Chris Purcell

Mathematician, computer scientist, bassist, knitter?

Last Notes

2026-01-21 16:14:35 GMT

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@nprofile…vhdw ok I'll see what I can do! I'll ask my colleague about printing the monochromatic version, we can always try the 2-colour version some time in the future.

2026-01-21 16:00:30 GMT

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@nprofile…tfwq He hasn't done it in school; we've mentioned square numbers before. This is conceptually similar: https://oeis.org/A014080 numbers which are the sum of the factorials of their digits.

2026-01-21 11:47:22 GMT

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@nprofile…vhdw I can't promise anything but I might be able to get one of these printed and given to prof Jendrol. I know someone with a 3d printer and while I don't know him personally, I have several short paths to him (e.g., he gave a talk here in Plzen in 2024). I don't know the first thing about 3d printing, is it very expensive these days?

2026-01-21 11:18:11 GMT

What are some examples of non-trivial finite sequences, whose definition could be understood by a child? I'll leave the definition of non-trivial vague, but as an example "positive integers less than 10" is trivial.
I mentioned that there are infinitely many prime numbers to the 7yo (he had heard that 67 is prime), and he said that that was obvious since there are infinitely many numbers. I think a good non-example would help here. I don't mind if the proof is too difficult.
#math

2025-08-24 07:12:15 GMT

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@nprofile…hszg @nprofile…pd3k I'm using the tusky app but I can mute for certain periods of time, so no need to remember. The options are 1h 6h 1d 7d 30d, it's not fully customisable. 1d or 7d should probably cover most threads.

2025-08-13 18:42:14 GMT

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@nprofile…fa4k do you (or any reader) think there's a reason for the east to west effect? Just chance or is it something to do with the way the image is generated. Long chats with chatgpt tend to degenerate in quality in my subjective experience. I could believe that something like that is at play here.

2025-08-05 13:40:26 GMT

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@nprofile…0tj7 one way to dispense with this unspoken assumption is by observing that 2/4 = 1/2 (or similar).

2025-06-18 10:24:14 GMT

Do people tend to underestimate or overestimate thirds? Does it vary a lot?
I consistently underestimate thirds; when tasked with cutting an orange into thirds I cut off what looks like a third then cut the remainder in half. The first third is consistently smaller.
Actually I cut oranges into sixths by first cutting an orange in half and cutting the halves into thirds. I am assuming this makes no difference? Would I do the same with rectangle? Not sure.

2025-04-17 08:23:59 GMT

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@nprofile…yu7x sorry should have just read to the end before commenting!! thanks.

2025-04-17 08:20:00 GMT

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@nprofile…fa4k I suppose that was too much to hope for... thanks for the link I will check it out! It's still a little odd to me that there are no obvious cones here, nor in @greganSF 's list of proofs. Am I just thinking about it in the wrong way? Can we extract some cone from the calculations or are all "fitting" cones sort of equivalent here? Perhaps there's some obvious correspondence between circles in 4d and cones in 3d I'm not getting.

2025-04-17 08:14:48 GMT

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@nprofile…yu7x wow thanks. Interestingly, this thought was planted in my head at our local science museum-type-thing-for-kids (Techmania in Plzen), where they have a big cone full of blue liquid you can rotate to see the conic sections. Right next to it they have an ellipse-shaped pool table to demonstrate the "whispering gallery" property. And they have multiple real "whispering galleries" achieved by parabolic dishes etc.

2025-04-17 07:03:02 GMT

Here's a stupid #geometry / #astronomy question. Planetary orbits are ellipses (under the obvious simplifying assumptions) which are conic sections. So what's the cone? There are infinitely many cones that fit of course. But is there one that best explains? To put it another way, is there a neat geometric argument that an orbit should be an ellipse, that doesn't require too much physics? #mathematics

2025-03-21 15:01:28 GMT

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@nprofile…6kk0 "Deathrow"

2025-03-20 10:03:29 GMT

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@nprofile…fa4k it's an extreme case, and macabre, but the Warsaw Ghetto Hunger Study comes to mind. I hope (and believe) conditions in America will not get that bad, but it's an example of hunkering down for sure. They also managed to overcome a typhus epidemic, which is rather topical. The wikipedia page has references, but I haven't read them to be honest.