \sum\limits^{\infty}_0 a_{n} e^{2 \pi i n t} ## [1] and ## g(t) = \sum\limits^{\infty}_0 b_{n} e^{ \pi i n t} ## [2] I want to show that ##b_n = a _{2n} ## Homework Equations see above. The Attempt at a Solution [/B] So obviously you want to use the...
  23. FeDeX_LaTeX

    I Discrete Convolution of Continuous Fourier Coefficients

    Suppose that we have a 2\pi-periodic, integrable function f: \mathbb{R} \rightarrow \mathbb{R}, whose continuous Fourier coefficients \hat{f} are known. The convolution theorem tells us that: $$\displaystyle \widehat{{f^2}} = \widehat{f \cdot f} = \hat{f} \ast \hat{f},$$ where \ast denotes the...
  24. D

    Fourier coefficients - dirichlet problem for annulus

    Hello 1. Homework Statement Find the Fourrier coefficients in the annulus problem of the text. uxx+uyy=0 in 0<a²<x²+y²<b² u=g(θ) for x²+y²=a² u=h(θ) for x²+y²=b² Homework Equations The solution is The Attempt at a Solution I have the solutions but when I solved it for...
  25. RJLiberator

    Understanding Fourier Coefficients using PDE

    Homework Statement In my PDE course we have a homework question stating the following: Let ϑ(x) = x in the interval [-pi, pi ]. Find its Fourier Coefficients. Homework Equations From my notes on this type of question: a_o = 2c_o = 1/pi * integral from -pi to pi [f(x) dx] a_n = c_n + c_(-n)...
  26. I

    Convolutions, Fourier coefficients

    Homework Statement When ##f## and ##g## are ##2\pi##-periodic Riemann integrable functions define their convolution by ##(f*g)(x) = \frac{1}{2\pi} \int_0^{2\pi} f(y)g(x-y)dy## Denoting Fourier coefficients by ##c_n(f)## show that ##c_n(f * g) = c_n(f)c_n(g)##. Homework Equations ##c_n =...
  27. I

    How was this equation simplified? (Fourier coefficients)

    Homework Statement http://puu.sh/gGhdb.jpg Solution:[/B] http://puu.sh/gGh3E.jpg Homework EquationsThe Attempt at a Solution How did they get that solution for the Fourier coefficient? When I evaluate the integral I can only seem to get it to: (1/-jk2π)[2*exp(-jkπt)-exp(-jk2πt)-1]
  28. \Theta

    Finding the Fourier Coefficients for mechanics homework

    Homework Statement Find the Fourier Coefficients for the triangular wave equation shown in this picture: Homework Equations ##f(t)= a_0 + \sum_{n=1}^\infty a_{n}cos(n{\omega}t) + \sum_{n=1}^\infty b_{n}sin(n{\omega}t)## ##a_0 = \frac{1}{\tau}\int_{-\tau/2}^{\tau/2} f(t) \, dt## ## \omega =...
  29. moriheru

    Fourier coefficients in string theory

    If a function f'(u) has Fourier coefficients anμ and bnμ, by integration one can make new coefficients Anμ ,Bnμ which include constants of integration. My question how can I verify that : Anμcos nτ + Bnμ sin nτ= -i/2 ((Bnμ) -Anμi) einτ- (Bnμ-iAnμ) e-inτ I assume this is the complex form of...
  30. DavideGenoa

    Identical Fourier coefficients of continuous ##f,\varphi\Rightarrow f=\varphi##

    Hi, friends! Let ##f:[a,b]\to\mathbb{C}## be an http://librarum.org/book/10022/173 periodic function and let its derivative be Lebesgue square-integrable ##f'\in L^2[a,b]##. I have read a proof (p. 413 here) by Kolmogorov and Fomin of the fact that its Fourier series uniformly converges to a...
  31. M

    How do we find the Fourier coefficient for a cosine term in a Fourier series?

    hey pf! okay, so if you've studied PDEs you know the value of a Fourier series, and the difficulty of determining a Fourier coefficient. my question relates to finding this coefficient. briefly, i'll define a Fourier series as f(x)=\sum_{n=0}^{\infty} A_n\cos\frac{n\pi x}{L}+B_n\sin\frac{n\pi...
  32. D

    Solving Real Valued Fourier Coefficients

    Homework Statement Let ##f## be a ##2\pi## periodic function. Let ##\hat{f}(n)## be the Fourier coefficient of ##f## defined by $$ \hat{f}(n)=\frac{1}{2\pi}\int_{a}^{b}f(x)e^{-inx}dx. $$ for ##n\in\mathbb{N}##. If ##\overline{\hat{f}(n)}=\hat{f}(-n)## show that ##f## is real valued. The...
  33. D

    Fourier coefficients and partial sum of Fejer

    Homework Statement f(t) a continuously differentiable function twice over the circle T1 cr its Fourier coefficients and σn(f,t) partial sum of Fejer. a.Demonstrate that http://imageshack.us/a/img94/5992/ds35.png b. Consider k as -n≤k≤n , using cr coefficients calculate...
  34. jbrussell93

    Can You Determine \(a_0\) in Fourier Series by Plugging \(n=0\) into \(a_n\)?

    I'm having trouble finding a definite answer to this question: When finding the Fourier series of a function is it always possible to find ##a_0## by first finding ##a_n## and just plugging in ##n=0##?
  35. M

    Sum of a serie involving Fourier coefficients

    Homework Statement Let \hat{u}_k the Fourier coefficients of 2-periodic function u(t)=t with t\in [0,2). Evaluate the sum of the serie: \sum_{k=-\infty}^{\infty}\hat{u}_k e^{\pi i k t} for t= 2 Ok, I think there is a trick that I don't know... \sum_{k=-\infty}^{\infty}\hat{u}_k...
  36. S

    Finding Fourier coefficients and Fourier Series

    Homework Statement Find the Fourier coefficients for the function *Should be a piecewise function, not sure how to write one in [itex /itex] tags* f(x) = |x|, |x| < 1, 1, 1≤|x|< 2; f(x+4) = f(x) and Find the Fourier series for f(x) = cos1/2\pi x, -1≤x<1...
  37. fluidistic

    Solving Fourier Coefficients for A_m & B_m

    Homework Statement I've reached a relation but then I need to obtain the coefficients ##A_m## and ##B_m##'s, those are the only unknowns. Here's the expression: ##\sum _{m=0}^\infty a^m [A_m \cos (m \theta ) + B_m \sin (m \theta )]=T_0\sin ^3 \theta##. Homework Equations Fourier...
  38. Z

    Wave Equation (Fourier Coefficients)

    For the wave equation I managed to get the coefficient of f: a1=2 and the coefficient of g: \frac{12pi}{2pi*2}=B2 Is these answers right, since my B2 does not match the answer I was given. Thank you
  39. D

    MHB Is $|f(\theta)|$ less than or equal to the sum of the absolute values of $A_n$?

    Supposing $f$ is bounded and $A_n$ is given by 1-8, prove that $\sup_n|A_n|$ is finite. $$ f(\theta) = \sum_{n = -\infty}^{\infty}A_ne^{in\theta} $$ Since $f$ is bounded, $|f| < M = |z|\in\mathbb{C}$. Since it could be $\mathbb{C}$, $M$ would be the modulus correct? We know that the modulus of...
  40. K

    Fourier Coefficients of Continuous functions are square summable.

    Homework Statement If C^1(\mathbb T) denotes the space of continuously differentiable functions on the circle and f \in C^1(\mathbb T) show that \sum_{n\in\mathbb Z} n^2 |\hat f(n)|^2 < \infty where \hat f(n) is the Fourier coefficient of f. The Attempt at a Solution Since f is...
  41. M

    MHB Proving Uniqueness of Fourier Coefficients for Continuous Periodic Functions

    Let $f:\mathbb R\to\mathbb R$ be a continuous function of period $2\pi.$ Prove that if $\displaystyle\int_0^{2\pi}f(x)\cos(nx)\,dx=0$ for $n=0,1,\ldots$ and $\displaystyle\int_0^{2\pi}f(x)\sin(nx)\,dx=0$ for $n=1,2,\ldots,$ then $f(x)=0$ for all $x\in\mathbb R.$ I know this has to do with the...
  42. G

    Compute Sin Coeffs for f(x)=e^(-x^2) on [0,2π]

    Compute the sine coefficients for f(x)=e^{-x^{2}} on the interval [0,2\pi]. Does this mean f(x+2\pi k)=f(x), k\in\mathbb{Z}? Can x\in[0,\infty)?
  43. J

    Fourier Coefficients of an Even Function

    Homework Statement For a given periodic function F(x), the coefficients An of its Fourier expansion can be found using the formulas (Form1) and (Form2). Consider a periodic square pulse and verify that the Fourier coefficients are as claimed: An =(\frac{2}{πn})sin(\frac{πan}{λ}) for n≥1 and...
  44. M

    Fourier Coefficients for asymmetric interval

    Homework Statement Fidn the Fourier expansion for f of period 2Pi that corresponds to y=x/3 on the interval [0,2Pi) Im just a little confused about if I am setting up the integration properly. The asymmetric interval is kind of confusing me here. The Attempt at a Solution a0 = 1/Pi ∫ x/3 dx =...
  45. N

    Basic question on fourier coefficients

    If f(x) has a period of 2*pi and |f(x)-f(y)| <= c*|x-y|^a where a and c are positive constants, why are are n-th Fourier coefficients <= c*(pi/n)^a ? Help or hints would be appreciated.
  46. W

    Finding Fourier Coefficients for a Function's Waveform

    Homework Statement For an even function, the Fourier series takes the form ^{\infty}_{n=0}\Sigma A_n cos(\frac{2\pi n x}{\lambda}) where \lambda is the wavelength of the function. In this problem you will see how to find the Fourier coefficients A_n. a.) Prove that A_0 =...
  47. D

    Calculate RMS Voltage & Fourier Coefficients for 10Ω Load & 240V 50Hz

    A single-phase ac voltage regulator has a resistive load of R= 10 ohms, and the input voltage is Vs = 240 V, 50 Hz. The delay angle of each of the Thyristors is ɑ = Π/2. Determine: (a) The rms value of the output voltage. (b) The Fourier coefficients of the fundamental, 11th and 13th current...
  48. I

    Value of the infinite sum of fourier coefficients?

    Homework Statement Calculate the exact value of the sum from minus infinity to infinity of Ck.Homework Equations Ck =...
  49. S

    Fourier coefficients in a discrete curve

    I'm struggling in an application of Fourier transform.here is my problem: a series of points from experimental data plotted as a cruve. I'm planning to do a Fourier transform to see how smooth the curve is? my question is: is it possible/useful to calculate the Fourier coefficients? if yes, how...
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