The Intellectual Wilderness There is nothing more useless than doing efficiently that which should not be done at all.

2019.11.21 17:24

Testing Textually Composed Numbers for Primality

Filed under: Computing,Science & Tech — Tags: , , , , , — zxq9 @ 17:24

Last night on Twitter one of my favorite accounts, @fermatslibrary, put up an interesting post:

Start at 82 and write it down, then 81, write that down, etc. until you reach one, and you’ve written a (huge) prime number. Wow!

This seemed so strange to me, so of course I got curious:

After some doodling around I wrote a script that checks whether numbers constructed by the method above are prime numbers when the number construction is performed in various numeric bases (from 2 to 36, limited to 36 because for now I’m cheating using Erlang’s integer_to_list/2). It prints the results to the screen as the process handling each base finishes and writes a text file “texty_primes-result-[timestamp].eterms” at the end for convenience so you can use file:consult/1 on it later to mess around.

There are a few strange aspects to large primes, one of them being that checking whether or not they are prime can be a computationally intense task (and nobody knows a shortcut to this). To this end I wrote the Miller-Rabin primality test into the script and allow the caller to decide how many rounds of Miller-Rabin to run against the numbers to check them. So far the numbers that have come out have matched what is expected, but once the numbers get extremely large (and they get pretty big in a hurry!) there is only some degree of confidence that they are really prime, so don’t take the output as gospel.

I wrote the program in Erlang as an escript, so if you want to run it yourself just download the script and execute it.
The script can be found here: texty_primes

A results file containing the (very likely) prime constructions in bases 2 through 36 using “count-back from X” where X is 1 to 500 can be found here: texty_primes-result-20191121171755.eterms
Analyzing from 1 to 500 in bases 2 through 36 took about 25 minutes on a mid-grade 8-core system (Ryzen5). There are some loooooooooong numbers in that file… It would be interesting to test the largest of them for primality in more depth.

(Note that while the script runs you will receive unordered “Base X Result” messages printed to stdout. This is because every base is handed off to a separate process for analysis and they finish at very different times somewhat unpredictably. When all processing is complete the text file will contain a sorted list of {Base, ListOfPrimes} that is easier to browse.)

An interesting phenomenon I observed while doing this is that some numeric bases seem simply unsuited to producing primes when numbers are generated in this manner, bases that themselves are prime numbers in particular. Other bases seem to be rather fruitful places to search for this phenomenon.

Another interesting phenomenon is the wide number of numeric bases in which the numbers “21”, “321”, “4321” and “5321” turn out to be prime. “21” and “4321” in particular turn up quite a lot.

Perhaps most strangely of all is that base 10 is not a very good place to look for these kinds of primes! In fact, the “count back from 82” prime is the only one that can be constructed starting between 1 and 500 that I’ve found. It is remarkable that anyone discovered that at all, and also remarkable that it doesn’t happen to start at 14,562 instead of 82 — I’m sure nobody would have noticed this were any number much higher than 82 the magic starting point for constructing a prime this way.

This was fun! If you have any insights, questions, challenges or improvements, please let me know in the comments.

2019.10.17 10:58

“Kintergarten Cop”: Amazing Efficiency

Filed under: Films and Media — Tags: , — zxq9 @ 10:58

I recently re-watched Kindergarten Cop, a 1990’s action comedy with Arnold Schwarzenegger where he (duh) plays a hard-boiled cop who becomes a kindergarten teacher for a while.

Aside from the kid in class who utters the infamous line about how one can identify between boys and girls (his father is a gynecologist, see?) and how utterly EVIL such a statement of common sense knowledge being uttered by a child would be in today’s world of identity warfare, the main thing that stuck out to me was how incredibly efficient the story was.

The movie followed the law of Chekhov’s Gun to the extreme. Every single device introduced was actually followed up on. Every setup had a payoff. Every joke had a setup (whoa!). Every bit of “Oh, I wonder if…” detail in the movie was actually used. It was sort of amazing to watch just how incredibly tidy the entire production was.

I haven’t looked into who was involved in the production, but the level of skill involved in creating such a tight, concise bit of storytelling (even if predictable and leaning hard on a few somewhat worn-out character tropes, even by 1990’s standards) indicates a vast experience in either writing efficient stories or maybe a master editor involved at the end.

I wish I was that clean about designing systems!

2019.09.11 12:09

H-IIB No.8: Fire on the launch pad

Filed under: Space — Tags: , , , , , — zxq9 @ 12:09

There was a fire on the launch pad today during the final preparation for launch of MHI’s H-IIB No.8, sending HTV-8 to the International Space Station.

Not much is known about the cause yet, but the really amazing thing is that the rocket didn’t blow up, safety systems all worked as designed, and they will get a chance to try again soon.

A (very poor) screen capture from a printout given at the post-cancellation debriefing showing the location of the fire relative to the rocket.

Video of the launch attempt (indexed just before the fire becomes visible):

Press conference (Japanese language only — if you have any questions ask in comments and I can translate):

2019.08.3 05:20

Building Erlang 22.0 on Debian/Ubuntu

Filed under: Computing,Science & Tech — Tags: , , — zxq9 @ 05:20

Every time I switch to a new system and have to build a new release of Erlang with kerl I sit and scratch my head to remember which dependencies are required. Once you’re set up or have a prep script it is just too easy to forget which thing is needed for what over the next few years.

Here is my list of pre-build package installs on Ubuntu 18.04 — note that they are in three groups instead of just being a single long apt install command (why apt couldn’t manage to install these all at once is beyond me…):

Group1:

  • gcc
  • curl

Group2:

  • g++
  • dpkg-dev

Group 3:

  • build-essential
  • automake
  • autoconf
  • libncurses5-dev
  • libssl-dev
  • flex
  • xsltproc
  • libwxgtk3.0-dev

2019.07.23 08:56

Erlang: R22.0 doc Mirror Updated

Filed under: Computing — Tags: , , , , , — zxq9 @ 08:56

The Erlang doc mirror linked here has been updated to include the R22.0 docs.

Note that some of the internal links and labels say “ERTS-10.4” and “Version 10.4” instead of “ERTS-11.0” and “Version 11.0”. This is an error. The docs refer to ERTS 11.0 but that detail seems to not have been updated when these docs were generated (whoops!). I was looking at fixing that throughout the docs and links, but it turns out to be a lot more complicated than I’m willing to deal with because of the number of references that include the string “10.4” (and some of them are in PDFs and other things more annoying to update than HTML pages). When the R22.1 docs come out that will probably be fixed and I’ll update to avoid confusion in the distant future.

2019.06.26 09:40

Before the Fall

Filed under: Society — Tags: , , , , — zxq9 @ 09:40

Times are bad. Children no longer obey their parents, and everyone is writing a book.

Marcus Tullius Cicero (106-43 BCE)

2019.06.22 23:20

Social Prophecy

If you want to feel creeped out in 2019…

Backstory setting for the Cyberpunk 2020 game rulebook Neo Tribes: The Nomads of North America, published in 1994.
Sagan nailed it in The Demon Haunted World.

Mojibake3

Filed under: Uncategorized — zxq9 @ 14:29

After updating the MySQL tables to a UTF8 multibyte encoding…

どうなるかな?

Hopefully that isn’t just a bunch of nonsense.

UPDATE:

WOW! Now things work like they are supposed to!
Unfortunately, however, it seems that all of my previous posts that used UTF-8 are lost… and I have no idea how to recover them. That’s super disappointing.

DDoS

Filed under: Society — Tags: , , , , — zxq9 @ 04:37

My site has been under sustained attack for the last week.
I don’t really care, I just think it is silly.

The main question that has been floating through my mind is, strangely, “When did ‘pwned’ revert back to ‘owned’ for the sake of normies?”

2019.06.20 22:39

Perspective

Filed under: Science & Tech — Tags: , , , , — zxq9 @ 22:39

SpaceX has made rocket booster landings so routine they are almost boring for anyone other than a space tech geek (but I’m a space tech geek, so there’s that). Blue Origin has a full-blown lunar cargo and machinery ferrying service in the works. The first 60 Starlink satellites have already launched. Acoustic simulation is now essentially a solved problem. VR latencies are finally being measured in single milliseconds. AMD is rapidly approaching its goal of having 1000+ cores available on the consumer end of computing devices.

These aren’t prospective projects, they are in development now and most of the underlying tech already works now. It is an absolutely amazing time to be alive.

And yet today I saw a picture of a US Navy destroyer mounted completely dry, high above the water, on a specialized cargoship… (a “wet” drydock? I have no idea what this is even called) and in spite of all the amazing things going on elsewhere in technology I am totally stunned at this. I’m sure it is just a normal thing for the operators of such craft to see, but wow that’s amazing:

ZOMG!
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