|| Grade
|| Name
|| Notes

| 3
| Algorithms
| https://github.com/cirosantilli/algorithm-cheat[Cheatsheet and implementations]

| 3
| Linux internals
| https://github.com/cirosantilli/linux-kernel-module-cheat[Linux Kernel Module Cheat]

| 5
| Git
| \a[git-tutorial][Tutorial]{external}

| 4
| Buildroot
| https://github.com/cirosantilli/buildroot-configs[Some .configs], https://github.com/cirosantilli/linux-kernel-module-cheat[Linux Kernel Module Cheat uses it a lot]

| 3
| OpenGL
| https://github.com/cirosantilli/cpp-cheat/tree/master/opengl[Cheatsheet and mini projects]

| 3
| Vim
| https://github.com/cirosantilli/dotfiles/blob/master/home/.vimrc[.vimrc + cheatsheet at end]

| 3
| Django
| https://github.com/cirosantilli/django-cheat[Cheatsheet and mini project]

| 2
| Android
| https://github.com/cirosantilli/android-cheat[Cheatsheet]

| 2
| OpenCL
| https://github.com/cirosantilli/cpp-cheat/tree/d14107f7c0b5e03e85d3f01b16f61271c260ae03/opencl[Cheatsheet]

| 3
| QEMU
| https://github.com/cirosantilli/linux-cheat/blob/492dbf28213c0c92fc4e034181a36734a50a7a24/qemu.md[QEMU recipes], https://stackoverflow.com/a/44612957/895245[basic devices]

| 1
| https://www.getchef.com/chef/[Chef]
| For <projects>[GitLab Contributions]

| 1
| AWS, Heroku
| EC2, SES

| 1
| Media formats
| Video, Images, https://stackoverflow.com/search?tab=votes&q=user%3a895245%20%5Bffmpeg%5D[FFmpeg]

| 1
| Networking
| https://github.com/cirosantilli/networking-cheat[Cheatsheet], https://github.com/cirosantilli/cpp-cheat/tree/d14107f7c0b5e03e85d3f01b16f61271c260ae03/posix[basic POSIX networking]


Other technologies

{c}
{wiki}

https://askubuntu.com/questions/220802/no-sound-zynaddsubfx-and-jack-wont-run/1297988\#1297988

Contains a large database of instruments, and allows you to edit them. This is a fun toy.

Instruments are edited on a <GUI>. It is a multi-window program, and you open new windows from new windows from new windows, all filled with hundreds of virtual knobs that you drag with your keyboard, and which would be better done from textual software like <Csound>. It is a thing of beauty.

It does not seem possible to program arbitrary <modular synthesizer> circuits therefore. But if you understand <additive synthesis> and <subtractive synthesis> well, you can make some funky sounds with it.

It is basically a superset of all popular <hardware synthesizers> ever made.

Has its own built-in MIDI keyboard which is nice.

On Ubuntu 20.04 Version: 3.0.5:
``
sudo apt install zynaddsubfx
zynaddsubfx -O alsa
``
as per https://askubuntu.com/questions/220802/no-sound-zynaddsubfx-and-jack-wont-run/1297988\#1297988
To do anything of interest, switch to the Advanced UI:
* Misc
* Switch Interface Mode

The UI is completely different form what is shown on the website as of 2020: https://zynaddsubfx.sourceforge.io/[], it looks instead like: https://www.youtube.com/watch?v=iVPr6iUuO3g[] Maybe on the website it is the new zyn-fusion UI... https://www.reddit.com/r/linuxaudio/comments/bxn3ur/some_help_for_installing_zynfusion_zynaddsubfx/ so confusing.

And they have some crappy policy of asking for 45 USD for binary downloads.

Compiling from source:
``
git clone https://github.com/zynaddsubfx/zynaddsubfx
cd zynaddsubfx
git checkout a789866de4d45a784c1f4d95fcf5a1938347baef
sudo apt build-dep zynaddsubfx
mkdir build
cd build
cmake ..
make -j`nproc`
``
fails with:
``
Traceback (most recent call last):
  File "/usr/bin/cxxtestgen", line 7, in <module>
    import cxxtest.cxxtestgen
  File "/usr/share/cxxtest/cxxtest/__init__.py", line 33, in <module>
    from cxxtest.cxxtestgen import *
  File "/usr/share/cxxtest/cxxtest/cxxtestgen.py", line 18, in <module>
    import __release__
ModuleNotFoundError: No module named '__release__'
``
Ciro gives up for now.


ZynAddSubFX

{wiki}

= Atomic bomb
{synonym}

= Nuclear bomb
{synonym}

\Image[https://upload.wikimedia.org/wikipedia/commons/thumb/0/0f/Plutonium_ring.jpg/505px-Plutonium_ring.jpg]
{title=A weapons-grade ring of electrorefined plutonium, typical of the rings refined at Los Alamos and sent to Rocky Flats for fabrication}
{description=The ring has a purity of 99.96%, weighs 5.3 kg, and is approx 11 cm in diameter. It is enough plutonium for one bomb core. Which city shall we blow up today?}

<Ciro Santilli> is mildly obsessed by nuclear reactions, because they are so <Ciro Santilli's self perceived creative personality>[quirky]. How can a little ball destroy a city? How can putting too much of it together produce criticality and kill people like in the https://en.wikipedia.org/wiki/Demon_core[Slotin accident] or the https://www.youtube.com/watch?v=ZWomuWd7-to[Tokaimura criticality accident]. It is <mind blowing> really.

<Uranium> vs <plutonium>: <uranium vs plutonium Quora answer by Ciro Santilli>{full}.

More fun nuclear stuff to watch:
* <Dr. Strangelove (1964)>
* https://en.wikipedia.org/wiki/Chernobyl_(miniseries)[]
* <The World Of Enrico Fermi by Harvard Project Physics (1970)>
* https://en.wikipedia.org/wiki/Fat_Man_and_Little_Boy[Fat Man and Little Boy (1987)] shows a possibly reasonably realistic of the history of the development of the <Trinity (nuclear test)>

\Video[https://www.youtube.com/watch?v=1E3sFfnAUQs]
{title=Tour of a nuclear misile silo from the 60's by Arizona Highways TV (2019)}

\Video[https://www.youtube.com/watch?v=8uIPQBOCJ64]
{title=The Ultimate Guide to Nuclear Weapons by hypohystericalhistory (2022)}
{description=
Good overall summary. Some interesting points:
* https://youtu.be/8uIPQBOCJ64?t=2946 talks about the difference between <tactical and strategic nuclear weapons>
* https://youtu.be/8uIPQBOCJ64?t=3291 mentions <variable yield> devices, this is the main new thing <Ciro Santilli> learned from this video
* https://youtu.be/8uIPQBOCJ64?t=3416 discusses if a <strategic nuclear weapon> usage would inevitably lead to <tactical nuclear weapon> escalation. It then mentions one case in which a possibly comparable escalation didn't happen: the abstainance of using <chemical weapon> during <World War II>.
}


Nuclear weapon

{wiki}

= Superposition
{synonym}

Quantum superposition is really weird because it is fundamentally different than "either definite state but I don't know which", because the superposition state leads to different measurements than the non-superposition state.

Examples:
* https://www.youtube.com/watch?v=tt8gVXDsh7Q "Interference in quantum mechanics" by <Looking Glass Universe> (2015) shows how a left-right <spin (physics)> measurement has a defined value for a superposed half up half down state, but not for a pure up state.

  TODO can this be conducted? As mentioned in the video, this is closely linked to the fact that you can describe the wave function in multiple different bases (up/down or left/right), which is also at the root of the <uncertainty principle>.
* <video Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013)> gives a similar photon version
* it seems that the <single particle double slit experiment> can also be thought of as in terms of a superposition of "the particle goes through the right" and "the particle goes through the right", although it is a bit harder to thing about as it is not a <discrete> process


Quantum superposition

{title2=RTL}
{wiki}

Register transfer level is the abstraction level at which computer chips are mostly designed.

The only two truly relevant RTL languages as of 2020 are: <Verilog> and <VHDL>. Everything else compiles to those, because that's all that <EDA vendors> support.

Much like a <C (language)> compiler abstracts away the <CPU> assembly to:
* increase portability across ISAs
* do optimizations that programmers can't feasibly do without going crazy

Compilers for RTL languages such as Verilog and <VHDL> abstract away the details of the specific <semiconductor physical implementation>[semiconductor technology] used for those exact same reasons.

The compilers essentially compile the RTL languages into a <standard cell library>.

Examples of companies that work at this level include:
* <Intel>. Intel also has <semiconductor fabrication plants> however.
* <Arm (company)> which does not have <fabs>, and is therefore called a "<fabless>" company.


Register transfer level

{title2=$N \rtimes H$}
{wiki}

As per https://en.wikipedia.org/w/index.php?title=Semidirect_product&oldid=1040813965\#Properties[], unlike the <Direct product>, the semidirect product of two goups is neither <unique>, nor does it always <exist>, and there is no known algorithmic way way to tell if one exists or not.

This is because reaching the "output" of the semidirect produt of two groups requires extra non-obvious information that might not exist. This is because the semi-direct product is based on the <product of group subsets>. So you start with two small and completely independent groups, and it is not obvious how to join them up, i.e. how to define the group operation of the product group that is compatible with that of the two smaller input groups. Contrast this with the <Direct product>, where the composition is simple: just use the group operation of each group on either side.

Product of group subsets

So in other words, it is not a <function (mathematics)> like the <Direct product>. The semidiret product is therefore more like a property of three groups. 

The semidirect product is more general than the <direct product of groups> when thinking about the <group extension problem>, because with the <direct product of groups>, both subgroups of the larger group are necessarily also normal (trivial projection <group homomorphism> on either side), while for the semidirect product, only one of them does.

Conversely, https://en.wikipedia.org/w/index.php?title=Semidirect_product&oldid=1040813965 explains that if $G = N \rtimes H$, and besides the implied requirement that N is normal, H is also normal, then $G = N \times H$.

Smallest example: $D_6 = C_3 \rtimes C_2$ where $D$ is a <dihedral group> and $C$ are <cyclic groups>. $C_3$ (the rotation) is a normal subgroup of $D_6$, but $C_2$ (the flip) is not.

Note that with the <Direct product> instead we get $C_6$ and not $D_6$, i.e. $C_3 \times C_2 = C_6$ as per <the direct product of two cyclic groups of coprime order is another cyclic group>.

TODO:
* why does one of the groups have to be normal in the definition?
* what is the smallest example of a non-<simple group> that is neither a direct nor a semi-direct product of any two other groups?

Bibliography: https://math.stackexchange.com/questions/1726939/is-this-intuition-for-the-semidirect-product-of-groups-correct


Semidirect product

{disambiguate=software}
{c}
{wiki}

Weight: heavy.

<MIDI> support is kind of secondary: https://www.youtube.com/watch?v=vnkJ0uYXMG8[], e.g. how to export MIDI? https://discourse.ardour.org/t/export-an-entire-project-into-a-midi-file/88116

\Video[https://www.youtube.com/watch?v=bfTAKv4htDE]
{title=Ardour 6 Quickstart (recording, editing, mixing and exporting) by unfa (2020)}
{description=unfa is a helpful Ardour master and <open source software> junkie at your <YouTube> service.}

Ardour <vmpk> input: https://askubuntu.com/questions/709673/save-as-midi-when-playing-from-vmpk-qsynth/1298231\#1298231


Ardour

{c}
{tag=1973 Nobel Prize in Physics}
{title2=1962}
{wiki}

<Discrete> <quantum> effect observed in <superconductors> with a small insulating layer, a device known as a <Josephson junction>.

To understand the behaviour effect, it is important to look at the <Josephson equations> consider the following <Josephson effect regimes> separately:
* <DC Josephson effect>
* <AC Josephson effect>
* <Inverse AC Josephson effect>

A good summary from Wikipedia by physicist Andrew Whitaker:
\Q[at a junction of two superconductors, a current will flow even if there is no drop in voltage; that when there is a voltage drop, the current should oscillate at a frequency related to the drop in voltage; and that there is a dependence on any magnetic field]

Bibliography:
* https://www.youtube.com/watch?v=cnZ6exn2CkE "Superconductivity: Professor <Brian Josephson>". Several random excerpts from Cambridge people talking about the Josephson effect


Josephson effect

The type of laser described at: <video How Lasers Work by Scientized (2017)>, notably https://youtu.be/_JOchLyNO_w?t=581[]. Mentioned at: https://youtu.be/_JOchLyNO_w?t=759[] That point also mentions that 4-level lasers also exist and are more efficient. TODO dominance? Alternatives?

\Video[https://www.youtube.com/watch?v=6c89tsJHuWc]
{title=<Three-level laser> system by Dr. Nissar Ahmad (2021)}

Bibliography:
* https://www.britannica.com/technology/three-level-laser


Three-level laser

{wiki}

= Representation
{disambiguate=group theory}
{synonym}

Basically, a "representation" means associating each group element as an invertible <matrices>, i.e. a matrix in (possibly some subset of) <GL(n)>, that has the same properties as the group.

Or in other words, associating to the more abstract notion of a <group (mathematics)> more concrete objects with which we are familiar (e.g. a matrix). 

Each such matrix then represents one specific element of the group.

This is basically what everyone does (or should do!) when starting to study <Lie groups>: we start looking at <matrix Lie groups>, which are very concrete.

Or more precisely, mapping each group element to a <linear map> over some <vector field> $V$ (which can be represented by a matrix infinite dimension), in a way that respects the group operations:
$$
R(g) : G \to GL(V)
$$

As shown at <Physics from Symmetry by Jakob Schwichtenberg (2015)>
* page 51, a representation is not unique, we can even use matrices of different dimensions to represent the same group
* 3.6 classifies the <representations of SU(2)>. There is only one possibility per dimension!
* 3.7 "The Lorentz Group O(1,3)" mentions that even for a "simple" group such as the <Lorentz group>, not all representations can be described in terms of matrices, and that we can construct such representations with the help of <Lie group> theory, and that they have fundamental physical application

Motivation:
* https://math.stackexchange.com/questions/1628464/what-is-representation-theory

Bibliography:
* https://www.youtube.com/watch?v=9rDzaKASMTM "RT1: Representation Theory Basics" by <MathDoctorBob> (2011). Too much theory, give me the motivation!
* https://www.quantamagazine.org/the-useless-perspective-that-transformed-mathematics-20200609 The "Useless" Perspective That Transformed Mathematics by <Quanta Magazine> (2020). Maybe there is something in there amidst the "the reader might not know what a <matrix> is" stuff.


Representation theory

{wiki}

TODO what's the point of it.

Bibliography:
* https://www.youtube.com/watch?v=j1PAxNKB_Zc Manifolds \#6 - Tangent Space (Detail) by WHYB maths (2020). This is worth looking into.
  * https://www.youtube.com/watch?v=oxB4aH8h5j4 actually gives a more concrete example. Basically, the vectors are defined by saying "we are doing the <Directional derivative> of any function along this direction".

    One thing to remember is that of course, the most convenient way to define a function $f$ and to specify a direction, is by using one of the <coordinate charts>.

    We can then just switch between charts by change of basis.
* http://jakobschwichtenberg.com/lie-algebra-able-describe-group/ by <Jakob Schwichtenberg>
* https://math.stackexchange.com/questions/1388144/what-exactly-is-a-tangent-vector/2714944 What exactly is a tangent vector? on <Stack Exchange>


Tangent space

{wiki}

Name of the <clade> of <archaea> plus <eukarya> proposed at: https://www.frontiersin.org/articles/10.3389/fmicb.2015.00717/full[]. Much better term than <prokaryote> as that is not a <clade>. Let's hope it catches on!


Arkarya

* upload all of https://cirosantilli.com to https://ourbigbook.com[]. I will do this by implementing an import from filesystem functionality based on the <OurBigBook CLI>. This will also require implementing slit headeres on the server to work well, I'll need to create one `Article` for every header on render.
* get `\x` and `\Include` working on the live web preview editor. This will require creating a new simple API, currently the editor jus shows broken references, but final render works because it goes through the database backend
* implement email verification signup. Finally! Maybe add some notifications too, e.g. on new comments or likes.


Next steps

{wiki}

* https://www.politico.com/news/magazine/2021/11/02/hard-tax-rich-518383
* https://www.washingtonpost.com/news/monkey-cage/wp/2018/02/27/why-the-poor-dont-vote-to-soak-the-rich/

\Video[https://www.youtube.com/watch?v=t6V9i8fFADI]
{title=How the rich avoid paying taxes by Vox (2021)}
{description=Features interview with Morris Pearl, a rich dude that is campaigning to tax the rich. He also participates in an association called https://patrioticmillionaires.org/about/["Patriotic Milionairs"] to further this agenda.}

\Video[https://www.youtube.com/watch?v=5_-Ac68FKG4]
{title=What Eating the Rich Did For Japan by <Asianometry> (2021)}


Tax the rich

<Ciro Santilli>'s general feeling is that university should not own IP, it should belong to the researchers. Instead, university should help researchers make their startups, so they can become big, and then we can tax them and reinvest in the universities.

Of course, this goes through the <nonprofit impact measurement> difficulty. Maybe we could instead limit the IP to some reasonably small percentage, like 10%?

But still, as of 2020, if feels like universities are way too greedy.

* https://youtu.be/ji5_MqicxSo?t=1406 Achieving Your Childhood Dreams by Randy Pausch (2007). At this timestamp he tells a story about how university IP issues almost ruined a collaboration he was passionate about.


University IP ownership

{tag=Evil}
{tag=Exam}

These are the most evil examinations society has.

They mean that until you are 18, you have to study a bunch of generic crap you hate just to get into university. Rather than <Self-directed learning>[studying whatever it is that you truly love to become a God at it as fast as possible and have any chance of advancing the field].

And then, if you decide that you want to change, which is not unlikely since you haven't really try to study what you signed up for before then, it can be very hard and time consuming, leading to a bunch of adults with useless degress they will never use at work.

With the invention of the <Internet>, all teaching material can be free and open source. Only <the only reason for universities to exist should be the laboratories>[laboratory space] has any cost (besides the opportunity cost of participation in actual projects in a research team).

Per-country list: https://en.wikipedia.org/wiki/List_of_admission_tests_to_colleges_and_universities


University entry exam

{wiki=Bra–ket_notation}

= Bra-ket
{synonym}

Notation used in <quantum mechanics>.

Ket is just a <vector (mathematics)>. Though generally in the context of <quantum mechanics>, this is an infinite dimensional vector in a <Hilbert space> like <l2>.

Bra is just the <dual vector> corresponding to a ket, or in other words <projection (mathematics)> <linear operator>, i.e. a linear function which can act on a given vector and returns a single <complex number>. Also known as... <dot product>.

For example:
$$
(\bra{x}) \ket{y}
$$
is basically a fancy way of saying:
$$
x \cdot y
$$
that is: we are taking the projection of $y$ along the $x$ direction. Note that in the ordinary dot product notation however, we don't differentiate as clearly what is a vector and what is an operator, while the bra-ket notation makes it clear.

The projection operator is completely specified by the vector that we are projecting it on. This is why the bracket notation makes sense.

It also has the merit of clearly differentiating vectors from operators. E.g. it is not very clear in $x \cdot y$ that $x$ is an operator and $y$ is a vector, except due to the relative position to the dot. This is especially bad when we start manipulating operators by themselves without vectors.

This notation is widely used in <quantum mechanics> because calculating the <probability> of getting a certain outcome for an experiment is calculated by taking the projection of a state on one an <eigenvalue> basis vector as explained at: <mathematical formulation of quantum mechanics>{full}.

Making the projection operator "look like a thing" (the bra) is nice because we can add and multiply them much like we can for vectors (they also form a <vector space>), e.g.:
$$
\bra{x} + \bra{y}
$$
just means taking the projection along the $x + y$ direction.

<Ciro Santilli> thinks that this notation is a bit over-engineered. Notably the bra's are just vectors, which we should just write as usual with $\va{v}$... the bra thing makes it look scarier than it needs to be. And then we should just find a different notation for the projection part.

Maybe <Dirac> chose it because of the appeal of the women's piece of clothing: <bra>, in an irresistible call from <British humour>.

But in any case, alas, we are now stuck with it.


Bra-ket notation

For numerical algorithms and to get a more low level understanding of the equations, we can expand all terms to the simpler and more explicit form:
$$
\pdv{E_x}{x} + \pdv{E_y}{x} +
\pdv{E_z}{x} =
\frac{\rho}{\vacuumPermittivity}
\\

\pdv{B_x}{x} +
\pdv{B_y}{x} +
\pdv{B_z}{x} =
0
\\

\pdv{E_z}{y} - \pdv{E_y}{z} = -\pdv{B_x}{t} \\
\pdv{E_x}{z} - \pdv{E_z}{x} = -\pdv{B_y}{t} \\
\pdv{E_y}{x} - \pdv{E_x}{y} = -\pdv{B_z}{t} \\

\pdv{B_z}{y} - \pdv{B_y}{z} = \vacuumPermeability \left(J_x + \vacuumPermittivity \pdv{E_x}{t} \right) \\
\pdv{B_x}{z} - \pdv{B_z}{x} = \vacuumPermeability \left(J_y + \vacuumPermittivity \pdv{E_y}{t} \right) \\
\pdv{B_y}{x} - \pdv{B_x}{y} = \vacuumPermeability \left(J_z + \vacuumPermittivity \pdv{E_z}{t} \right) \\
$$
{id=equation-maxwells-equation-explicit}


Explicit scalar form of the Maxwell's equations

{c}
{wiki=Euler–Mascheroni constant}

<Convergence (mathematics)>: https://math.stackexchange.com/questions/629630/simple-proof-euler-mascheroni-gamma-constant


Title	Created	Updated
Other technologies	1970-01-01	1970-01-01
ZynAddSubFX	1970-01-01	1970-01-01
Nuclear weapon	1970-01-01	1970-01-01
Quantum superposition	1970-01-01	1970-01-01
Register transfer level	1970-01-01	1970-01-01
Semidirect product	1970-01-01	1970-01-01
Spy	1970-01-01	1970-01-01
Ardour	1970-01-01	1970-01-01
Josephson effect	1970-01-01	1970-01-01
Three-level laser	1970-01-01	1970-01-01
Representation theory	1970-01-01	1970-01-01
Tangent space	1970-01-01	1970-01-01
Arkarya	1970-01-01	1970-01-01
Next steps	1970-01-01	1970-01-01
Tax the rich	1970-01-01	1970-01-01
University IP ownership	1970-01-01	1970-01-01
University entry exam	1970-01-01	1970-01-01
Bra-ket notation	1970-01-01	1970-01-01
Explicit scalar form of the Maxwell's equations	1970-01-01	1970-01-01
Euler-Mascheroni constant	1970-01-01	1970-01-01

Ciro Santilli (@cirosantilli, 32)