Questions tagged [fourier-transform]
The fourier-transform tag has no usage guidance.
54
questions
1
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1answer
112 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
23 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, ...
0
votes
1answer
99 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
31 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: $$f(x) = \mathrm{exp}\left[-\left(\frac{x-5}{2}...
2
votes
1answer
72 views
Incorporating a potential barrier in a wave-packet simulation (Fourier Transform method)
I'm trying to simulate the scattering of a wave-packet at a potential barrier in Python. I'm using a Fourier Transform method (not sure if its the same as the Split-Step method), where I apply Fourier ...
3
votes
2answers
113 views
DFT of $g(\omega) \exp(i C \omega^2)$. How to do it ,if uniform sampling requires too much memory?
I have a following problem : I want to transform a function $g(\omega) \exp(i C \omega^2)$. $g(\omega)$ is real and limited. It changes slowly compered to $\exp(i C \omega^2)$. I have a black box that ...
1
vote
0answers
46 views
The sign of Schrodinger equation
I have a question for the format of Schrodinger equation
$$\psi(x,t) = \int_0^\infty c_n e^{-iE_nt/\hbar} \psi_n(x)$$
Why do we have $i$ instead of $-i$?
2
votes
1answer
49 views
FFT of “implicitly” uniform data
I am trying to take a Fourier transform of a density field estimated from mock galaxy survey catalogs.
Basically, you start with a list of galaxy positions, then you bin these positions over some ...
5
votes
0answers
63 views
Discrete sine and cosine transform for mixed derivatives
Using sine and cosine transforms to solve Poisson's equation with Dirichlet boundary conditions seem quite standard nowadays (see, e.g., here or Table 2 in this paper). In the case of Poisson's ...
1
vote
0answers
32 views
What is the inverse Laplace transform algorithm that is most accurate given the fewest frequencies considered?
Based on your empirical knowledge.
This paper suggests a nonlinearly accelerated Fourier series approach, such as the one proposed here, but I have one constraint: we should be able to express the ...
0
votes
2answers
100 views
How to solve diffusion equation in Fourier space when mobility is not constant
I want to solve non-classic diffusion equation in Fourier space. The equation is $$∂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
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1answer
240 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.
...
1
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0answers
88 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
vote
1answer
61 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
74 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
vote
0answers
51 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
140 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
206 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 ...
0
votes
1answer
76 views
0 Hz (quite sharp) peak in FFT and division by 0
In a previous question, link, I asked about how I could most effectively do a Fourier Transform of a radial function given at certain values and which we knew the asymptotical behaviour of. The ...
3
votes
1answer
1k views
Fast(er) computation of dot product of two convolutions?
Let $a,b,c,d\in\mathbb{R}^n$. Is it possible to compute
$$\langle a*b,c*d\rangle$$
faster than 6 FFTs?
I can do it with 6 FFTs by doing normal convolutions, 3 FFTs each.
In my application I know $b, ...
3
votes
2answers
284 views
How to calculate efficiently and accurately the Fourier transform of a radial function in Fortran
As my question states, I want to calculate the Fourier transform $F(q)$ of a radial function $f(r)$ (defined on $[0,\infty)$ and which decays like an exponential $\exp(-Ar+b)$ at large $r$) as ...
0
votes
2answers
249 views
Generating initial velocity field
I am trying to generate initial velocity field which satisfies incompressible flows condition. To formulate this I am using Rogallo's procedure formulation which is given by
$$
\tilde{u}(k) = \frac{\...
1
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0answers
69 views
Spherical Harmonics: band-limited representations of a vector field on a sphere
I have used pyShtools in the past to expand scalar functions to spherical harmonics and to synthesize band-limited representations of them. However, I am not too sure how to achieve this for a vector ...
3
votes
1answer
212 views
MPI support for discrete Fourier transform (DFT) in Python
I am looking for a discrete Fourier transform (DFT) library that can be run with MPI on Python.
Usually, in other languages (C, Fortran) FFTW is used. There's a Python wrapper for FFTW called pyFFTW, ...
2
votes
1answer
143 views
Approximating improper integral with FFT
In d'Halluin et al. (2005) (http://imajna.oxfordjournals.org/content/25/1/87.short) the authors claim that the correlation integral
$$ I(x) = \int_{-\infty}^{\infty} V(x+y) f(y) dy $$
can be ...
2
votes
0answers
184 views
Solving a 3D (almost radial) convolution with FFT
I have a 3D integral that is almost a radial convolution of the form
$$ \int d^{3}k'h(\mathbf{k'})g(|\mathbf{k-k'}|) $$
and I am looking for a fast and efficient algorithm (e.g. FFT) to solve it ...
2
votes
3answers
192 views
Amplitude at a given frequency in a wide band signal
Could anyone suggest the most computationally efficient method for finding amplitude at a given frequency having a noisy wide band signal.
To be more specific about a task. I have some physical ...
4
votes
2answers
9k views
C++ libraries for Fast Fourier Transform in high precision
I am looking for a C++ library for Fast Fourier Transform (FFT) in high precision (e.g., using high precision real data types similar to mpfr_t in MPFR or ...
2
votes
1answer
269 views
How do I avoid divide-by-zero when solving the Poisson equation with Fourier transforms?
I wanted to try to implement part of the method in the following article using Fourier transforms.
http://www.shodor.org/media/content/jocse/student_submissions/nocito2010/nocito2010_pdf
Right now I ...
2
votes
1answer
659 views
Numerically computing the advection equation
I am trying to write a program to compute the advection equation.
$$u_t +u_x = 0$$
I use the spectral method for the spatial derivative $u_x$ and the leapfrog method for the time derivative $u_t$.
...
4
votes
1answer
514 views
Help with Fourier beam propagation method
I am working on implementing the Fourier beam propagation method in C++. I am really more of a programmer than a physicist but I think I have a good understanding of what I am trying to do. Here is ...
3
votes
0answers
273 views
How to obtain values in physical space for a given spectrum?
My question falls under purview of turbulent flows. I want to add an initial perturbation, for which I have a given energy spectrum (say$ E(k)=ak^4e^{-bk^2} $). The steps involved in getting these ...
1
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0answers
448 views
3D plot from Fourier coefficients in Matlab
I would like to construct a surface on a 2D mesh, given a set of Fourier coefficients for the original function $V(x,y)$.
Given some $\mathbb{K}\subset \mathbb{Z}^2$ I have a partial double sum of a ...
-2
votes
1answer
588 views
Understanding and implementing the Fast Fourier Transform
I need to write the fast fourier transform in C++ and I am referring to this formula from wikipedia:
But for some reason I am not getting the correct output when I simply enter (1,1) (1,1) (1,1) (1,...
3
votes
1answer
54 views
Hankel transform with high accuracy
Short version
I'm computing the zero-order Hankel transform of a function $f(r)$,
$$F(k) = \int_0^\infty f(r)J_0(kr)r\,\mathrm{d}r$$
I know $f(r_n)$ at selected $r_n$ but it is impractical to compute ...
2
votes
0answers
44 views
Constructing 2 fold oversampled cosine basis in MATLAB
So I'm trying to construct a 2 fold oversampled cosine basis in MATLAB. I know how to construct the basis as a square matrix using the following command:
...
3
votes
1answer
159 views
How to get Fourier transform of 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 ...
3
votes
1answer
405 views
My calculated laser pulse duration is too large. Where am I wrong?
I am currently writing a small Python script to estimate the pulse duration from the optical spectrum.
At the end, the idea is to observe the effects of the spectral phase on the pulse duration and ...
1
vote
1answer
428 views
FFT on non-orthogonal lattice ( for computing convolutions and solving PDEs )
I saw many examples of application of FFT for computation of convolutions and solving PDEs ( like Poisson equation ). It is very strightforward and efficient if I work with rectangular (orthogonal) ...
3
votes
1answer
252 views
precision loss in non-trigonometric, periodic functions using FFTW and NaNs after marching forward in time (Fortran)
I have developed a pseudospectral solver of the Navier-Stokes equations using FFTW. I tested my formulation of right hand sides (RHS) of the NS equations against standard trigonometric functions (...
2
votes
0answers
336 views
Fast Forward Laplace transform
There are examples for fast numerical inversion of the Laplace transforms. For example here:
http://www.mathworks.com/matlabcentral/fileexchange/32824-numerical-inversion-of-laplace-transforms-in-...
0
votes
1answer
46 views
Particle mesh Ewald: recommended splitting into short and long range
Particle mesh Ewald method for acceleration of solving pairwise interaction by long range forces (electrostatic, gravitational ... ) seem to be very general and easy to implement. The basic principle ...
1
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0answers
153 views
Am I using the incorrect implementation of the fast Chebyshev transform?
I was told that the fast Chebyshev transform has superior spectral convergence, but I am unable to verify its rumored convergence. I was given plots of its spectral convergence, where the signal's ...
2
votes
1answer
522 views
FFT convolution vs direct convolution
Recently I tried to compare results for 1D direct convolution and convolution via FFT. I expected to get absolutely the same result, however I faced with a problem that results are different, ...
1
vote
1answer
72 views
Coordinate transformations and analytic form of non-uniformly gridded fourier transform
My question concerns coordinate maps and non-equally spaced fourier transforms.
I have dependent variables $(X(\xi),Y(\xi))$, where $\xi\in(0,2\pi)$. In general, $Y$ is assumed even and expanded as a ...
0
votes
1answer
645 views
Scaling factor of the inverse Fourier Transform (for convolution purposes)
I have a certain 2-D function. More properly, I have not the function itself, but the matrices $[X,Y,Z]$, where $X,Y$ are $1\times n$, and $Z$ is $n \times n$.
Now, I want to calculate a a new matrix,...
0
votes
1answer
197 views
Diagonalize a circulant-plus-rank-one matrix
Suppose $A=uv^T$ where $u$ and $v$ are non-zero column vectors in $\mathbb{R}^n, n\geq 3$. $\lambda=0$ is an eigenvalue of $A$ since $A$ is not of full rank. $\lambda=v^Tu$ is also an eigenvalue of $A$...
4
votes
1answer
470 views
Two approaches to solving diffusion equation in Fourier space
I want to numerically solve the diffusion equation $\partial_t u = D \partial_x^2 u$ in Fourier space, and can think of multiple ways to do it.
Setup
Option 1
Differentiating $u$ twice in Fourier ...
2
votes
1answer
568 views
Calculating high-order derivatives using FFT in Matlab
I'm trying to calculate the derivative of a function using FFT in Matlab. I've coded the following function to calculate it:
...
