Ciro Santilli @cirosantilli 37

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Disk encryption password handover plausible deniability Updated 2025-07-16

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Can we do better than "wrong password implies random bytes"?

Can the last disk access times be checked via forensic methods?

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Disk read-and-write head Updated 2025-07-16

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Distribution (mathematics) Updated 2025-07-16

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Generalize function to allow adding some useful things which people wanted to be classical functions but which are not,

It therefore requires you to redefine and reprove all of calculus.

For this reason, most people are tempted to assume that all the hand wavy intuitive arguments undergrad teachers give are true and just move on with life. And they generally are.

One notable example where distributions pop up are the eigenvectors of the position operator in quantum mechanics, which are given by Dirac delta functions, which is most commonly rigorously defined in terms of distribution.

Distributions are also defined in a way that allows you to do calculus on them. Notably, you can define a derivative, and the derivative of the Heaviside step function is the Dirac delta function.

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Divergence in Einstein notation Updated 2025-07-16

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First we write a vector field as:

F (x_{0}, x_{1}, x_{2}) = (F^{0} (x_{0}, x_{1}, x_{2}), F^{1} (x_{0}, x_{1}, x_{2}), F^{2} (x_{0}, x_{1}, x_{2})) : R^{3} \to R^{3}

(1)

Note how we are denoting each component of

F

F^{i}

with a raised index.

Then, the divergence can be written in Einstein notation as:

\nabla \cdot F = \frac{\partial F ^{0} ( x _{0} , x _{1} , x _{2} )}{\partial x _{0}} + \frac{\partial F ^{1} ( x _{0} , x _{1} , x _{2} )}{\partial x _{1}} + \frac{\partial F ^{2} ( x _{0} , x _{1} , x _{2} )}{\partial x _{2}} = \partial_{i} F^{i} (x_{0}, x_{1}, x_{2}) = \frac{\partial F ^{i} ( x _{0} , x _{1} , x _{2} )}{\partial x ^{i}}

(2)

It is common to just omit the variables of the function, so we tend to just say:

\nabla \cdot F = \partial_{i} F^{i}

(3)

or equivalently when referring just to the operator:

\nabla \cdot = \partial_{i}

(4)

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Don Lincoln Updated 2025-07-16

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www.youtube.com/watch?v=vIJTwYOZrGU&list=PLCfRa7MXBEsoJuAM8s6D8oKDPyBepBosS

Publishes through the Fermilab YouTube channel under the playlist "Fermilab - Videos by Don Lincoln"

Some insights, but too much on the popular science side of things.

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Do one thing and do it well Updated 2025-07-16

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Is this exact sentence really from UNIX philosophy? en.wikipedia.org/wiki/Unix_philosophy#Do_One_Thing_and_Do_It_Well

Of course, it has precedents, e.g. jack of all trades, master of none

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Do superconductors carry infinite current? Updated 2025-07-16

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No, see: superconductor I-V curve.

Bibliography:

Video 1.

Superconducting Short Circuits across Batteries by Eugene Khutoryansky (2020)

Source. Well, internal battery resistance acts as the only resistor, and voltage drops to zero immediately outside of the battery. And you get a huge current.

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Double-slit experiment Updated 2025-07-16

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Amazingly confirms the wave particle duality of quantum mechanics.

The effect is even more remarkable when done with individual particles such individual photons or electrons.

Richard Feynman liked to stress how this experiment can illustrate the core ideas of quantum mechanics. Notably, he night have created the infinitely many slits thought experiment which illustrates the path integral formulation.

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Nested set model Updated 2025-07-16

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This is particularly important in SQL: Nested set model in SQL, as it is an efficient way to transverse trees there, since querying parents every time would require multiple disk accesses.

The ASCII art visualizations from stackoverflow.com/questions/192220/what-is-the-most-efficient-elegant-way-to-parse-a-flat-table-into-a-tree/194031#194031 are worth reproducing.

As a tree:

Root 1
- Child 1.1
  - Child 1.1.1
  - Child 1.1.2
- Child 1.2
  - Child 1.2.1
  - Child 1.2.2

As the sets:

 __________________________________________________________________________
|  Root 1                                                                  |
|   ________________________________    ________________________________   |
|  |  Child 1.1                     |  |  Child 1.2                     |  |
|  |   ___________    ___________   |  |   ___________    ___________   |  |
|  |  |  C 1.1.1  |  |  C 1.1.2  |  |  |  |  C 1.2.1  |  |  C 1.2.2  |  |  |
1  2  3___________4  5___________6  7  8  9___________10 11__________12 13 14
|  |________________________________|  |________________________________|  |
|__________________________________________________________________________|

Consider the following nested set:

0, 8, root
  1, 7, mathematics
    2, 3, geometry
      3, 6, calculus
        4, 5, derivative
        5, 6, integral
      6, 7, algebra
  7, 8, physics

When we want to insert one element, e.g. limit, normally under calculus, we have to specify:

parent
index within parent

so we have a method:

insert(parent, previousSibling)

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Nested set model in SQL Updated 2025-07-16

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How to implement Nested set model in SQL:

stackoverflow.com/questions/192220/what-is-the-most-efficient-elegant-way-to-parse-a-flat-table-into-a-tree/42781302#42781302 contains the correct left/size representation and update queries, which makes it much easier to maintain the tree without having to worry about the sizes of siblings which are constant
stackoverflow.com/questions/192220/what-is-the-most-efficient-elegant-way-to-parse-a-flat-table-into-a-tree/194031#194031 amazing ASCII art representations of the structure. Unfortunately uses a wonky left/right representation, rather than the much more natural left/size representation from the other post

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Neuralink Updated 2025-07-16

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Elon Musk's attempt.

Video 1.

Neuralink: Elon Musk's entire brain chip presentation in 14 minutes (supercut) by CNET (2020)

Source.

youtu.be/CLUWDLKAF1M?t=380 Shows a pig with the implant, and live signals are shown when its nose touches something.
youtu.be/CLUWDLKAF1M?t=536 shows a pre-recorded pig study correlating really the joint positions while walking with the neuralink signals

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NeuroMechFly Updated 2025-07-16

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Just imagine this together with a Drosophila connectome on a single brain-in-the-loop simulation.

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Dr. Strangelove Updated 2025-07-16

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Video 1.

Inside the Making of 'Dr. Strangelove' part 2

. Source.

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Dual space Updated 2025-07-16

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The dual space of a vector space

V

, sometimes denoted

V^{*}

, is the vector space of all linear forms over

V

with the obvious addition and scalar multiplication operations defined.

Since a linear form is completely determined by how it acts on a basis, and since for each basis element it is specified by a scalar, at least in finite dimension, the dimension of the dual space is the same as the

V

, and so they are isomorphic because all vector spaces of the same dimension on a given field are isomorphic, and so the dual is quite a boring concept in the context of finite dimension.

Infinite dimension seems more interesting however, see: en.wikipedia.org/w/index.php?title=Dual_space&oldid=1046421278#Infinite-dimensional_case

One place where duals are different from the non-duals however is when dealing with tensors, because they transform differently than vectors from the base space

V

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Dulong-Petit law Updated 2025-07-16

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Observation that all solids appear to have the same constant heat capacity per mole.

It can be seen as the limit case of an Einstein solid at high temperatures. At lower temperatures, the heat capacity depends on temperature.

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Dungeon Crawl Stone Soup Updated 2025-07-16

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Appears to be the best classic open source roguelike of the 2020's.

This website is really cool! crawl.akrasiac.org:8080/#lobby You can spectate players live and chat! Also has statistics.

Devs of this game are smart, they have one good in-tree tileset, unlike some other text-based games that didn't have an in-tree option...

Build on Ubuntu 21.10:

sudo apt install build-essential libncursesw5-dev bison flex liblua5.1-0-dev \
libsqlite3-dev libz-dev pkg-config python3-yaml binutils-gold python-is-python3 \
libsdl2-image-dev libsdl2-mixer-dev libsdl2-dev libfreetype6-dev libpng-dev \
fonts-dejavu-core advancecomp pngcrush

git clone --depth 1 --branch 0.28.0 https://github.com/crawl/crawl
cd crawl/crawl-ref/source
echo 0.28-a > util/release_ver
make -j`nproc` TILES=y
./crawl

This launches the UI version already for you.

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Dutch university Updated 2025-07-16

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Early expansions of hominins out of Africa Updated 2025-07-16

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Early human migrations Updated 2025-07-16

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