# Questions tagged [fourier-transform]

For questions about Fourier transforms, how they are used, and implementation details.

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### Derivative using torch.fft oscilates on the boundary

I was trying to use the torch.fft to compute derivatives. The issue is that even for a simple example ($f = \sin(x)$), I have weird oscillations on the boundaries. ...
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### Why not use the convolution theorem for explicit timestepping?

Consider the advection equation $$\frac{\partial C}{\partial t} + u\frac{\partial C}{\partial x} + v\frac{\partial C}{\partial y} = 0$$ I want to do a forward time, center ...
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### Bounds condition for IFT to obtain a $1/f$ time-series

I am coding a function to obtain a randomized time-series from a given $\frac{1}{f}$ law. The randomization is obtained by introducing a random phase in the function. I experience a problem in the ...
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1 vote
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### How do people deal with resized grid steps while numerically integrating using discrete Fourier Transform?

I am trying to simulate light propagation on python using FFT following the Fresnel diffraction equation given on Wikipedia: The problem with this is that the output matrix from the DFT would be ...
1 vote
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### How do you correctly implement Scipy's FFT procedures to produce a low-pass filter - image processing

I'm following this low-pass filter example in the text "Image Operators: Image Processing in Python 1st Edition" by Jason M. Kinser, but can't seem to duplicate their results. The text's ...
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### 3D Cooley-Tukey FFT

To compute the $N$-point DFT $$X[k] = \sum_{n=0}^{N-1} x[n] W_N^{kn}$$ where $N = N_1 N_2$, we can write the indices as $n = N_2 n_1 + n_2$ and $k = k_1 + N_1 k_2$, (effectively packing the data ...
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1 vote
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### 2D DFT for lower frequencies only; is there something significantly faster than numpy.fft.fft2 (throwing away high frequencies)?

I do a lot of 2D discrete FFT in python using np.fft.fftshift(np.fft.fft2(y)), then throw away 90% or more of the array, keeping only the central low-frequency area....
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1 vote
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1 vote
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### 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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1 vote
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