Questions tagged [fourier-transform]
The fourier-transform tag has no usage guidance.
86
questions
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1answer
70 views
Reason behind different outputs for Fast Fourier Transform in Numpy and Matlab
Here is the output of Numpy
np.fft.ifft([0, 4, 0, 0])
array([ 1.+0.j, 0.+1.j, -1.+0.j, 0.-1.j]) # may vary
Here is the output of Matlab
...
1
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0answers
37 views
Fourier transform in finite element
I have a finite element solver where I am using tetrahedral elements. I am solving for electric potentials and then calculate the current densities in each element, which are constant in each element. ...
1
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2answers
79 views
The derivative of a gauss function via FFT and IFFT in Python
I have a problem with computing a derivative of a Gauss function using FFT and IFFT from NumPy library. I use the fact that
$$
\begin{equation}
\frac{d}{dx}f(x) = \frac{1}{\sqrt{2\pi}}\int{ike^{ikx}\...
0
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0answers
20 views
Temporal/spectral conversion for large fields - best approach?
I am currently working on a more efficient implementation of a pulse propagation algorithm. The propagation is done in the spectral domain, but several evaluations (such as energy calculations) are ...
1
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1answer
294 views
Understanding why scipy.fft.fft (fast Fourier transform) doesn't work as expected
I write the following fast Fourier transform code into my Python notebook expecting to see a plot wherein there's a spike at $1/2\pi$ since that's the frequency of the sin function, but instead I get ...
0
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0answers
184 views
How to take convolution of two arrays in Python by using NumPy?
Generally, we know that if we have this relation between Fourier transforms of three functions in frequency domain as:
$$\mathfrak{F}\{\mathsf{P}(t)\} = \mathfrak{F}\{\mathsf{Z}(t)\}\mathfrak{F}\{\...
1
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0answers
58 views
Problems with simulation of a spatial filter 4f setup (Python)
I have a question about my code which computes numerically the output field of a 4f setup with a pinhole in the middle which works as a spatial filter.
My setup consists of two lenses with 50mm focal ...
0
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0answers
28 views
Scale of x-axis for Fourier transform
Consider a function $f(t)$ and its Fourier transform $F(\omega)$. The amplitude of the Fourier transform $F(\omega)$ depends on the frequency $\omega$ and thus also depends on the scale of the $t$-...
3
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2answers
420 views
Computing numeric derivative via FFT - SciPy
I wrote the following code to compute the approximate derivative of a function using FFT:
...
4
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1answer
146 views
How to define a dimensionless Objective function for determining how peaked a curve is?
I have attached 2 plots for FFT spectra. One is considered good and one is bad.
The good one is classified on the basis of how closely spaced the frequencies and the bad is based on how multiple ...
0
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0answers
43 views
Boundary conditions for triangular lattice in comsol
I am trying to simulate an infinite 2D triangular lattice in Comsol but I am confused that how should I use periodic floquet boundary conditions on the unit-cell.
A unit cell that I am using is given ...
1
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0answers
34 views
Solving a spectral system by reducing it to a single frequency - Feasability of approach?
I'm trying to solve the linear non-paraxial pulse propagation equation
$$\partial_z\hat{E}=ik_z\hat{E}$$
for a field defined as $$E=E(r, t, z)$$
The equation given above uses $$\hat{E}=\hat{E}(k_\perp,...
3
votes
1answer
100 views
Problem implementing convolutions exactly with the FFT
I'm trying to perform convolutions as defined mathematically $f \star g (\tau)= \int_{\mathcal{R}}f(t-\tau)g(t) dt$ in a numerical simulation. Hence, my signal is a sampling of points $f(x_i)$.
I ...
2
votes
1answer
103 views
Fourier spectral method for coordinate transformed heat equation
As the title said, I want to solve a coordinate transformed heat equation using fourier spectral method. In particular, I am interested in transforming an uniform grid into an adaptive non-uniform ...
1
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0answers
47 views
Advantage of fractional Fourier transform over multiscale wavelet?
What could be the arguments of using fractional Fourier transform instead of multiscale wavelet for data analysis ?
Optimization of the good time-frequency domain parameter? good in the sens of best ...
4
votes
1answer
144 views
Computation of triple nested loops as a convolution product?
I'm trying to compute efficiently the following
\begin{equation}
A_j = \sum_{l'=1}^{\infty}\sum_{k= 0}^{K-1} L_{l'}T_ke^{2\pi i \frac{k}{K}j}\epsilon_{l',k}
\end{equation}
for $j = 0,1, \ldots, K-2,K-...
1
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0answers
36 views
FFT convolution works only with certain domain length
in my quest to understand how I can use FFT to compute integrals (see my other question click, still no answer there), I came across the fact that a convolution of two functions can be calculated by ...
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35 views
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79 views
Calculate integrals using numpy.fft
Good evening,
I would like to understand why I do not get the correct result:
I assume that I know my function on discrete data points and expand it as a discrete Fourier transform: $\text{sin}(x)=\...
3
votes
2answers
237 views
Chebyshev differentiation via FFT with a domain [a,b]
I want to ask something about Chebyshev differentiation via FFT, which can be used to obtain with spectral accuracy the derivative of a smooth function. See for instance this code in python, which ...
2
votes
0answers
18 views
Normalising DFTs Correctly
I have been playing around with convolutions in scipy's signal package:
...
4
votes
1answer
454 views
How to use matlab command 'fft' to solve Ax=b arising from Poisson equation?
I want to ask a question about fast solver to the Poisson equation with Homogenous boundary conditions as follows:
$$-\Delta u = f.$$
After centered difference using $n+2$ equidistance points in all ...
0
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0answers
99 views
Smoothing FFT result
I am trying to calculate the spectrum of Bremmstrahlung, which involves calculating the Fourier transformed acceleration. I am solving a non-linear ODE to numerically calculate the acceleration in the ...
4
votes
1answer
147 views
Differences between Discrete Fourier Transform and Continuous Fourier Transform?
I am trying to visualize the time dependence of a free particle given an initial wave-function using Python and I just wanted to know if I could use the in built FFT implementation from NumPy to find ...
2
votes
0answers
29 views
Palmprint Identification - Why do we align the images before we use the Fourier Transform?
I am currently reading the paper PALMPRINT IDENTIFICATION BY FOURIER TRANSFORM by WENXIN LI, DAVID ZHANG and ZHUOQUN XU about identifying persons based on an image of their palm, one version of the ...
1
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1answer
63 views
Obtain velocity from imposed energy spectrum using the inverse FFT
I am trying to obtain the spatial representation of $u(x)$ (e.g. velocity) from its energy spectrum $E(k)=k^4\exp(-(k/k_0)^2)$, which is given in the frequency domain, provided $|u(k)|=\sqrt{2E(k)}$. ...
1
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0answers
51 views
Fusing callbacks with FFTs: an open-source GPU FFT implementation?
I'm using cuFFT to do some 2D FFTs on matrices of size 2048x2048 or larger. The FFTs are preceded and followed by various scaling operations.
These scaling operations are memory-bound, so they take ...
3
votes
1answer
121 views
Calculating the Convolution Using DFT (FFT)
I have the following convolution as part of a numerical simulation.
$$T(r)=\int \mathrm{d}^3r_2\, p(r_2)f(r_2)\alpha(r-r_2)\, .$$
My problem is that the analytical expressions for $f$ and $p$ do ...
1
vote
2answers
359 views
Numpy FFT gives me a pulse shorter than it should be. Not sure what I am doing wrong
I've created a code (Python, numpy) that defines an ultrashort laser pulse in the frequency domain (pulse duration should be 4 fs), but when I perform the Fourier Transform using DFT, my pulse in the ...
0
votes
0answers
26 views
Simulating Anderson model, have problem with momentum representation (MATLAB)
I want to change from real-space representation to momentum-space representation I have a Hamilton-operator (Anderson-model), and I calculated some kind of entropy of its eigenstates (this is working, ...
1
vote
1answer
999 views
Split-step Fourier method applied on Schrodinger equation
I'm trying to solve a Schrodinger equation of the form $i\frac{\partial}{\partial t}\psi=-\frac{\partial^2}{\partial x^2}\psi + (V(x)+\alpha|\psi|^2)\psi$ using the split-step Fourier method ...
0
votes
1answer
76 views
Specifying mesh spacing for DFT in numpy
I was testing the .fft package of numpy 1.16.1 in Python 3.7.2. In particular I was trying to verify that the transform resembles the analytical one for: $$f(x) = \mathrm{exp}\left[-\left(\frac{x-5}{2}...
2
votes
1answer
385 views
Incorporating a potential barrier in a wave-packet simulation (Fourier Transform method)
I'm trying to simulate the scattering of a wave-packet at a potential barrier in Python. I'm using a Fourier Transform method (not sure if its the same as the Split-Step method), where I apply Fourier ...
3
votes
2answers
126 views
DFT of $g(\omega) \exp(i C \omega^2)$. How to do it ,if uniform sampling requires too much memory?
I have a following problem : I want to transform a function $g(\omega) \exp(i C \omega^2)$. $g(\omega)$ is real and limited. It changes slowly compered to $\exp(i C \omega^2)$. I have a black box that ...
1
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0answers
47 views
The sign of Schrodinger equation
I have a question for the format of Schrodinger equation
$$\psi(x,t) = \int_0^\infty c_n e^{-iE_nt/\hbar} \psi_n(x)$$
Why do we have $i$ instead of $-i$?
2
votes
1answer
60 views
FFT of “implicitly” uniform data
I am trying to take a Fourier transform of a density field estimated from mock galaxy survey catalogs.
Basically, you start with a list of galaxy positions, then you bin these positions over some ...
5
votes
0answers
93 views
Discrete sine and cosine transform for mixed derivatives
Using sine and cosine transforms to solve Poisson's equation with Dirichlet boundary conditions seem quite standard nowadays (see, e.g., here or Table 2 in this paper). In the case of Poisson's ...
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0answers
33 views
What is the inverse Laplace transform algorithm that is most accurate given the fewest frequencies considered?
Based on your empirical knowledge.
This paper suggests a nonlinearly accelerated Fourier series approach, such as the one proposed here, but I have one constraint: we should be able to express the ...
0
votes
2answers
125 views
How to solve diffusion equation in Fourier space when mobility is not constant
I want to solve non-classic diffusion equation in Fourier space. The equation is $$∂c/∂t=-∇.J$$ Where $J$ is $$J= -M.∇μ$$ Where M is mobility. It depends on c and $\mu$ is $$ μ= g(c) - \nabla^2c $$ ...
0
votes
1answer
362 views
High precision Discrete Fourier Transform in c
I'm trying to do a high precision discrete fourier transform on a signal. To examine the precision, I use a gaussian function as the signal, because the fourier transform is also a gaussian function.
...
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0answers
98 views
Code for solving the heat equation on the semi-infinite rod
Cross posted in mathematica.SE.
Question : I want to test the solution which is given below is right by Matlab/Maple/Mathematica.
Please look the post in mathstackexhange
or
Please look below.
...
1
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1answer
80 views
fft with non uneven spacing between the value of the signal
I am trying to implement in C or C++ a solution for a fft and Ifft when the signal values are not obtained at a constant rate, making it having a desviation between the values and the periodic ones. I ...
1
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0answers
80 views
Can x-ray back-projection be converted to hard-field magnetic induction tomography?
This is a question about hard-field back-projection as used in x-ray tomography, applied magnetic induction tomography. Al-Zeibak and Saunders have shown that x-ray filtered backprojection can be ...
1
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0answers
53 views
Beam propagation over extremely short distances
I like to simulate the free-space propagation of an electric field (beam radius ~ 1-10 cm) over extremely short distances (~ 5-10 cm). The reason is that the field is focused by a lens with a very ...
0
votes
1answer
145 views
Hankel transform from paper works only for certain functions
I implemented an algorithm for solving the Hankel transformation based on a paper. My problem is: It works very good for the functions suggested in the paper (as test functions), it works pretty ok ...
1
vote
1answer
322 views
Power spectrum incorrectly yielding negative values
I have a real signal in time given by:
And I am simply trying to compute its power spectrum, which is the Fourier transform of the autocorrelation of the signal, and is also a purely real and ...
3
votes
2answers
1k views
Fourier transform by FFT : by using cubic splines to interpolate between data points, do we change the frequency content of the Fourier transform?
I have a data file with some points equally spaced. These represent some function. I have to calculate the Fourier transform of this set of points. The thing is, I'm tempted to take a cubic spline of ...
