Inscription (blockchain) Updated 2025-11-18
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Data that is inscribed in a blockchain as a way to perpetuate the data, rather than to follow the main intended purpose of the given blockchain, e.g. ASCII art instead of financial transactions on the Bitcoin blockchain.
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Uncanny valley Updated 2025-11-05
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Side project time Updated 2025-11-05
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Rotate object around a point in Inkscape Updated 2025-11-05
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There's a tiny little crosshair that you can drag around to set the center of rotation.
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Set SVG background color in Inkscape Updated 2025-11-05
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Silk Road (marketplace) Updated 2025-11-05
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Ciro Santilli has become slightly obsessed with this story, and the main mastermind Ross Ulbricht.
Figure 1.
Ross Ulbricht's open laptop shortly after his arrest at the Francisco Public Library
. Source. He was running some GNOME based distro, could be Ubuntu from that photo, and likely is given that Ross once recommended Ubuntu to his flatmate.
The best article available so far is: www.theregister.co.uk/2019/01/29/how_i_caught_silk_road_mastermind (archive) which summarizes what one of the investigators said in a 2019 French computer security conference.
The key living posts are:
The big question is of course how libertarian free market ideologically motivated the website was, and how purely criminal greed it was.
The magnitude of the early operational security mistakes does make Ciro think that Ross did it "because he could" and "for the lolz" in a real world Breaking Bad way.
[i]n 2011," [I believe I will be] "creating a year of prosperity and power beyond what I have ever experienced before,
Silk Road is going to become a phenomenon and at least one person will tell me about it, unknowing that I was its creator."
Having this kind of feeling, is the greatest thing any human can have, and what motivates all great things.
Capitalizing in illegal things though is a cheat, big things take longer than a few years to reach, but reaching them is that much more satisfying as well.
Other interesting quotes:
I hated working for someone else and trading my time for money with no investment in myself.
which Ciro also feels, see don't be a pussy, and:
Everyone knows I am working on a bitcoin exchange. I always thought honesty was the best policy and now I didn't know what to do. I should have just told everyone I am a freelance programmer or something, but I had to tell half truths. It felt wrong to lie completely so I tried to tell the truth without revealing the bad part, but now I am in a jam. Everyone knows too much. Dammit.
The murder for hire allegations are also interesting: mashable.com/2013/10/03/silk-road-hits, he paid 80k dollars to undercover DEA agents!
Except for the fact that Ross was an 80 million Dollar drug lord, those accounts sound exactly like what you would expect from any other nerdy startup founder! The:
  • "just do it" strategy effectively going to a minimal viable product (manual transaction management!), while making many mistakes along the way, including hiring mistakes and successes when scaling is needed
  • the hardship of self bootstrapping your own social network (here with some kilos of mushrooms)
  • the variety of periods, from relatively calm, to hair pulling stress during big changes
It is also amusing to see very concretely the obvious fact that the FBI can get a subpoena for all accounts you ever had, e.g. they knew his laptop model from Amazon and brought a corresponding power cable to the arrest! If you are going to be a cyber criminal, don't use your real name, ever!
Should justice be blind? Maybe. But it does hurt for mere non-blind men to see it sometimes. Especially when drug liberalization is involved.
Video 1.
One Mistake Took Down a 29-Yr-Old Dark Web Drug Lord by Newsthink (2022)
Source. Wow nice video, covers most of the interesting annecdotes and the (alledged) investigation procedure.
Video 2.
What is BusKill?
Source.
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Inkscape Updated 2025-11-05
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Inkscape is a a good software for editing/creating SVG files.
Its functionality is fundamental for as a software for drawing geometry diagrams, as it is a good middle ground between algorithmic generation, and raster graphics.
At 1.0.2, its UI is a bit terrible:
  • the way the menus open on the right with title below the window...
  • several defaults are atrocious, e.g. export drawing rather than page
And it crashes from time to time on Ubuntu 21.04. And it has some glaring bugs, e.g.:
But still, it is a very good initiative.
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Personality type Updated 2025-11-05
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How to mine Monero Updated 2025-11-05
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Ubuntu 20.10 as per xmrig.com/docs/miner/build/ubuntu:
sudo apt install git build-essential cmake libuv1-dev libssl-dev libhwloc-dev
git clone https://github.com/xmrig/xmrig.git
mkdir xmrig/build && cd xmrig/build
cmake ..
make -j$(nproc)
At minexmr.com/#getting_started we see that all you then need is a single CLI command:
xmrig -o pool.minexmr.com:4444 -u <your-monero-address>
Seems simple, well done devs!
Benchmark on Lenovo ThinkPad P51 (2017) as per xmrig.com/docs/miner/benchmark:
./xmrig --bench=1M
gives:
948.1 h/s
which according to the minexmr.com mining pool would generate 0.0005 XMR/day, which at the February 2021 rate of 140 USD/XMR is 0.07 USD/day. The minimum payout in that pool is 0.004 XMR so it would take 8 days to reach that.
So clearly, application-specific integrated circuit mining is the only viable way of doing this.
www.makeuseof.com/cryptos-you-can-mine-at-home/ is a completely full of bullshit article that says otherwise. How can someone publish that!
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Sponsor Ciro Santilli's work on OurBigBook.com / Progress updates Updated 2025-11-21
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Ciro Santilli wants to make sure that you get clear highlights of what your donation money is going into!
Sponsors also receive the "Ciro's Edit" email newsletter if desired, which contains similar updates to the above, but delivered directly to their inbox.
Video 1.
OurBigBook Project 2025 one year funded work debrief by Ciro Santilli
. Source. This video debriefs the end of the 1-year-funded 1000 Monero donation work in 2025, in addition to some personal remarks and what might come next.
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Home Updated 2025-11-21
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Check out: OurBigBook.com, the best way to publish your scientific knowledge. It's an open source note taking system that can publish from lightweight markup files in your computer both to a multi-user mind melding dynamic website, or as a static website. It's like Wikipedia + GitHub + Stack Overflow + Obsidian mashed up. Source code: github.com/ourbigbook/ourbigbook.
Sponsor me to work on this project. For 300k USD I will quit my job or not get a new job and work on OurBigBook full time for a second year to try and kickstart The Higher Education Revolution. Status: ~44k / 300k USD. At 2M USD I retire/tenure and work on open STEM forever. How to donate: Section "Sponsor Ciro Santilli's work on OurBigBook.com".
I first quit my job 1st June 2024 to work on the project for 1 year after I reached my initial 100k goal mostly via a 1000 Monero donation. For a second follow up year, I increased my requirement to 300k USD to give me more peace of mind. So the total donation so far is 244k, and if I reach a total of 400k USD, then I'll work on the project for a second year. A second year greatly improve chances of success: year one I improved my tech, year two I come guns blazing to solve courses and expand further.
Mission: to live in a world where you can learn university-level mathematics, physics, chemistry, biology and engineering from perfect free open source books that anyone can write to get famous. More rationale: Section "OurBigBook.com"
Explaining things is my superpower, e.g. I was top user #39 on Stack Overflow in 2023[ref][ref] and I have a few 1k+ star educational GitHub repositories[ref][ref][ref][ref]. Now I want to bring that level of awesomeness to masters level Mathematics and Physics. But I can't do it alone! So I created OurBigBook.com to allow everyone to work together towards the perfect book of everything.
My life's goal is to bring hardcore university-level STEM open educational content to all ages. Sponsor me at github.com/sponsors/cirosantilli starting from 1$/month so I can work full time on it. Further information: Section "Sponsor Ciro Santilli's work on OurBigBook.com". Achieving what I call "free gifted education" is my Nirvana.
This website is written in OurBigBook Markup, and it is published on both cirosantilli.com (static website) and outbigbook.om/cirosantilli (multi-user OurBigBook Web instance). Its source code is located at: github.com/cirosantilli/cirosantilli.github.io and also at cirosantilli.com/_dir and it is licensed under CC BY-SA 4.0 unless otherwise noted.
To contact Ciro, see: Section "How to contact Ciro Santilli". He likes to talk with random people of the Internet.
https://raw.githubusercontent.com/cirosantilli/media/master/ID_photo_of_Ciro_Santilli_taken_in_2013.jpg https://raw.githubusercontent.com/cirosantilli/media/master/Ciro_Santilli's_learn_teach_apply_logo.png
Besides that, I'm also a freedom of speech slacktivist and recreational cyclist. I like Chinese traditional music and classic Brazilian pop. Opinions are my own, but they could be yours too. Tax the rich.
I offer:
My approach is to:
For minors, parents are welcome to join video calls, and all interactions with the student will be recorded and made available to parents.
I am particularly excited about pointing people to the potential next big things, my top picks these days are:
I am also generally interested in:
Figure 1.
Ciro Santilli's amazing Stack Overflow profile
. Ciro contributes almost exclusively by answering question he Googles into out of his own need, and never by refreshing the newest question of big tags for low hanging fruit! More information at: Section "Ciro Santilli's Stack Overflow contributions".
Figure 2.
Ciro Santilli's Stack Overflow stats
. Further methodology details at: Figure "Ciro Santilli's Stack Overflow stats".
Video 1.
The problem with education by Ciro Santilli
. Source. In this video Ciro Santilli exposes his fundamental philosophy regarding why Education is broken. This philosophy was the key motivation behind the failed OurBigBook Project.
Video 2.
Introduction to the OurBigBook Project
. Source.
Video 3.
OurBigBook Web topics demo
. Source. The OurBigBook topic feature allows users to "merge their minds" in a "sort by upvote"-stack overflow-like manner for each subject. This is the killer feature of OurBigBook Web. More information at: docs.ourbigbook.com/ourbigbook-web-topics.
Video 4.
OurBigBook dynamic article tree demo
. Source. The OurBigBook dynamic tree feature allows any of your headers to be the toplevel h1 header of a page, while still displaying its descendants. SEO loves this, and it also allows users to always get their content on the correct granularity. More information at: docs.ourbigbook.com/ourbigbook-web-dynamic-article-tree.
Video 5.
OurBigBook local editing and publishing demo
. Source. With OurBigBook you can store your content as plaintext files in a Lightweight markup, and then publish that to either OurBigBook.com to get awesome multi-user features, or as a static website where you are in full control. More information at: docs.ourbigbook.com/publish-your-content.
Video 6.
Top Down 2D continuous game with Urho3D C++ SDL and Box2D for Reinforcement learning by Ciro Santilli (2018)
Source. More information: Section "Ciro's 2D reinforcement learning games". This is Ciro's underwhelming stab at the fundamental question: Can AGI be trained in simulations?. This project could be taken much further.
Video 7.
Water Margin tribute to Chinese dissidents by Ciro Santilli (2022)
Source. Part of Ciro Santilli's campaign for freedom of speech in China, see also: cirosantilli.com/china-dictatorship/water-margin.
Figure 3.
Lenovo ThinkPad T430 running a BIOS hello world
. This uses Ciro Santilli's project x86 bare metal examples with source code at: github.com/cirosantilli/x86-bare-metal-examples
Video 8.
Linux Kernel Module Cheat presentation
. Source. This project, with source code at: github.com/cirosantilli/linux-kernel-module-cheat, aims to be the perfect emulation setup to study and develop the Linux kernel, kernel modules, QEMU, gem5 as well as x86_64, ARM userland and baremetal assembly and more.
Video 9.
My Bitcoin inscription museum by Ciro Santilli
. Source. Introductory video to Section "Cool data embedded in the Bitcoin blockchain".
 -------------------------------------
|  Force of Will               3 U U  |
|  ---------------------------------  |
| |                  ////////////   | |
| |                ////() ()\////\  | |
| |               ///_\ (--) \///\  | |
| |        )      ////  \_____///\\ | |
| |       ) \      /   /   /    /   | |
| |    ) /   \     |   |  /   _/    | |
| |   ) \  (  (   /   / /   / \     | |
| |  / ) ( )  / (    )/(    )  \    | |
| |  \(_)/(_)/  /UUUU \  \\\/   |   | |
| .---------------------------------. |
| Interrupt                           |
| ,---------------------------------, |
| | You may pay 1 life and remove a | |
| | blue card in your hand from the | |
| | game instead of paying Force of | |
| | Will's casting cost.  Effects   | |
| | that prevent or redirect damage | |
| | cannot be used to counter this  | |
| | loss of life.                   | |
| | Counter target spell.           | |
| `---------------------------------` |
|                                     l
| Illus.  Terese Nelsen               |
 -------------------------------------
Code 1.
ASCII art of a Force of Will Magic: The Gathering card inscribed in the Bitcoin blockchain
.
Artist unknown, uploaded December 2014. Part of Section "Cool data embedded in the Bitcoin blockchain" where Ciro Santilli maintains a curated list of such interesting inscriptions.
This was a small project done by Ciro for artistic purposes that received some attention due to the incredible hype surrounding cryptocurrencies at the time. Ciro Santilli's views on cryptocurrencies are summarized at: Section "Are cryptocurrencies useful?".
Figure 4.
YellowRobot.jpg
. Source.
JPG image fully embedded in the Bitcoin blockchain depicting some kind of cut material art depicting a yellow robot, inscribed on January 29, 2017.
Ciro Santilli found this image and others during his research for Section "Cool data embedded in the Bitcoin blockchain" by searching for image fingerprints on every transaction payload of the blockchain with a script.
The image was uploaded by EMBII, co-creator of the AtomSea & EMBII upload mechanism, which was responsible for a large part of the image inscriptions in the Bitcoin blockchain.
The associated message reads:
Chiharu [EMBII's Japanese wife] and I found this little yellow robot while exploring Chicago. It will be covered by tar or eventually removed but this tribute will remain. N 41.880778 E -87.629210
This is one of Ciro Santilli's favorite AtomSea & EMBII uploads, as it perfectly encapsules the "medium as an art form" approach to blockchain art, where even non-novel works can be recontextualized into something interesting, here depicting an opposition between the ephemeral and the immutable.
At twitter.com/EMBII4U/status/1615389973343268871 EMBII announced that he would be giving off shares of that image on Sup!?, a Bitcoin-backed NFT system he was; making. In December 2023, he gave some shares of the robot to Ciro Santilli.
Figure 5.
2010 Wayback Machine archive of starwarsweb.net
.
This website was used as one of the CIA 2010 covert communication websites, a covert system the CIA used to communicate with its assets. More details at: Section "CIA 2010 covert communication websites".
Ciro Santilli had some naughty OSINT fun finding some of the websites of this defunct network in 2023 after he heard about the 2022 Reuters report on the matter, which for the first time gave away 7 concrete websites out of a claimed 885 total found. As of November 2023, Ciro had found about 350 of them.
Figure 6.
2010 Wayback Machine archive of noticiasmusica.net
.
This is another website that was used as one of the CIA 2010 covert communication websites. This website is written in Brazilian Portuguese, and therefore suggests that the CIA had assets in Brazil at the time, and thus was spying on a "fellow democracy".
Although Snowden's revelations made it extremely obvious to the world that the USA spies upon everyone outside of the Five Eyes, including fellow democracies, it is rare to have such a direct a concrete proof of it visible live right on the Wayback Machine. Other targeted democracies include France, Germany, Italy and Spain. More details at: USA spying on its own allies.
This investigative report by Ciro Santilli was featured on the Daily Mail after 404 Media reported on it in 2025.
Video 10.
How I found a Star Wars website made by the CIA by Ciro Santilli
. Source. Slightly edited VOD of the talk Aratu Week 2024 Talk by Ciro Santilli: My Best Random Projects about the CIA 2010 covert communication websites.
Figure 7.
Diagram of the fundamental theorem on homomorphisms by Ciro Santilli (2020)
Shows the relationship between group homomorphisms and normal subgroups.
Figure 8.
Spacetime diagram illustrating how faster-than-light travel implies time travel by Ciro Santilli (2021)
Used in the Stack Exchange answer to Does faster than light travel imply travelling back in time?. One of Section "The best articles by Ciro Santilli".
Figure 9.
Average insertion time into heaps, binary search tree and hash maps of the C++ standard library by Ciro Santilli (2015)
Source. Used in the Stack Overflow answer to Heap vs Binary Search Tree (BST). One of Section "The best articles by Ciro Santilli".
Video 11.
Birch and Swinnerton-Dyer conjecture in two minutes by Ciro Santilli
. Source. Quick and direct explanation of the statement of the BSD conjecture for people who know basic university mathematics. This is one of the Millennium Prize Problems, and you will get a million dollars if you can solve it! This therefore falls in the Simple to state but hard to prove of Ciro Santilli's the beauty of mathematics aesthetics.
Figure 10.
Top view of an open Oxford Nanopore MinION
. Source. This is Ciro Santilli's hand on the Wikipedia article: en.wikipedia.org/wiki/Oxford_Nanopore_Technologies. He put it there after working a bit on Section "How to use an Oxford Nanopore MinION to extract DNA from river water and determine which bacteria live in it" :-) And he would love to document more experiments like that one Section "Videos of all key physics experiments", but opportunities are extremely rare.
A quick 2D continuous AI game prototype for reinforcement learning written in Matter.js, you can view it on a separate page at cirosantilli.com/_raw/js/matterjs/examples.html#top-down-asdw-fixed-viewport. This is a for-fun-only prototype for Ciro's 2D reinforcement learning games, C++ or maybe Python (for the deep learning ecosystem) seems inevitable for a serious version of such a project. But it is cute how much you can do with a few lines of Matter.js!
HTML snippet:
<iframe src="_raw/js/matterjs/examples.html#top-down-asdw-fixed-viewport" width="1000" height="850"></iframe>
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Sponsor Ciro Santilli's work on OurBigBook.com Updated 2025-11-21
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Ciro Santilli is actively looking for donations and contracts so he can continue to work full time on OurBigBook.com sustainably, and develop free hardcore university-level STEM education for all ages!
For 300k USD I will quit my job or not get a new job and work on OurBigBook full time for a second year to try and kickstart The Higher Education Revolution. Status: ~44k / 300k USD. At 2M USD I retire and work on open STEM forever.
I first quit my job 1st June 2024 to work on the project for 1 year after I reached my initial 100k goal mostly via a 1000 Monero donation. In this first year I improved project tech, but didn't go and solve university courses to create super targeted content, and didn't obtain a single contributing user except myself, see a summary at Section "OurBigBook Project Update March 2025".
For a followup, it would be an interesting experiment to spend on year full time solving as many courses as I can from a world class university in the city where I live in the UK, and adding that as content to the platform to see if that would attract interest, and reaching out directly to course takers at their university environment to try and help them. I intended to do that in year one but my got distracted by tech. It is quite possible that no one has ever done that before in history: a highly motivated technical person with the time and opportunity to do one single thing: help top university students learn their STEM courses better and have more fun doing so. I have in particular identified one course where this would be particularly feasible: the mathematics course, given that much of their course materials, and also their building are quite open. I do sometimes wonder if doing this would be just a waste of my life. But part of me tells me it could generate big interest and is worth a try. It would also be fun for you to watch me continue to commit career seppuku.
For a second follow up year, I increased my requirement to 300k USD to give me more peace of mind. So the total donation so far is 244k, and if I reach a total of 300k USD, then I'll work on the project for a second year.
At 2M USD I retire and work on open STEM projects forever. At these timelines, I can't guarantee it will be specifically on education technology specifically all the way, but I guarantee that whatever it is it will be open and extremely well explained as usual.
It's necessary to be slightly underemployed if you are to do something significant - James Watson
Total donations to date ~244k USD. Donation breakdown:
More details: Section "Accounting method"
How to give:
And if you have a different preferred payment mechanism not listed above, please contact Ciro, and he will set it up.
Ciro's current ambitions require him to remain in developed countries, because Ciro wants to document advanced science and technology by liaising with top universities, and there is not nearly as much high technology in poor countries. Remaining in developed countries is also a required due to family reasons.
Note to potential anonymous crypto donors: I live in the UK, and after some messy back and forth that included freezing my account at one point, Barclays finally decided that they do not allow me to receive anonymous donations. And I'm pretty sure that the same would happen on any other British bank sooner or later. Therefore, while I continue to accept anonymous donations, they will not count towards my "if I get X I do Y"-type donation goals, since I can't reliably spend that money. If you want to donate in crypto, and clearly give me your real identity and an explanation of how you got the money, then that is fine, it can count. Just be warned that I will need to give that information to my bank and clear it with them beforehand, and if they are still not happy with it, I'll just give it all back. Your identity does not have to be publicly disclosed, only the dates and amounts. But my bank has to know.
If you would like public acknowledgement for your support, Ciro will very gladly give it, just let Ciro know how you'd prefer it. Due to Ciro Santilli's campaign for freedom of speech in China, many supporters have chosen to be anonymous, and that is totally fine, not everyone is interested in politics, or has a situation where going public is acceptable, so we don't have a standard setup yet, let's build it together. A acknowledgement section at the bottom of this page would be a minimum, but I for larger donations we could add a your advertisement in a locations such as:
100k USD/year is a semi arbitrary amount that sounds nice. My last day job total compensation as of 2024 was about 150k USD/year.
Video 1.
The problem with education by Ciro Santilli
. Source. In this video Ciro Santilli exposes his fundamental philosophy regarding why Education is broken. This philosophy was the key motivation behind the failed OurBigBook Project.
Video 2.
Intro to the OurBigBook Project
. Source.
Figure 1.
Ciro Santilli playing with a pipette at the University of Cambridge circa 2017
. Although totally disqualified for it, Ciro would really like to understand and explain cool scientific experiments in insane detail much as he does with computer software, related:
Maybe if he ever gets enough credibility, such opportunities would actually materialize. It could be a bit like Periodic Videos, but for molecular biology and physics, and backed by OurBigBook text/tree with minimal openly licensed videos. The fact that such opportunities are essentially impossible outside of the boredom of the university system is something we should really change about education.
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Education is broken Updated 2025-11-21
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Once Ciro was at a University course practical session, and a graduate was around helping out. Ciro asked if what the graduate did anything specifically related to the course, and they replied they didn't. And they added that:
One has to put the bread on the table.
Even though Ciro was already completely disillusioned by then, that still made an impression on him. Something is really wrong with this shit.
Other people that think that the educational system is currently bullshit as of 2020:
Video 1.
The problem with education by Ciro Santilli
. Source. In this video Ciro Santilli exposes his fundamental philosophy regarding why Education is broken. This philosophy was the key motivation behind the failed OurBigBook Project.
Figure 1.
Educational systems are carried by Indian YouTubers meme
. Source. Over a Dmitriy Khaladzhi carrying a horse over his shoulders meme template.
Video 2.
Peter Gregory from Silicon Valley shows his hate for university in a fake TED talk
. Source. Key moment: someone from the crowd cries:
The true value of a college education is intangible!
to which the speaker replies:
The true value of snake oil is intangible as well.
IMDb says it's not a cameo. It really looked like one, good acting, but what a missed opportunity. Imagine a Xavier Niel appearance.
Video 3.
David Deutsch on Education interviewed by Aidan McCullen (2019)
Source.
Key quote that hits the nail:
[...] the existing assumptions behind educational systems are that the purpose of education is to transmit valuable knowledge faithfully from on generation to the next. From people who already have that knowledge, to people who don't.
So the knowledge is conceived of as a kind of valuable fluid, which you pour from one generation to the next, pour it into their brains.
So right... the purpose of education is not to teach facts. The purpose of education is to propose ways of thinking, which students themselves must try to apply and decide if it suits them! And use the patterns of thinking that are useful to reach their goals.
Like Noam Chomsky, he proposes education has been a system of indoctrination more than anything else e.g. twitter.com/daviddeutschoxf/status/1406374921748496386:
All compulsory education, "tough" or not, "love" or not, in camps or not, and whether it "traumatises" or not, is a violation of human rights.
At twitter.com/DavidDeutschOxf/status/1051475227476185089 another good quote by Churchill:
Headmasters have powers at their disposal with which Prime Ministers have never yet been invested.
The same video also mentions in passing that john Wheeler used to be Deutsch's boss, but I can't find a reference for it very easily.
Video 4.
Quote selection by Charles Bukowski (2016)
Source.
Generally speaking, you're free until you're about 4 years old. Then you go to grammar school and then you start becoming... oriented and shoved into areas. You lose what individualism you have, if you have enough of course, you retain some of it... Then you work the 8 hour job with almost a feeling of goodness, like you're doing something. Then you get married like marriage is a victory, and you have children like children is a victory... Marriage, birth, children. It's something they have to do because there's nothing else to do. There's no glory in it, there's no steam, there's no fire. It's very, very flat... You get caught into the stricture of what you're supposed to be and you have no other choice. You're finally molded and melded into what you're supposed to be. I didn't like this.
Video 5.
Was MIT worth it? by Joseph Li
. Source.
Video 6.
I "Crashed Out" After Studying Physics In College by Alex Wei
. Source. A large part of his problem are High expectations Asian father issues. But society does absolutely nothing to help either, treating Education as an IQ test.
Video 7.
The decline of play by Peter Gray TEDxNavesink talk
. Source.
Video 8.
Can you get an MIT education for $2,000? by Scott Young
. Source.
Video 9.
Graduating from Chinese College by Arcnimation
. Source.
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Who are the developers that are making the most money through GitHub sponsors? Updated 2025-07-16
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No-go theorem Updated 2025-07-16
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ELF Hello World Tutorial / Global file structure Updated 2025-07-16
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An ELF file contains the following parts:
  • ELF header. Points to the position of the section header table and the program header table.
  • Section header table (optional on executable). Each has e_shnum section headers, each pointing to the position of a section.
  • N sections, with N <= e_shnum (optional on executable)
  • Program header table (only on executable). Each has e_phnum program headers, each pointing to the position of a segment.
  • N segments, with N <= e_phnum (only on executable)
The order of those parts is not fixed: the only fixed thing is the ELF header that must be the first thing on the file: Generic docs say:
Although the figure shows the program header table immediately after the ELF header, and the section header table following the sections, actual files may differ. Moreover, sections and segments have no specified order. Only the ELF header has a fixed position in the file.
In pictures: sample object file with three sections:
            +-------------------+
            | ELF header        |---+
+---------> +-------------------+   | e_shoff
|           |                   |<--+
| Section   | Section header 0  |
|           |                   |---+ sh_offset
| Header    +-------------------+   |
|           | Section header 1  |---|--+ sh_offset
| Table     +-------------------+   |  |
|           | Section header 2  |---|--|--+
+---------> +-------------------+   |  |  |
            | Section 0         |<--+  |  |
            +-------------------+      |  | sh_offset
            | Section 1         |<-----+  |
            +-------------------+         |
            | Section 2         |<--------+
            +-------------------+
But nothing (except sanity) prevents the following topology:
            +-------------------+
            | ELF header        |---+ e_shoff
            +-------------------+   |
            | Section 1         |<--|--+
+---------> +-------------------+   |  |
|           |                   |<--+  | sh_offset
| Section   | Section header 0  |      |
|           |                   |------|---------+
| Header    +-------------------+      |         |
|           | Section header 1  |------+         |
| Table     +-------------------+                |
|           | Section header 2  |---+            | sh_offset
+---------> +-------------------+   | sh_offset  |
            | Section 2         |<--+            |
            +-------------------+                |
            | Section 0         |<---------------+
            +-------------------+
But some newbies may prefer PNGs :-)
Figure 1.
ELF Executable and Linkable Format diagram by Ange Albertini
. Source.
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ELF Hello World Tutorial / Implementations Updated 2025-07-16
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  • Operating systems read and run ELF files.
    Kernels cannot link to a library nor use the C stlib, so they are more likely to implement it themselves.
    This is the case of the Linux kernel 4.2 which implements it in th file fs/binfmt_elf.c.
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ELF Hello World Tutorial / Object hd Updated 2025-07-16
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Running:
hd hello_world.o
gives:
00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  01 00 3e 00 01 00 00 00  00 00 00 00 00 00 00 00  |..>.............|
00000020  00 00 00 00 00 00 00 00  40 00 00 00 00 00 00 00  |........@.......|
00000030  00 00 00 00 40 00 00 00  00 00 40 00 07 00 03 00  |....@.....@.....|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000080  01 00 00 00 01 00 00 00  03 00 00 00 00 00 00 00  |................|
00000090  00 00 00 00 00 00 00 00  00 02 00 00 00 00 00 00  |................|
000000a0  0d 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000b0  04 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000c0  07 00 00 00 01 00 00 00  06 00 00 00 00 00 00 00  |................|
000000d0  00 00 00 00 00 00 00 00  10 02 00 00 00 00 00 00  |................|
000000e0  27 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |'...............|
000000f0  10 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000100  0d 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000110  00 00 00 00 00 00 00 00  40 02 00 00 00 00 00 00  |........@.......|
00000120  32 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |2...............|
00000130  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000140  17 00 00 00 02 00 00 00  00 00 00 00 00 00 00 00  |................|
00000150  00 00 00 00 00 00 00 00  80 02 00 00 00 00 00 00  |................|
00000160  a8 00 00 00 00 00 00 00  05 00 00 00 06 00 00 00  |................|
00000170  04 00 00 00 00 00 00 00  18 00 00 00 00 00 00 00  |................|
00000180  1f 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000190  00 00 00 00 00 00 00 00  30 03 00 00 00 00 00 00  |........0.......|
000001a0  34 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |4...............|
000001b0  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001c0  27 00 00 00 04 00 00 00  00 00 00 00 00 00 00 00  |'...............|
000001d0  00 00 00 00 00 00 00 00  70 03 00 00 00 00 00 00  |........p.......|
000001e0  18 00 00 00 00 00 00 00  04 00 00 00 02 00 00 00  |................|
000001f0  04 00 00 00 00 00 00 00  18 00 00 00 00 00 00 00  |................|
00000200  48 65 6c 6c 6f 20 77 6f  72 6c 64 21 0a 00 00 00  |Hello world!....|
00000210  b8 01 00 00 00 bf 01 00  00 00 48 be 00 00 00 00  |..........H.....|
00000220  00 00 00 00 ba 0d 00 00  00 0f 05 b8 3c 00 00 00  |............<...|
00000230  bf 00 00 00 00 0f 05 00  00 00 00 00 00 00 00 00  |................|
00000240  00 2e 64 61 74 61 00 2e  74 65 78 74 00 2e 73 68  |..data..text..sh|
00000250  73 74 72 74 61 62 00 2e  73 79 6d 74 61 62 00 2e  |strtab..symtab..|
00000260  73 74 72 74 61 62 00 2e  72 65 6c 61 2e 74 65 78  |strtab..rela.tex|
00000270  74 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |t...............|
00000280  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000290  00 00 00 00 00 00 00 00  01 00 00 00 04 00 f1 ff  |................|
000002a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000002b0  00 00 00 00 03 00 01 00  00 00 00 00 00 00 00 00  |................|
000002c0  00 00 00 00 00 00 00 00  00 00 00 00 03 00 02 00  |................|
000002d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000002e0  11 00 00 00 00 00 01 00  00 00 00 00 00 00 00 00  |................|
000002f0  00 00 00 00 00 00 00 00  1d 00 00 00 00 00 f1 ff  |................|
00000300  0d 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000310  2d 00 00 00 10 00 02 00  00 00 00 00 00 00 00 00  |-...............|
00000320  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000330  00 68 65 6c 6c 6f 5f 77  6f 72 6c 64 2e 61 73 6d  |.hello_world.asm|
00000340  00 68 65 6c 6c 6f 5f 77  6f 72 6c 64 00 68 65 6c  |.hello_world.hel|
00000350  6c 6f 5f 77 6f 72 6c 64  5f 6c 65 6e 00 5f 73 74  |lo_world_len._st|
00000360  61 72 74 00 00 00 00 00  00 00 00 00 00 00 00 00  |art.............|
00000370  0c 00 00 00 00 00 00 00  01 00 00 00 02 00 00 00  |................|
00000380  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000390
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ELF Hello World Tutorial / Program header table Updated 2025-07-16
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Only appears in the executable.
Contains information of how the executable should be put into the process virtual memory.
The executable is generated from object files by the linker. The main jobs that the linker does are:
  • determine which sections of the object files will go into which segments of the executable.
    In Binutils, this comes down to parsing a linker script, and dealing with a bunch of defaults.
    You can get the linker script used with ld --verbose, and set a custom one with ld -T.
  • do relocation according to the .rela.text section. This depends on how the multiple sections are put into memory.
readelf -l hello_world.out gives:
Elf file type is EXEC (Executable file)
Entry point 0x4000b0
There are 2 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000000d7 0x00000000000000d7  R E    200000
  LOAD           0x00000000000000d8 0x00000000006000d8 0x00000000006000d8
                 0x000000000000000d 0x000000000000000d  RW     200000

 Section to Segment mapping:
  Segment Sections...
   00     .text
   01     .data
On the ELF header, e_phoff, e_phnum and e_phentsize told us that there are 2 program headers, which start at 0x40 and are 0x38 bytes long each, so they are:
00000040  01 00 00 00 05 00 00 00  00 00 00 00 00 00 00 00  |................|
00000050  00 00 40 00 00 00 00 00  00 00 40 00 00 00 00 00  |..@.......@.....|
00000060  d7 00 00 00 00 00 00 00  d7 00 00 00 00 00 00 00  |................|
00000070  00 00 20 00 00 00 00 00                           |.. .....        |
and:
00000070                           01 00 00 00 06 00 00 00  |        ........|
00000080  d8 00 00 00 00 00 00 00  d8 00 60 00 00 00 00 00  |..........`.....|
00000090  d8 00 60 00 00 00 00 00  0d 00 00 00 00 00 00 00  |..`.............|
000000a0  0d 00 00 00 00 00 00 00  00 00 20 00 00 00 00 00  |.......... .....|
Structure represented www.sco.com/developers/gabi/2003-12-17/ch5.pheader.html:
typedef struct {
    Elf64_Word  p_type;
    Elf64_Word  p_flags;
    Elf64_Off   p_offset;
    Elf64_Addr  p_vaddr;
    Elf64_Addr  p_paddr;
    Elf64_Xword p_filesz;
    Elf64_Xword p_memsz;
    Elf64_Xword p_align;
} Elf64_Phdr;
Breakdown of the first one:
  • 40 0: p_type = 01 00 00 00 = PT_LOAD: this is a regular segment that will get loaded in memory.
  • 40 4: p_flags = 05 00 00 00 = execute and read permissions. No write: we cannot modify the text segment. A classic way to do this in C is with string literals: stackoverflow.com/a/30662565/895245 This allows kernels to do certain optimizations, like sharing the segment amongst processes.
  • 40 8: p_offset = 8x 00 TODO: what is this? Standard says:
    This member gives the offset from the beginning of the file at which the first byte of the segment resides.
    But it looks like offsets from the beginning of segments, not file?
  • 50 0: p_vaddr = 00 00 40 00 00 00 00 00: initial virtual memory address to load this segment to
  • 50 8: p_paddr = 00 00 40 00 00 00 00 00: unspecified effect. Intended for systems in which physical addressing matters. TODO example?
  • 60 0: p_filesz = d7 00 00 00 00 00 00 00: size that the segment occupies in memory. If smaller than p_memsz, the OS fills it with zeroes to fit when loading the program. This is how BSS data is implemented to save space on executable files. i368 ABI says on PT_LOAD:
    The bytes from the file are mapped to the beginning of the memory segment. If the segment’s memory size (p_memsz) is larger than the file size (p_filesz), the ‘‘extra’’ bytes are defined to hold the value 0 and to follow the segment’s initialized area. The file size may not be larger than the memory size.
  • 60 8: p_memsz = d7 00 00 00 00 00 00 00: size that the segment occupies in memory
  • 70 0: p_align = 00 00 20 00 00 00 00 00: 0 or 1 mean no alignment required. TODO why is this required? Why not just use p_addr directly, and get that right? Docs also say:
    p_vaddr should equal p_offset, modulo p_align
The second segment (.data) is analogous. TODO: why use offset 0x0000d8 and address 0x00000000006000d8? Why not just use 0 and 0x00000000006000d8?
Then the:
 Section to Segment mapping:
section of the readelf tells us that:
  • 0 is the .text segment. Aha, so this is why it is executable, and not writable
  • 1 is the .data segment.
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Noisy-channel coding theorem Updated 2025-07-16
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Setting: you are sending bits through a communication channel, each bit has a random probability of getting flipped, and so you use some error correction code to achieve some minimal error, at the expense of longer messages.
This theorem sets an upper bound on how efficient you can be in your encoding, for any encoding.
The next big question, which the theorem does not cover is how to construct codes that reach or approach the limit. Important such codes include:
But besides this, there is also the practical consideration of if you can encode/decode fast enough to keep up with the coded bandwidth given your hardware capabilities.
news.mit.edu/2010/gallager-codes-0121 explains how turbo codes were first reached without a very good mathematical proof behind them, but were still revolutionary in experimental performance, e.g. turbo codes were used in 3G/4G.
But this motivated researchers to find other such algorithms that they would be able to prove things about, and so they rediscovered the much earlier low-density parity-check code, which had been published in the 60's but was forgotten, partially because it was computationally expensive.
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