Diophantine equation Updated 2025-07-16
Sometimes systems of Diophantine equations are considered.
Problems generally involve finding integer solutions to the equations, notably determining if any solution exists, and if infinitely solutions exist.
The general problem is known to be undecidable: Hilbert's tenth problem.
The Pythagorean triples, and its generalization Fermat's last theorem, are the quintessential examples.
Dipole antenna Updated 2025-07-16
Radio Wave Properties: Electric and Magnetic Dipole Antennae by Harvard Natural Sciences Lecture Demonstrations (2020)
Source. The dude lights bulbs on an antenna made of a single piece of copper, powered with EM radiation. Amazing.
Dirac equation vs quantum electrodynamics Updated 2025-07-16
TODO: in high level terms, why is QED more general than just solving the Dirac equation, and therefore explaining quantum electrodynamics experiments?
Also, is it just a bunch of differential equation (like the Dirac equation itself), or does it have some other more complicated mathematical formulation, as seems to be the case? Why do we need something more complicated than
The main high level insight seems to be that The Dirac equation does not work for more than one electron.
Bibliography:
- physics.stackexchange.com/questions/101307/dirac-equation-in-qft-vs-relativistic-qm
- physics.stackexchange.com/questions/44188/what-is-the-relativistic-particle-in-a-box/44309#44309 says:
- physics.stackexchange.com/questions/64206/why-is-the-dirac-equation-not-used-for-calculations
- www.physicsforums.com/threads/is-diracs-equation-still-useful-after-qed-is-developed.663994/
Dirac Lagrangian Updated 2025-07-16
where:
Remember that is a 4-vetor, gamma matrices are 4x4 matrices, so the whole thing comes down to a dot product of two 4-vectors, with a modified by matrix multiplication/derivatives, and the result is a scalar, as expected for a Lagrangian.
Like any other Lagrangian, you can then recover the Dirac equation, which is the corresponding equations of motion, by applying the Euler-Lagrange equation to the Lagrangian.
Directional derivative Updated 2025-07-16
Dirichlet boundary condition Updated 2025-07-16
Specifies fixed values.
Can be used for elliptic partial differential equations and parabolic partial differential equations.
Numerical examples:
Discord email notifications Updated 2025-07-16
Discord is useless if you want to participate in more than one large group because of this. It is impossible to get email notification for selected threads you care about.
No way to get email notifications for missed activity? support.discord.com/hc/en-us/community/posts/360041806392-Can-we-get-an-email-notification-option-for-messages-
Discrete Fourier transform of a real signal Updated 2025-07-16
See sections: "Example 1 - N even", "Example 2 - N odd" and "Representation in terms of sines and cosines" of www.statlect.com/matrix-algebra/discrete-Fourier-transform-of-a-real-signal
The transform still has complex numbers.
Summary:Therefore, we only need about half of to represent the signal, as the other half can be derived by conjugation.
- is real
"Representation in terms of sines and cosines" from www.statlect.com/matrix-algebra/discrete-Fourier-transform-of-a-real-signal then gives explicit formulas in terms of .
NumPy for example has "Real FFTs" for this: numpy.org/doc/1.24/reference/routines.fft.html#real-ffts
DFT of with 25 points
. Source at: numpy/fft_plot.py. This plot illustrates how the DFT of a real signal is symmetric around the middle point, and so only half of the transform points are needed to reconstruct the original signal. We also see how the phase of the sinusoids determines if their DFT components are real or imaginary. Disk encryption password handover plausible deniability Updated 2025-07-16
Can the last disk access times be checked via forensic methods?
Bitcoin mining pool Updated 2025-07-16
Python package manager Updated 2025-07-16
Chinese poetry Updated 2025-07-16
Bitcoin blockchain parser Updated 2025-07-16
This section is about partial implementations that are only able to read the blocks, ususally coming from Bitcoin Core, to interpret the data:
Bitcoin Core snap Updated 2025-07-16
Officially supported installation method on Ubuntu 23.10.
ffplay multiple input files Updated 2025-07-16
TODO possible? superuser.com/questions/559768/ffplay-how-to-play-together-separate-video-and-audio-files
For synthesized streams like but it does not seem to accept multiple fails with:
sine we can do it e.g.ffplay -autoexit -nodisp -f lavfi -i '
sine=frequency=500[a];
sine=frequency=1000[b];
[a][b]amerge, atrim=end=2
'-i for some reason. So is there a way to open a file from some filter? E.g.:ffplay -i tmp.wav -i tmp.mkv -filter_complex "[0:a]atrim=end=2[a];[1:v]trim=end=2[v]" -map '[a]' -map '[v]'Argument 'tmp.mkv' provided as input filename, but 'tmp.wav' was already specified. Exif tag Updated 2025-07-16
Andy Matuschak Updated 2025-07-16
Proponent of evergreen notes.
He's also curious about quantum computing: quantum.country/ like Ciro Santilli. Some crazy overlaps we get.
Bitcoin Forum Updated 2025-07-16
Founded by Satoshi Nakamoto, making it the earliest and one of the most important Bitcoin communities. TODO official in any way? Who founded it?
Some notable appearances:
- in 2010, it is where Laszlo's pizzas offer was announced
- it was used e.g. on the Mt. Gox investigation: youtu.be/tJ-TsrK6SuY?t=2018
- Jimmy Zhong's investigation: youtu.be/pxvd1YOMGxU?t=1004
Connect to other computer in LAN by hostname Updated 2025-07-16
Silk Road investigator Updated 2025-09-09
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