Fourier Definition and 1000 Threads

In mathematics, a Fourier transform (FT) is a mathematical transform that decomposes functions depending on space or time into functions depending on spatial or temporal frequency, such as the expression of a musical chord in terms of the volumes and frequencies of its constituent notes. The term Fourier transform refers to both the frequency domain representation and the mathematical operation that associates the frequency domain representation to a function of space or time.
The Fourier transform of a function of time is a complex-valued function of frequency, whose magnitude (absolute value) represents the amount of that frequency present in the original function, and whose argument is the phase offset of the basic sinusoid in that frequency. The Fourier transform is not limited to functions of time, but the domain of the original function is commonly referred to as the time domain. There is also an inverse Fourier transform that mathematically synthesizes the original function from its frequency domain representation, as proven by the Fourier inversion theorem.

Linear operations performed in one domain (time or frequency) have corresponding operations in the other domain, which are sometimes easier to perform. The operation of differentiation in the time domain corresponds to multiplication by the frequency, so some differential equations are easier to analyze in the frequency domain. Also, convolution in the time domain corresponds to ordinary multiplication in the frequency domain (see Convolution theorem). After performing the desired operations, transformation of the result can be made back to the time domain. Harmonic analysis is the systematic study of the relationship between the frequency and time domains, including the kinds of functions or operations that are "simpler" in one or the other, and has deep connections to many areas of modern mathematics.
Functions that are localized in the time domain have Fourier transforms that are spread out across the frequency domain and vice versa, a phenomenon known as the uncertainty principle. The critical case for this principle is the Gaussian function, of substantial importance in probability theory and statistics as well as in the study of physical phenomena exhibiting normal distribution (e.g., diffusion). The Fourier transform of a Gaussian function is another Gaussian function. Joseph Fourier introduced the transform in his study of heat transfer, where Gaussian functions appear as solutions of the heat equation.
The Fourier transform can be formally defined as an improper Riemann integral, making it an integral transform, although this definition is not suitable for many applications requiring a more sophisticated integration theory. For example, many relatively simple applications use the Dirac delta function, which can be treated formally as if it were a function, but the justification requires a mathematically more sophisticated viewpoint. The Fourier transform can also be generalized to functions of several variables on Euclidean space, sending a function of 3-dimensional 'position space' to a function of 3-dimensional momentum (or a function of space and time to a function of 4-momentum). This idea makes the spatial Fourier transform very natural in the study of waves, as well as in quantum mechanics, where it is important to be able to represent wave solutions as functions of either position or momentum and sometimes both. In general, functions to which Fourier methods are applicable are complex-valued, and possibly vector-valued. Still further generalization is possible to functions on groups, which, besides the original Fourier transform on R or Rn (viewed as groups under addition), notably includes the discrete-time Fourier transform (DTFT, group = Z), the discrete Fourier transform (DFT, group = Z mod N) and the Fourier series or circular Fourier transform (group = S1, the unit circle ≈ closed finite interval with endpoints identified). The latter is routinely employed to handle periodic functions. The fast Fourier transform (FFT) is an algorithm for computing the DFT.

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  1. Fosheimdet

    A Fourier transform equation question

    In my QFT homework I was asked to prove that $$\int d^3x \int \frac{d^3k}{(2\pi)^3} e^{i\mathbf{k} \cdot (\mathbf{x} - \mathbf{y})} k_j f(\mathbf{x}) = i \frac{df}{dx_j}(\mathbf{y}) $$ Using ##\frac{\partial e^{i\mathbf{k} \cdot (\mathbf{x} - \mathbf{y})}}{\partial x^j} = i k_j e^{i\mathbf{k}...
  2. G

    MHB Fourier and inverse fourier transform

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  3. 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=...
  4. N

    I Separation of variables - Getting the Fourier coefficients

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  5. S

    Compute the given Fourier transform by using the given tables

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  6. Samuelcomeau

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  7. A

    Units of Fourier Transform (CTFT) vs spectral density

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  8. Ineedhelp0

    I Parseval's theorem and Fourier Transform proof

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  9. rude man

    Compact summary of Fourier series equations

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  10. CBuphyx

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  11. arcTomato

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  12. J

    Fourier transform fallacy? (Optics)

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  13. arcTomato

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  14. arcTomato

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  15. Haynes Kwon

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  16. P

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  17. A

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  18. M

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  19. B

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  20. M

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  21. PainterGuy

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  22. Phys pilot

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  23. Haorong Wu

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  24. Wrichik Basu

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  25. E

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  26. K

    I Understanding Waves: The Importance of Fourier Analysis in Undergraduate Physics

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  27. PainterGuy

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    Hi, A function which could be represented using Fourier series should be periodic and bounded. I'd say that the function should also integrate to zero over its period ignoring the DC component. For many functions area from -π to 0 cancels out the area from 0 to π. For example, Fourier series...
  28. M

    Understanding Fourier Coefficients in the Fourier Transform of a Function

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  29. E

    Fourier series for a series of functions

    ## ## Well I start with equation 1): ## e^{b\theta }=\frac{sinh(b\pi )}{\pi }\sum_{-\infty }^{\infty }\frac{(-1)^{n}}{b-in}e^{in\theta } ## If ## \theta =0 ## ##e^{b(0)}=\frac{sinh(b\pi )}{\pi }\sum_{-\infty }^{\infty }\frac{(-1)^{n}}{b-in}e^{in(0) }## ##1=\frac{sinh(b\pi )}{\pi...
  30. redtree

    I Covariance of Fourier conjugates for Gaussian distributions

    Given two variables ##x## and ##k##, the covariance between the variables is as follows, where ##E## denotes the expected value: \begin{equation} \begin{split} COV(x,k)&= E[x k]-E[x]E[k] \end{split} \end{equation} If ##x## and ##k## are Foureir conjugates and ##f(x)## and ##\hat{f}(k)## are...
  31. J

    I Why Is My Fourier Transform of a Gaussian Incorrect?

    Attached is a personal problem that I spent last night working on for about 2 hours and something is going wrong, I just can not figure it out what. The answer by the big X is what I wound up with but it's obviously not correct. Could someone please guide me through solving this? The starting...
  32. L

    B How to calculate the Fourier transform of sin(a*t)*exp(-t/b) ?

    Hi all, I need to calculate Fourier transform of the following function: sin(a*t)*exp(-t/b), where 'a' and 'b' are constants. I used WolphramAlpha site to find the solution, it gave the result that you can see following the link...
  33. Phys pilot

    Getting the coefficients of inhomogeneous PDE using Fourier method

    Hello, I posted the same in the partial differential equations section but I'm not getting responses and maybe this section is better for help with homework. I have to solve this problem: $$u_t=ku_{xx}+h \; \;\; \; \; 0<x<1 \; \; \,\; t>0$$ $$u(x,0)=u_0(1-\cos{\pi x}) \; \;\; \; \; 0\leq x \leq...
  34. Phys pilot

    I Problem getting the coefficients of a non-homogeneous PDE using the Fourier method

    Hello, I have to solve this problem: $$u_t=ku_{xx}+h \; \;\; \; \; 0<x<1 \; \; \,\; t>0$$ $$u(x,0)=u_0(1-\cos{\pi x}) \; \;\; \; \; 0\leq x \leq 1$$ $$u(0,t)=0 \; \;\; \; \; u(1,t)=2u_0 \; \;\; \; \; t\geq0$$ So I know that I can split the solution in two (I don't know the reason. I would...
  35. Morbidly_Green

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  36. K

    How can I use spherical coordinates to simplify the Fourier transform equation?

    By applying the Fourier transform equation, and expanding the dot product, I get a sum of terms of the form: $$V(k)=\sigma_1^x\nabla_1^x\sigma_2^y\nabla_2^y\frac{1}{|\vec{r_2}-\vec{r_1}|}e^{-m|\vec{r_2}-\vec{r_1}|}e^{-ik(r_2-r_1)} =...
  37. G

    I Symmetry of an exponential result from a Fourier transform

    I used a matrix to calculate the Fourier transform of a lorentzian and it did generate a decaying exponential but that was followed by the mirror image of the exponential going up. I am referring to the real part of the exponential. If I use an fft instead I also see this. Shouldn't the...
  38. Safder Aree

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  39. 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...
  40. NatanijelVasic

    I Fourier Transform of a Probability Distribution

    Hi all :oldbiggrin: Yesterday I was thinking about the central limit theorem, and in doing so, I reached a conclusion that I found surprising. It could just be that my arguments are wrong, but this was my process: 1. First, define a continuous probability distribution X. 2. Define a new...
  41. S

    MHB Solving wave equation using Fourier Transform

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  42. M

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  43. K

    Why Does the Fourier Series of |sin(x)| Treat n=1 Differently?

    Homework Statement Hello, i am trying to do find the Fourier series of abs(sin(x)), but have some problems. As the function is even, bn = 0. I have calculated a0, and I am now working on calculating an. However, when looking at the solution manual, they have set up one calculation for n > 1...
  44. merlyn

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    Hi all. Could someone work out for me how equation 21 in attachment left side becomes right side. Please show in detail if you could. It's for exponential Fourier series. Drforbin thank you
  45. redtree

    I Is There a Generalized Fourier Transform for All Manifolds?

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  46. tanaygupta2000

    Proof of Parseval's Identity for a Fourier Sine/Cosine transform

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  47. Behrouz

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  48. Behrouz

    I Fourier transform, same frequencies, different amplitudes

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  49. J

    B Why the Fourier series doesn't work to solve any differential equation?

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  50. redtree

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