So, it appears that the irascible gods of Twitter granted us yet another feature that nobody wanted: downvotes! Except that I have no idea why the downvote button appears on certain tweets and not others, or what it does. I don't really want to know, but that's not my point.
My point is: THE FACT THAT THE DOWNVOTE BUTTON APPEARS ON CERTAIN TWEETS AND NOT OTHERS MEANS THAT THE “LIKE” BUTTON IS NOT ALWAYS IN THE SAME PLACE. And that, in turn, means that, inevitably, I'm going to downvote some tweets that I meant to like. Great job, guys!
PS: If you don't see a “downvote” button yet, it's because they are, apparently, still in the process of rolling it out. But be assured, like covid and all evil things, the downvote button will come to you as well.
(Actually, I suspect it's just a placebo. Which would be smart!)
Twitter: downvotes will “help inform us of the content people want to see”. Ah, so you mean you're going to ignore this feedback like you ignore feedback about everything you do and what your users want? Definitely just a placebo!
J'essaie de calculer la vitesse à laquelle ma moto (Yamaha Tracer 9) avance en fonction de la vitesse de rotation du moteur et j'arrive à un truc assez absurde. J'ai dû faire une erreur débile dans ce qui suit ⤵️, est-ce que quelqu'un a une idée de ce qui ne va pas? •1/9
Je vais faire le calcul pour la vitesse à la ligne rouge en 1er rapport. La ligne rouge est à 10600rpm, donc 10 600 rotations par minute, soit 177 tours par seconde. On est bien d'accord que c'est des tours de l'axe en sortie de moteur, ça? •2/9
Ensuite, il y a une démultiplication par la boîte de vitesse. Ce facteur m'est donné par Yamaha dans le manuel en fonction du rapport:
⁃ 1er: 2.571 (36/14)
⁃ 2e: 1.947 (37/19)
⁃ 3e: 1.619 (34/21)
⁃ 4e: 1.381 (29/21)
⁃ 5e: 1.190 (25/21)
⁃ 6e: 1.037 (28/27)
•3/9
Last week I got a lot of attention because of the claim that the “color of something infinitely hot” is (148,177,255) in sRGB space, which was reproduced by John C. Baez 🔽. Some have wondered as to the exact value, let me explain this and why. 🧵⤵️ •1/37
Now for some background: first, I did this computation back around 2005, when writing a page about colors and colorimetry — madore.org/~david/misc/co… — in my usual style of trying to understand something and writing all about it so I can forget it afterwards. •2/37
So if you want some more detailed explanations about colors and CIE matching functions and what “white” and so on mean, I refer to the page mentioned in the previous tweet. For more about the blackbody spectrum and the Rayleigh-Jeans law, see J. C. Baez's explanations. •3/37
If ℳ is a nonstandard model of PA (Peano arithmetic), with ℕ identified with the standard subset of ℳ, we can define six sets of subsets of ℕ which might be called “non-standard computable in ℳ”, namely:
ⓐ those X⊆ℕ for which there is e∈ℳ such that ∀i∈ℕ. ℳ⊧(e•i)↓ and ∀i∈ℕ. (i∈X⇔ℳ⊧(e•i)=1),
ⓑ those X⊆ℕ for which there is e∈ℕ such that ∀i∈ℕ. ℳ⊧(e•i)↓ and ∀i∈ℕ. (i∈X⇔ℳ⊧(e•i)=1),
ⓒ those X⊆ℕ for which there is e∈ℳ such that ∀i∈ℳ. ℳ⊧(e•i)↓ and ∀i∈ℕ. (i∈X⇔ℳ⊧(e•i)=1),
ⓓ those X⊆ℕ for which there is e∈ℕ such that ∀i∈ℳ. ℳ⊧(e•i)↓ and ∀i∈ℕ. (i∈X⇔ℳ⊧(e•i)=1),
Ces gens sont vraiment obstinés dans leur connerie. Tout le monde sait qu'imposer le port du masque en extérieur est inutile, ça peut même être nuisible, un juge administratif a suspendu la mesure idiote, mais ils veulent quand même persister. Mais POURQUOI???
Mais c'est surtout cette obstination dans la bêtise qui me laisse perplexe. La mesure n'est réclamée ni soutenue par personne, aucun scientifique, aucun groupe significatif, il n'y a pas de popularité à récolter à la prendre… je ne comprends vraiment pas.
This leads me to ask: is there a specific name for a morphism φ:A→B of commutative rings (or maybe just an inclusion) such that 𝔪 ↦ φ^−1(𝔪) defines a bijection between maximal ideals of B and those of A (i.e., Specmax(B) → Specmax(A))?
(This is the case of the inclusion of the ring of smooth real functions inside the ring of continuous ones in the context of the tweet cited above.)
There are a number of sub-questions or variants, here:
‣ When does (pullback by) φ take a maximal ideal to a maximal ideal?
‣ When does it define a bijection on the prime ideals?
‣ Does the conjunction of the two above imply that it defines a bijection on the maximal ideals?