<Ciro Santilli> is a <UK> resident. He will register as a "solo trader" (slightly funny legal term) and treat donations that he uses for projects as grants, which pay regular income tax:
* <Gift vs Grant vs Contract in the United Kingdom>
* https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim41810 notably:
  \Q[https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim41810]
The rates are given at: https://www.gov.uk/income-tax-rates and are as of writing:
* 0 - £12,570	0%
* £12,571 - £50,270: 20%
* £50,271 - to £125,140: 40%
* £125,140: 45%

National insurance is also likely going to be paid: https://www.gov.uk/self-employed-national-insurance-rates:
* 6% on profits between £12,570 and £50,270
* 2% on profits over £50,270

Fortunately however VAT does not need to be paid.

The ammount that will be declared is the same as he grant ammount that was requested, e.g. if 100k USD is requested for 1 year, then 100k USD will be pro-rata declared on that year.

Any remaining donationa that don't yet meet specific grant goals will be initially treated as cash gifts which pay no tax. If in the future they are used as grant money after further goal ammounts are reached, then they will taxed as grants.

Note however that if the donor is UK-based and dies within 7 years of the gift being given, inheritance tax has to be paid on them as per: https://www.gov.uk/inheritance-tax/gifts[], at a maximum of 32% and going to to 0% at 7 years, so let me know from the afterlife.


Taxation

<Ciro Santilli>'s preferred visualization of the real projective plane is a small variant of the standard "lines through origin in <\R^3>".

Take a open half <sphere> e.g. a sphere but only the points with $z > 0$.

Each point in the half sphere identifies a unique line through the origin.

Then, the only lines missing are the lines in the x-y plane itself.

For those sphere points in the <circle> on the x-y plane, you should think of them as magic poins that are identified with the corresponding <antipodal point>, also on the x-y, but on the other side of the origin. So basically you you can teleport from one of those to the other side, and you are still in the same point.

Ciro likes this model because then all the magic is confined just to the $z=0$ part of the model, and everything else looks exactly like the sphere.

It is useful to contrast this with the sphere itself. In the sphere, all points in the circle $z = 0$ are the same point. But this is not the case for the <projective plane>. You cannot instantly go to any other point on the $z=0$ by just moving a little bit, you have to walk around that circle.

\Image[https://raw.githubusercontent.com/cirosantilli/media/master/spherical-cap-model-of-the-real-projective-plane.svg]
{title=Spherical cap model of the real projective plane}
{description=On the x-y plane, you can magically travel immediately between <antipodal points> such as A/A', B/B' and C/C'. Or equivalently, those pairs are the same point. Every other point outside the x-y plane is just a regular point like a normal <sphere>.}


Spherical cap model of the real projective plane

I had meant to make an update earlier, but I wanted to try and add some more "visible end-user changes" to </OurBigBook.com>.

Just noticed BTW that signup on the website is broken. Facepalm. Not that it matters much since it is not very useful in the current state, but still. Going to fix that soon. EDIT: nevermind, it wasn't broken, I just had JavaScript disabled on that website with an extension to test if pages are visible without JavaScript, and yes, they are perfectly visible, you can't tell the difference! But you can't login without JavaScript either!

I still haven't the user visible ones I wanted, but I've hit major milestones, and it feels like time for an update.

I have now finished all the <OurBigBook CLI> features that I wanted for 1.0, all of which will be automatically reused in ourbigbook.com.

The two big things since last email were the following:
* <group all SQL queries together>
* <the table of contents shows across different files via \Include>

A secondary but also important advance was: <further improvements to the website's base technology>.

I knew I was going to do them for several months now, and I knew they were going to hurt, and they did, but I did them.

These change caused two big bugs that I will solve next, one them infinite recursion in the database recursive query, but they shouldn't be too hard.


ourbigbook.com

The new default homepage for a logged out user how shows a list of the topics with the most articles.

This is a reasonable choice for default homepage, and it immediately exposes users to this central feature of the website: the topic system.

Doing this required in particular calculating the best title for a topic, since it is possible to have different titles with the same ID, the most common way being with capitalization changes, e.g.:
``
JavaScript
Javascript
``
would both have topic ID `javascript`.

With this in place we also added the preferred topic title to the top topic page.

The algorithm chosen is to pick the top 10 most upvoted topics, and select the most common title from amongst them. This should make topic title vandalism quite hard. This was made in a single <SQL> query, and became the most complext SQL query <Ciro Santilli> has ever written so far: https://twitter.com/cirosantilli2/status/1549721815832043522

\Image[https://raw.githubusercontent.com/cirosantilli/media/master/OurBigBook_topic_index_page.png]
{height=800}
{title=Screenshot showing the list of topics}
{description=The page is: https://ourbigbook.com[] for the logged out user, https://ourbigbook.com/go/topics[] for the logged in user.}

\Image[https://raw.githubusercontent.com/cirosantilli/media/master/OurBigBook_topic_page_with_title.png]
{height=400}
{title=Screenshot showing a topic page}
{description=The page is: https://ourbigbook.com/go/topic/vector-space[]. Before this sprint, we didn't have the "Vector Space" at the top, as it wasn't necessarily trivial to determine what the preferred title would be.}


List topics on home page

{wiki}

Do electrons spontaneously jump from high orbitals to lower ones emitting photons?

Explaining this was was one of the key initial achievements of the <Dirac equation>.

Yes, but this is not predicted by the <Schrödinger equation>, you need to go to the <Dirac equation>.

A critical application of this phenomena is <laser>.

See also:
* https://physics.stackexchange.com/questions/233330/why-do-electrons-jump-between-orbitals
* https://physics.stackexchange.com/questions/117417/quantum-mechanics-scattering-theory/522220\#522220
* https://physics.stackexchange.com/questions/430268/stimulated-emission-how-can-giving-energy-to-electrons-make-them-decay-to-a-low/430288


Spontaneous emission

{title2=$Sp(n, F)$}

Intuition, please? Example? https://mathoverflow.net/questions/278641/intuition-for-symplectic-groups The key motivation seems to be related to <Hamiltonian mechanics>. The two arguments of the <bilinear form> correspond to each set of variables in Hamiltonian mechanics: the generalized positions and generalized momentums, which appear in the same number each.

Seems to be set of matrices that preserve a <skew-symmetric bilinear form>, which is comparable to the <orthogonal group>, which preserves a <symmetric bilinear form>. More precisely, the orthogonal group has:
$$
O^T I O = I
$$
and its generalization the <indefinite orthogonal group> has:
$$
O^T S O = I
$$
where S is symmetric. So for the symplectic group we have matrices Y such as:
$$
Y^T A Y = I
$$
where A is antisymmetric. This is explained at: https://www.ucl.ac.uk/~ucahad0/7302_handout_13.pdf They also explain there that unlike as in the analogous <orthogonal group>, that definition ends up excluding determinant -1 automatically.

Therefore, just like the <special orthogonal group>, the symplectic group is also a <subgroup> of the <special linear group>.


Symplectic group

* https://github.com/mapbox/node-sqlite3
* https://www.npmjs.com/package/sqlite3

Includes its own copy of sqlite3, you don't use the system one, which is good to ensure compatibility. The version is shown at: https://github.com/mapbox/node-sqlite3/blob/918052b538b0effe6c4a44c74a16b2749c08a0d2/deps/common-sqlite.gypi\#L3 <SQLite> source is tracked compressed in-tree: https://github.com/mapbox/node-sqlite3/blob/918052b538b0effe6c4a44c74a16b2749c08a0d2/deps/sqlite-autoconf-3360000.tar.gz horrendous. This explains why it takes forever to clone that repository. People who don't believe in git submodules, there's even an official Git mirror at: https://github.com/sqlite/sqlite

It appears to spawn its own <threads> via its <C (programming language)> extension (since <JavaScript is single threaded> and and <SQLite> is not <server>-based), which allows for parallel queries using multiple threads: https://github.com/mapbox/node-sqlite3/blob/v5.0.2/src/threading.h

Hello world example: \a[nodejs/node-sqlite3/index.js].

As of 2021, this had slumped back a bit, as maintainers got tired. Unmerged pull requests started piling more, and <better-sqlite3 Node.js package> started pulling ahead a little.
* https://github.com/mapbox/node-sqlite3/issues/1381 `FATAL ERROR: Error::ThrowAsJavaScriptException napi_throw` with <Node.js worker_threads> vs <better-sqlite3 Node.js package> https://github.com/JoshuaWise/better-sqlite3/issues/237


`sqlite3` Node.js package

<code>sqlite3</code> Node.js package

{disambiguate=film}
{c}
{tag=Spy film}
{tag=Good film}
{title2=2011}

This is not bad, but some divergences to the better <Tinker Tailor Soldier Spy>[BBC miniseries], which presumably sticks more closely to the novel:
* in the film Jim Prideaux is captured in a cafe in Prague, in the series it's in the woods. It is therefore much more plausible that he would have been shot.
* in the film Peter Guillam is played by Benedict Cumberbatch, who feels a bit young to be Ricki Tarr's boss. Not impossible, but still.
* the series is much less chronological, and more flashback based, as new information becomes available. The film is more chronological, which makes it easier to understand, but less interesting at the same time.
* in the film they shoot the Russian girl Irina in front of Jim, in the series the fact that she was shot is only known through other sources. The film has more eye candy, which weakens it.
* Toby Esterhase is not threatened in an airfield, only in a safe ;house in London.

Related:
* https://www.reddit.com/r/TrueFilm/comments/s5f3f1/tinker_tailor_soldier_spy_2011_is_the_best_spy/


Tinker Tailor Soldier Spy

{wiki}

= Principle of least action
{synonym}
{title2}
{wiki}

As mentioned on the <Wikipedia> page https://en.wikipedia.org/w/index.php?title=Stationary_Action_Principle&oldid=1020413171[], "principle of least action" is not accurate since it could not necessarily be a minima, we could just be in a <maxima and minima>[saddle-point].


Stationary action principle

{c}
{title2=1921}
{wiki=Stern–Gerlach_experiment}

Originally done with silver in 1921, but even clearer theoretically was the hydrogen reproduction in 1927 by T.E. Phipps and J.B. Taylor.

The hydrogen experiment was apparently harder to do and the result is less visible, TODO why: https://physics.stackexchange.com/questions/33021/why-silver-atoms-were-used-in-stern-gerlach-experiment

Needs an inhomogenous magnetic field to move the atoms up or down: <magnetic dipole in an inhomogenous magnetic field>. TODO how it is generated?

\Video[https://www.youtube.com/watch?v=AcTqcyv-V1I]
{title=The Stern-Gerlach Experiment by Educational Services, Inc (1967)}
{description=Featuring <MIT> Professor Jerrold R. Zacharias. Amazing experimental setup demonstration, he takes apart much of the experiment to show what's going on.}


Stern-Gerlach experiment

{c}
{tag=Cirism}
{wiki}

= Restrict the fuck out of cars in cities
{synonym}
{title2}

= Restrict the fuck out of cars
{synonym}

Only people who need to drive a car should be allowed to drive a car anywhere near a city, e.g. people who work door to door, people who are disabled, etc.

Countryside driving is fine. If going to a city, you just have to drive to a parking outside of the city where you then take the <public transport>. And those who live in cities must leave their cars there too.

Everyone else must walk or cycle from home to <public transport>.

Cars just destroy everything, they make everything ugly:
* this was extremely clear to <Ciro Santilli> as a <Ciro Santilli's cycling>[cyclist]. He previously lived in a place with few cars and the countryside was so pleasant. Then he moved to a place with more cars and it was shocking. It's a mixture of pollution, noise, and the fact that roads cut up the countryside that just make things not pleasant at all. Dual lane roads in particular are just a terrible thing. You can hear them from afar, much before you see them.

  You can just see as tiny little villages surrounding the bit city and it's oversized motorways are more or less homogenized into one big city mass, the process is clearly visible as you cycle out of the big city and the villages become nicer and more unique as you go along further out.
* even within cities, cars completely dehumanize the streets. For example, Ciro once lived in a small dead end street, and he would have gladly opened his front window more often to meet the neighbours. But just the noise of cars passing by every so often makes it impractical to work like that.

The <Zatoichi effect> applies well to the problem of cyclists:
* they are not really pedestrians, and pedestrian paths are not suitable to them because they are too narrow, of not smooth, or curved. But pedestrians will always have enough political power to have their paths, because they live around the paths
* they are not really motor vehicles, because motor vehicle paths are too wide and too fast for them. But motor vehicles will always have enough political power to have their paths, because people are lazy and stupid, and because as the world stands, individually you just don't have any reasonable choice to go anywhere.
This is the main drama faced by cyclists.

Lobbying groups:

\Video[https://www.youtube.com/watch?v=gohSeOYheXg]
{title=Why isn't cycling normal in London? by Jay Foreman (2018)}


Street reclamation

{wiki}

The summary from https://www.geeksforgeeks.org/tree-traversals-inorder-preorder-and-postorder/ is a winner:
``
    1
   / \
  2   3
 / \
4   5
``
* <inorder DFS>: 4 2 5 1 3
* <preorder DFS>: 1 2 4 5 3
* <postorder DFS>: 4 5 2 3 1
* <breadth-first search>: 1 2 3 4 5

In principle one could talk about tree traversal of <unordered trees> as a number of possible traversals without a fixed order. But we won't consider that under this section, only deterministic <ordered tree> traversals.


Tree traversal

{c}
{wiki}

= Monkey King
{c}
{synonym}
{title2}

= 孙悟空
{synonym}
{title2}

= 美猴王
{synonym}
{title2}

Only after Ciro became an adult did he finally understand that <Sun Wukong> was the basis for <children cartoons Ciro Santilli liked to watch>[Dragon Ball] as mentioned at: <image 19th century illustration of the Journey to the West protagonist Sun Wukong>{full}. And that Sun Wukong was a million times more famous overall. <Mind blown>.

His given name "Wukong" literally means "the one who mastered the void" (Wu = Comprehend, Kong = void), so clearly a <Dharma name>. Edit: it is actually given in the novel, he was born without name. They seem to be Taoist however.

The family name sun1 孙 is the same character as "grandson", but most educated Chinese people seem to be able to recognize it as a reference to "monkey" from some archaic context not anymore in current usage.

\Image[https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Xiyou.PNG/421px-Xiyou.PNG]
{title=19th century illustration of <Sun Wukong>}

\Image[https://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Sun_Wukong_at_Beijing_opera_-_Journey_to_the_West.jpg/450px-Sun_Wukong_at_Beijing_opera_-_Journey_to_the_West.jpg]
{title=20 <Sun Wukong> depiction in Peking opera}


Sun Wukong

= The best Chinese traditional instrumental music of all time
{synonym}

\Video[https://www.youtube.com/watch?v=WbCjceRPXyw]
{disambiguate=best}
{title=<High Mountain and Flowing Water> performed on the <guzheng> by Xiang SiHua (2000)}

\Video[https://www.youtube.com/watch?v=uakOkv341P8]
{title=<Suwu herding sheep> performed on the <erhu> by <Song Fei (Erhu)> (2017)}

\Video[https://www.youtube.com/watch?v=2KftwzQOECA]
{title=<Ambush from ten sides> performed on the <pipa> by Jiaju Shen (2017)}

\Video[https://www.youtube.com/watch?v=8Ho_jNv8fUo]
{title=<White snow in sunny spring> performed on the <pipa> by <Liu Dehai>}

\Video[https://www.youtube.com/watch?v=aBP8q24Ddpc]
{title=<Dance of the Yi People> performed on the <pipa> by <Liu Dehai>}

\Video[https://www.youtube.com/watch?v=MeUiaxaRMK8]
{title=<Wang Jin (Water Margin character)> beats <Gao Qiu> theme music from <The Water Margin (1998 TV series)> featuring a <suona> solo}


The best Chinese traditional instrumental music

* <Google Maps> download offline maps. This works very reliably, you can select the area you want to download. The only downside is that Google maps can't reliably show a route offline, and it does not contain national cycle route routes. Or those features are impossible for a <software engineer> to get working after trying for about 2 hours.
* <OpenStreetMap> on browser with cycling layer: https://www.openstreetmap.org/\#map=5/49.582/1.934&layers=C This is the best visualization of cycling routes I've found so far, contains both <National Cycle Network> and National Byway and a few others, and they are shown extremely clearly. But as a website it doesn't reliably work offline
* the <OsmAnd> app for <Android> is the best offline free-ish OpenStreetMap viewer I've found so far. You only have to pay after reaching 5 region downloads, and it is very cheap if you want to do so. The cycle route view is not amazing, the routes are not so clearly marked and mixed with very similarly colored big roads, but with a bit of effort you can make them out. No routing though
* I've heard Komoot can keep a predefined route (possibly auto planed) reliably offline, but haven't used it myself. I was not able to see National Cycle Route clearly marked anywhere on it


The best cycling map app

{tag=Willis Lamb}
{title2=QED experiments}
{title2=Lamb and Kusch}

The key initial <quantum electrodynamics> experiments:
* <Willis Lamb>{child} for the <Lamb-Retherford experiment>{child}
* <Polykarp Kusch>{child} for the <anomalous magnetic dipole moment of the electron>{child}, key publication: <the Magnetic Moment of the Electron by Kusch and Foley (1948)>{child}


1955 Nobel Prize in Physics

{tag=Tensor product}

We don't need to understand a super generalized version of <tensor products> to know what they mean in basic <quantum computing>!

Intuitively, taking a <tensor product> of two <qubits> simply means putting them together on the same quantum system/computer.

When we write the <bra-ket notation>: $\ket{00}$ that is the same as $\ket{0} \otimes \ket{0}$.

The tensor product is called a "product" because it distributes over addition.

E.g. consider:
$$
(\frac{\ket{0} + \ket{1}}{\sqrt{2}}) \otimes \ket{0} =
\frac{\ket{0} \otimes \ket{0} + \ket{1} \otimes \ket{0}}{\sqrt{2}} =
\frac{\ket{00} + \ket{10}}{\sqrt{2}}
$$

Intuitively, in this operation we just put a <Hadamard gate> qubit together with a second pure $\ket{0}$ qubit.

And the outcome still has the second qubit as always 0, because we haven't made them interact.

The <quantum state> $\frac{\ket{00} + \ket{10}}{\sqrt{2}}$ is called a <separable state>, because it can be written as a single product of two different qubits. We have simply brought two qubits together, without making them interact.

If we then add a <CNOT gate> to make a <Bell state>:
$$
\frac{\ket{00} + \ket{11}}{\sqrt{2}} =
\frac{\ket{0} \otimes \ket{0} + \ket{1} \otimes \ket{1}}{\sqrt{2}}
$$
we can now see that the <Bell state> is <non-separable>: we've made the two qubits interact, and there is no way to write this state with a single <tensor product>. The qubits are fundamentally <entangled>.


Tensor product in quantum computing

{wiki}

This is a general principle of software/hardware design that Ciro feels holds wide applicability.

The most extreme case of this is of course the <integrated circuit> itself, in which it is essentially impossible (?) to observe the specific value of some indidual wire at some point.

Somewhat on the other extreme, we have high level programming languages running on top of an <operating system>: at this point, you can just <GDB step debug> your program, print the value of any variable/memory location, and fully understand anything that you want. Provided that you manage to easily reach that point of interest.

And for anything in between we have various intermediate levels of complication. The most notable perhaps being developing the operating system itself. At this level, you can't so easily step debug (although <Step debug the Linux kernel>[techniques do exist]). For early boot or <bootloaders> for example, you might want to use <JTAG> for example on real hardware.

In parallel to this, there is also another very important pair of closely linked tradeoffs:
* the lower level at which something is implemented, the faster it runs
* <emulation> gives you observability back, at the cost of slower runtime

Emulation also has another potential downside: unless you are very careful at implementing things correctly, your model might not be representative of the real thing. Also, there may be important tradeoffs between how much the model looks like the real thing, and how fast it runs. For example, <QEMU>'s use of <binary translation> allows it to run orders of magnitude faster than <gem5>. However, you are unable to make any predictions about system performance with QEMU, since you are not modelling key elements like the cache or CPU pipeline.

<Instrumentation (computer programming)> is another technique that has can be considered to achieve greater observability.


The lower level you go into a computer, the harder it is to observe things

{disambiguate=nuclear test}
{c}
{tag=Nuclear weapon test}
{title2=1945-07-16}
{title2=first nuclear weapon detonated}
{wiki}

<Plutonium>-based.

Its plutonium was produced at <Hanford site>.

\Image[https://upload.wikimedia.org/wikipedia/commons/9/95/TrinityDetonation1945GIF.gif]

\Video[https://www.youtube.com/watch?v=nN5q8i-kQj0]
{title=Trinity Test Preparations by AtomicHeritage (2016)}
{description=Appears to be a compilation of several videos, presumably each with their own separate <LA-UR>, though these are not noted. Credited: "Video courtesy of the Los Alamos National Laboratory Archives", TODO how to search that archive online?}

\Video[https://www.youtube.com/watch?v=7QxsehUI0rY]
{title=<Trinity (nuclear test)>[Trinity]: Getting The Job Done}
{description=
Good video, clarifies several interesting technical points:
* <Gun-type fission weapon> were much easier to build as you don't need super synchronized charges as in <implosion-type fission weapon>. But they are less efficient.
* <Plutonium> make much more efficient usage of <uranium>, because you don't need to highly enrich a bunch of <Uranium-235> in the first place, but rather just use way less enriched <Uranium-235> to produce a bunch of <Plutonium> by converting <Uranium-238>
}


Trinity

{tag=TU Delft}

<EdX> course. Meh! Just give me the <YouTube> list!!

But seriously, this is a valuable little list.

The course is basically exclusively about <transmons>.

\Video[https://www.youtube.com/watch?v=cb_f9KpYipk]
{title=The <transmon> qubit by Leo Di Carlo (2018)}
{description=Via <QuTech Academy>.}

\Video[https://www.youtube.com/watch?v=JmnpcWEuMJY]
{title=Circuit QED by Leo Di Carlo (2018)}
{description=Via <QuTech Academy>.}

\Video[https://www.youtube.com/watch?v=KDANvtOkEc4]
{title=Measurements on transmon qubits by Niels Bultink (2018)}
{description=Via <QuTech Academy>. I wish someone would show some actual equipment running! But this is of interest.}

\Video[https://www.youtube.com/watch?v=_MfABBLtBd0]
{title=<Single-qubit gate> by Brian Taraskinki (2018)}
{description=Good video! Basically you make a phase rotation by controlling the envelope of a pulse.}

\Video[https://www.youtube.com/watch?v=vwjlEdwi2LU]
{title=<Two-qubit gate>[Two qubit gates] by Adriaan Rol (2018)}

\Video[https://www.youtube.com/watch?v=BA5thMaxkyY]
{title=Assembling a Quantum Processor by Leo Di Carlo (2018)}
{description=Via <QuTech Academy>.}


Title	Created	Updated
Taxation	1970-01-01	1970-01-01
Spherical cap model of the real projective plane	1970-01-01	1970-01-01
ourbigbook.com	1970-01-01	1970-01-01
List topics on home page	1970-01-01	1970-01-01
Spontaneous emission	1970-01-01	1970-01-01
Symplectic group	1970-01-01	1970-01-01
`sqlite3` Node.js package	1970-01-01	1970-01-01
Tinker Tailor Soldier Spy	1970-01-01	1970-01-01
Stationary action principle	1970-01-01	1970-01-01
Stern-Gerlach experiment	1970-01-01	1970-01-01
Street reclamation	1970-01-01	1970-01-01
Tree traversal	1970-01-01	1970-01-01
Sun Wukong	1970-01-01	1970-01-01
The best Chinese traditional instrumental music	1970-01-01	1970-01-01
The best cycling map app	1970-01-01	1970-01-01
1955 Nobel Prize in Physics	1970-01-01	1970-01-01
Tensor product in quantum computing	1970-01-01	1970-01-01
The lower level you go into a computer, the harder it is to observe things	1970-01-01	1970-01-01
Trinity	1970-01-01	1970-01-01
The Hardware of a Quantum Computer by TU Delft	1970-01-01	1970-01-01

Ciro Santilli (@cirosantilli, 32)