Gro-Tsen Profile picture
23 Jan, 4 tweets, 1 min read
Given a topological space X, can we find another topological space ∇X (ideally in a functorial way…) so that continuous real-valued functions on ∇X coincide with locally constant real-valued functions on X?
mathoverflow.net/q/382011/17064
In more sophisticated (i.e., cryptic) terms, does the inclusion functor of the full subcategory of P-spaces (in the sense of Gillman-Henriksen) have a left adjoint?
But with or without sophistication, I don't know whether the ring of locally constant real-valued functions on ℚ can be written as the ring of (all) continuous real-valued functions on some topological space!
Oh, I already received a very nice answer to my question:
‣ the uniform norm can be defined purely algebraically on C(X),
‣ C(X) is always complete for that norm,
‣ but this is not the case for the ring of locally constant functions on ℚ, so it's not a C(X) for any X.

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More from @gro_tsen

24 Jan
Il est temps que je le reconnaisse publiquement: j'ai eu tort. J'ai eu, de façon répétée, par optimisme excessif, complètement tort. ⤵️ •1/36
Quand la Chine a confiné des dizaines de millions de personnes, il y a un an, j'ai été horrifié par la barbarie du procédé. Je me suis dit «ce sont des méthodes dignes d'un régime totalitaire: quand l'épidémie viendra en Europe, nous réagirons différemment». •2/36
Quand l'Italie a confiné toute la Lombardie, puis tout le pays, en mars 2020, avec des mesures de plus en plus fortes, j'ai été terrifié. Mais je me suis dit: ils sont débordés, ils ne savent pas ce qu'ils font, mais la raison l'emportera sûrement. •3/36
Read 36 tweets
21 Jan
De nombreux médias ont repris cette information selon laquelle le covid a fait baisser l'espérance de vie de plusieurs mois. Ce n'est pas exactement «faux», mais c'est EXTRÊMEMENT trompeur. Image
L'espérance de vie est quelque chose qui a un sens en régime permanent. Calculer l'espérance de vie sur une année exceptionnelle, c'est techniquement possible, mais ça donne une impression extrêmement fallacieuse.
J'ai expliqué ça un peu plus longuement dans ce fil (en anglais): — disons pour résumer que cette baisse d'espérance de vie correspondrait à ce qui se produirait si la même chose se produisait CHAQUE ANNÉE qu'en 2020.
Read 4 tweets
21 Jan
J'ai eu le malheur d'avoir eu besoin de passer dans mon bureau sur le plateau de Saclay au moment où Macron était dans le coin à se gargariser de son «plan quantique» juste de l'autre côté de la rue. Petit récit et quelques réflexions sur les déplacements présidentiels. ⤵️ •1/36
Je vais m'abstenir de dire ce que je pense de la conception du pilotage de la recherche consistant à choisir un buzzword (blockchain, intelligence artificielle, informatique quantique…) et à inciter tout le monde à se précipiter dedans pour essayer de récupérer les sous. •2/36
Disons juste que ma conception de la science ce serait plutôt de cultiver le maximum de variété dans les sujets explorés, à la fois pour préserver la diversité écologique de la recherche conçue comme un organisme vivant et interdépendant, … •3/36
Read 36 tweets
21 Jan
Two years later, this bug, which prevents opening single-page DjVu files with evince, is still present in Ubuntu 20.04 “Focal”, and now also in Debian 10 “Buster”. Despite trivial analysis, existence of a patch, and appropriate bug reports.
To wit, here is the bug-report in Ubuntu, which obviously NOBODY CARES ABOUT DESPITE THERE BEING AN OBVIOUS FIX IN THE BUG-REPORT: bugs.launchpad.net/ubuntu/+source…
The problem is:
⁃ for some idiotic reason, the MIME type of single-page DjVu files is reported as “image/vnd.djvu” and multipage DjVu as “image/vnd.djvu+multipage”,
⁃ for some idiotic reason, evince refuses to open the former (because of an explicit patch to break it‽),
Read 4 tweets
20 Jan
#TIL that the oaths of office for the president and vice-president of the United States are quite different. The president is supposed to “preserve, protect and defend”, whereas the vp will “support and defend”. Also, the vp's oath mentions “enemies”, not the president's.
The vp makes an oath of allegiance which the president does not. The president will “faithfully execute the Office of President” (does one execute an office? that's a bit strange, isn't it?) whereas the vp will “faithfully discharge the duties of the office”.
And, perhaps more importantly, the president vows to do all this “to the best of [their] abilities”, whereas the vp will do this “without any mental reservation”.

I get the reason for these differences: one oath is in the Constitution, the other is in a 19th century law.
Read 4 tweets
19 Jan
I stumbled upon this paper which performs multivariate linear regression analysis to try to infer the effectiveness of various policy measures on covid-19 mortality.
ajtmh.org/content/journa…
One of their conclusions is that mask mandates are very effective. Another is that lockdown duration has no statistically significant effect (the model doesn't even conclude whether the effect is positive or negative). Image
I'm very skeptical as to whether we can draw any useful conclusion from such multivariate linear regressions between countries. There will be simply too many unknown variables and confounders. But this isn't really my point here.
Read 7 tweets

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