Mark Dominus

I'm an expert computer programmer, looking for work. Check out my résumé: https://plover.com/~mjd/cv/Mark%20Jason%20Dominus.pdf

Also, an amateur mathematician, but not the angle-trisecting kind.

Good blog: https://blog.plover.com/
Crappy blog: https://shitpost.plover.com/
Math.SE: https://math.stackexchange.com/users/25554/mjd?tab=profile
Totemic animal spirit: Octopus

Last Notes

2026-09-09 00:26:34 UTC

Today I learned about the figure-8 immersion of the Klein bottle. Instead of starting with a tube and then sticking the tube through the wall of the tube to get it to join up in the opposite direction, you start with a double tube whose cross-section has the shape of a figure 8. Then you bend the double-tube around into a circle like a hula hoop and join the ends, but you give the double-tube a half-twist as you bend it, so that it turns into a single tube with a half twist.
The self-intersection is still a circle, but instead of being the place where the tube punches through the wall of the bottle, it's the set of crossing points of the figure-8s.
https://en.wikipedia.org/wiki/Klein_bottle#The_figure_8_immersion8
https://media.mathstodon.xyz/media_attachments/files/117/238/230/964/291/162/original/dbbed12ee840b406.png
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2026-07-18 13:34:07 UTC

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I was delighted to learn recently that in order to resign, a Member of Parliament needs the approval of the Chancellor of the Exchequer, approximately the equivalent of the U.S. Secretary of the Treasury.
I learned the reason why: MPs are not legally allowed to resign; this dates from a time when it was a troublesome duty that many people wanted to avoid. So when a modern MP does want to resign, it is accomplished by having the Chancellor appoint them to a post under the Crown, and officers of the Crown are disqualified from serving in Parliament.
One of the two obsolete posts they use for this legal maneuver is called the “Crown Steward and Bailiff of the Chiltern Hundreds”. Wikipedia has a list of its officeholders over the centuries which includes the eight who were all appointed on 17 December 1985, each immediately resigning the post to make way for the next appointee.
I hope you found this more amusing and interesting than annoying.
(In practice, the Chancellor almost never withholds consent.)
https://en.wikipedia.org/wiki/List_of_stewards_of_the_Chiltern_Hundreds

2026-06-16 20:13:42 UTC

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How *do* I edit a text file that contains Arabic, Markdown and text in Latin script?
Emacs is really, really not doing it—it seems to treat some lines as RTL with embedded LTR segments and others as LTR with embedded RTL segments, and the meaning of a key like right-cursor might mean either “next logical character” or “previous logical character” depending on the sense of the current line.
And it doesn't display the text in the order it will actually appear! (See below.)
Combined with the terminal's inability to even render the glyphs correctly when they have the vowel marks, it makes editing nearly impossible. In the Emacs screencap below, the cursor is the yellow rectangle. Is it predictable which character will be deleted when I press DEL? I'm not sure it is and in any case I keep being surprised.
I did a quick Google search and it produced a supremely unhelpful Reddit thread: “which would handle both scripts without too much hassle?” asked OP, and the top response is “I would say all of them. Did you try any?”
https://media.mathstodon.xyz/media_attachments/files/116/761/593/382/328/391/original/e452f436f9ae36bb.png
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2026-06-16 12:02:25 UTC

Why doesn't #Kitty render this #Arabic text correctly? I don't know Arabic, but even I know this ain't Arabic. None of the three phrases is even remotely correct. Many of the letters aren't even letters. (What's Arabic for “mojibake”?)
Font problem? Terminal problem? Emacs problem?
I can confirm that the data *in* the file is correct, because it renders correctly in Firefox.
https://media.mathstodon.xyz/media_attachments/files/116/759/662/420/140/376/original/3cd3223f6a06598d.png
https://media.mathstodon.xyz/media_attachments/files/116/759/666/735/577/532/original/c739c9415466a40b.png

2026-05-12 13:19:58 UTC

Today I learned the geometry term “antiparallel lines”.
And also, that parallel lines are antiparallel 😠.
https://en.wikipedia.org/wiki/Antiparallel_lines

2026-02-11 16:51:10 UTC

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Check out "theoretical gore" in this illustration. (Upper right.) "Gore" is actually a rather old word for a triangular-shaped piece of land. (Also a for triangular-shaped piece of fabric in a garment, and more generally for anything spear-shaped.)
In highway design, the “theoretical gore” is the triangular part of the road between the main way and the exit way, the part with white strips that you're not supposed to drive on.
The “physical gore” is the sort of armpit place _off_ the road that you will crash into if you keep driving on the theoretical gore.
I ❤️ #jargon
https://media.mathstodon.xyz/media_attachments/files/116/053/019/984/505/538/original/47e19d993881f0fc.png

2026-02-11 16:42:52 UTC

I was skimming through the Manual on Uniform Traffic Control Devices yesterday, and thinking, as I often do, that one of the finest things in the U.S. is our highway signage. No joke, I am really proud of our achievements here. They are a triumph of civic administration and a valuable application of our affluence for the public good.
(The manual is available online for free and will be of great delight to many varieties of nerd: https://mutcd.fhwa.dot.gov/kno_11th_Edition.htm )
And then I thought it's probably only a matter of time until the current administration figures out a way to line their pockets with highway sign funds, announces that highway signs are Fake News and Woke and fucks them up somehow or eliminates them entirely.
https://media.mathstodon.xyz/media_attachments/files/116/052/990/000/172/406/original/75921a9bac21279c.png

2025-12-28 18:37:44 UTC

There are six possible axial orientations of an almond in a nutcracker and to my mind one of them is clearly superior to the others.
(Inspired because yesterday I met a grown man who did not know how to crack an almond.)
https://media.mathstodon.xyz/media_attachments/files/115/798/642/328/774/545/original/e059158be6ee1913.jpg
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2025-12-15 22:43:32 UTC

I'm sitting on the sofa watching a recording of a talk I gave in 2014 and I realize that in the video I'm wearing the same sweater I have on right now.

2025-11-10 17:58:48 UTC

I haven't looked at the Higher-Order Perl errata pages for years, so I had forgotten this detail. All the other pages on the site have a picture of the book cover. But each errata page has instead a random selection from a set of 17 erroneous covers. Some examples:
https://media.mathstodon.xyz/media_attachments/files/115/526/694/930/724/578/original/234d8d6179c094bc.jpg
https://media.mathstodon.xyz/media_attachments/files/115/526/694/955/026/502/original/15139d7daaa763e7.jpg
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