@antoinechambertloir@mathstodon.xyz avatar

antoinechambertloir

@antoinechambertloir@mathstodon.xyz
  • Apprentice mathematician (Professor at Université Paris Cité),
    interested in algebraic geometry, number theory and (more recently) proof formalization
  • Apprentice musician (drums, tabla, cajon; previously Diderock; a rarely meeting jazz trio/quartet).
  • Apprentice progressist.

In love with math, jazz, indian music and poetry. ⏚

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ciredutempsEsme, to random French
@ciredutempsEsme@mamot.fr avatar

La retraite par capitalisation est un ponzi. Ça ne marche pas.
Sans génération suivante pour produire ce dont vous avez besoin ou s'il advient une grosse inflation, votre argent vaut zéro.

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@ciredutempsEsme
Avant les années 30, la retraite était par capitalisation, la crise de 29 a montré les limites du système et les 15 années suivantes ont servi à imaginer le système mis en place en 41 puis en 46...

antoinechambertloir, to random French
@antoinechambertloir@mathstodon.xyz avatar

P… L'ENS nous écrit « Transformez votre impôt en impact ». Du coup, je leur ai envoyé une petite avoinée:

De la part d'une fondation adossée à un établissement d'enseignement supérieur public, je me permets de vous écrire que je trouve votre slogan particulièrement malvenu.

Le principe des fondations contrevient explicitement au principe général de non-affectation de l'impôt.

L'impôt général finance les infrastructures publiques, dont l'enseignement supérieur et la recherche, de la façon dont la représentation nationale et le gouvernement décident de l'organiser, qu'on approuve ou non la politique mise en place.

Il est donc en particulier responsable de mon salaire (honorable) de des conditions de travail (moins luxueuses) de l'universitaire que je suis devenu grâce à l'École normale supérieure.

Par corollaire, la multiplication des fondations et niches fiscales qui permettent d'affecter cet impôt est responsable de la dégradation des conditions de vie de la fonction publique française, et je ne vois pas pourquoi j'aggraverai un peu plus cette situation.

Bien à vous

krazykitty, to random French
@krazykitty@mamot.fr avatar

Et à la septième anche, il vit que cela était bon et il se reposa.

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@krazykitty laisse tes mains sur mes anches...

MartinEscardo, to random
@MartinEscardo@mathstodon.xyz avatar

What is a topological space?

It is a mathematical device to define what a continuous function is, in a general setting.

  1. A topological space is a set X together with a collection of subsets of X, called open, such that finite intersections of open sets are open, and arbitrary unions of open sets are open.

  2. A function of topological spaces is continuous if inverse images of open sets are open.

What is the intuition behind (1) and (2)?

I claim that it is better to ask, instead, how mathematicians came up with (1) and (2).

1/

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@boarders @joshuagrochow @MartinEscardo
Indeed, it was already a surprise when Serre proved that the Zariski topology could recover the sheaf cohomology of coherent sheaves ,(FAC and GAGA papers), but that did not furnish the “correct” cohomology groups that would help proving the Weil conjectures (along the way suggested by Weil himself).

At some point, Serre observed that “algebraic fiber spaces” were not locally trivial for the Zariski topology, but we're made trivial after passing to an étale covering.
He made a talk at the Chevalley seminar about it

Grothendieck was there and, at the end of the talk, told Serre that they had (« on a ») the Weil cohomology.

http://www.numdam.org/item/SCC_1958__3__A1_0/

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@johncarlosbaez @boarders @joshuagrochow @MartinEscardo
There is no such phonetic proximity in french, revêtement et recouvrement are quite different words. However, both Serre and Grothendieck had thought a lot about topology, having thought about fiber spaces in particular.

In his PhD thesis, Serre generalized the concept of Hurewicz fibration to what we now call Serre fibration— requiring the lifting property for maps from cubes only.

The first work of Grothendieck in algebraic geometry was in 1955, about fiber spaces.

https://webusers.imj-prg.fr/~leila.schneps/grothendieckcircle/Kansasnotes.pdf

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@johncarlosbaez
Actually I never remember what is a cover and what is a covering!!!

@boarders @joshuagrochow @MartinEscardo

ColinTheMathmo, to random
@ColinTheMathmo@mathstodon.xyz avatar

Asking for a friend(*) ...

"I need to brush up on some Pure Maths for a thing, and I find I'm pretty rusty. Can any of m'Maths friends recommend a good book on rings and ideals, and that end of algebra?"

(*) No, really ...

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@ColinTheMathmo I wrote mine, (mostly) commutative algebra, with part of this profile in mind. My Field guide on algebra goes towards Galois theory but has less general algebra in it, it tries to bypass that.

christianp, to random
@christianp@mathstodon.xyz avatar

Watching the endless patience of Davide Cervone on the mathjax-users mailing list gives me something to aspire to

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@christianp by the way, what do you use to render math/latex on mathstodon? There are several other systems competing with mathjax, is there any reason for choosing one over another?

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@christianp I had the impression there had been another one.
The main interest of KaTeX seems to be its speed, isn't it? but maybe that changed a few years after.

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@christianp searching a little bit, I found jqmath and mathquill

https://mathscribe.com/author/jqmath.html
http://mathquill.com/

jeeynet, to random French
@jeeynet@framapiaf.org avatar

« Un adolescent est un mineur. Donc les parents ont le droit de fouiller dans le téléphone », a asséné Sabrina Agresti-Roubache, se disant stupéfaite que l’on puisse « penser qu’un adolescent a une vie privée ».

https://www.numerama.com/tech/1731624-un-parent-a-t-il-le-droit-de-fouiller-dans-le-telephone-de-son-enfant.html

Stupéfiant de stupidité. C'est épouvantable. Courage à cette jeunesse sur laquelle crache quotidiennement ce gouvernement

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@jeeynet
De stupidité, je ne sais pas; d'incompétence en matière juridique, certainement; de brutalité et de condescendance, encore plus.

johncarlosbaez, (edited ) to random
@johncarlosbaez@mathstodon.xyz avatar

The 'hexagonal tiling honeycomb' is a beautiful structure in 3-dimensional hyperbolic space. I'm trying to figure out something about it.

It contains infinitely many sheets of hexagons, tiling planes in the usual way hexagons do. These are flat Euclidean planes in 3d hyperbolic space, called 'horospheres'. I want to know the coordinates of the vertices of these hexagons. I have some clues.

The hexagonal tiling honeycomb has Schläfli symbol {6,3,3} . The Schläfli symbol is defined in a recursive way. The symbol for the hexagon is {6}. The symbol for the hexagonal tiling of the plane is {6,3} because 3 hexagons meet at each vertex. Finally, the hexagonal tiling honeycomb has symbol {6,3,3} because 3 hexagonal tilings meet at each edge!

The symmetry group of the hexagonal tiling honeycomb is the Coxeter group {6,3,3}. This is a discrete subgroup of the Lorentz group O(3,1), which acts on 3d hyperbolic space because that space is the set of points (t,x,y,z) in Minkowski spacetime with

t² − x² − y² − z² = 1 and t > 0

The Coxeter group {6,3,3} is generated by reflections, but its 'even part', generated by pairs of reflections, is a discrete subgroup of PSL(2,ℂ), because this is the identity component of the Lorentz group. In fact, this Coxeter group is almost PSL(2,𝔼), where 𝔼 is the ring of 'Eisenstein integers'. These are complex numbers of the form

a + bω

where a,b are integers and ω is a nontrivial cube root of 1. So there should be a nice description of the hexagonal tiling honeycomb using Eisenstein integers! And this is what I'm trying to find... quickly, before May 1st because I'm have a column due then. 😧

I should ask @roice3, who drew this....

(1/n)

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@johncarlosbaez @gregeganSF
Yeah, that's already like this for the hyperbolic plane (and maybe easier to visualize).

ciredutempsEsme, to random French
@ciredutempsEsme@mamot.fr avatar

Est ce qu'il y a des profs qui aiment corriger les copies ?
(Les profs de maths peut-être ?)

antoinechambertloir,
@antoinechambertloir@mathstodon.xyz avatar

@ciredutempsEsme
Pas moi en tout cas.
Je crois que je déteste surtout l'espèce de dialogue de sourds que constitue la paire devoir/correction. Un dialogue élève/professeur est une chose vitale, mais requiert disponibilité et liberté. Aucune des parties ne l'a.

Gala, to random French
@Gala@ludosphere.fr avatar

deleted_by_author

    •
    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar

    @Gala
    Cette part d'héritage et ce livret A seraient deux raisons raisonnables de demander à ton parent de te prêter l'argent à 0%, mais j'ai cru comprendre que c'était impossible à faire… (Et si, d'autorité, tu prenais l'argent nécessaire sur le livret A, quitte à le remettre ensuite?)

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar

    @SadKitten @Gala
    (Sous-titre, désolé pour le mansplaining éventuel, mais je viens de comprendre : je crois qu'on n'a droit qu'à un seul livret A par personne, donc l'existence de ce livret A t'interdit de fait d'en avoir un à toi…)

    antoinechambertloir, to random French
    @antoinechambertloir@mathstodon.xyz avatar
    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar
    antoinechambertloir, to random French
    @antoinechambertloir@mathstodon.xyz avatar

    @libe vient de publier plusieurs articles autour de la légion d'honneur (sic) récemment décernée par Emmanuel Macron à Thierry Ardisson. Je ne savais pas ce qui était arrivé à Christine Angot sur ce plateau de télévision, pas plus que je ne connaissais l'histoire de Nelly Arcan, et les expériences similaires, effrayantes, qu'elle avait subies sur le plateau d'Ardisson et sur le plateau canadien de Guy Lepage.
    Ces gens sont des criminels.
    Angot y a survécu, Arcan non…

    HydrePrever, to random
    @HydrePrever@mathstodon.xyz avatar

    Chaque fois que je fais un tour sur le marché des Lices, j'ai une phase "c'est bon y en a marre on déménage et je vais turboter" et puis après je me rends à l'évidence et je commence à ourdir des plans pour vitrifier la totalité de Paris et des deux premières couronnes

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar

    @krazykitty @HydrePrever
    Idem.

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar

    @HydrePrever @krazykitty
    Je ne sais pas dire.
    Monsieur est un actuel collègue, prof et turbote, et madame une ancienne collègue, maîtresse de conférences, et travaille sur place.

    antoinechambertloir, to jazz French
    @antoinechambertloir@mathstodon.xyz avatar

    There was a guy named Maxwell,
    who was born on that day, 100 years ago,
    even if his birth certificate doesn't say so.
    His second name was Lemuel.
    Whatever — everybody calls him Max.
    Let's embark for a 100 days celebration
    with Max Roach, here on Mastodon.
    You know what to do? — Follow the hashtag…

    #Jazz #MaxRoach100

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar
    1. Monk's Dream

    This is such a great concert.
    On a classic theme by Thelonious Monk, these two musicians just play. And laugh!

    https://www.youtube.com/watch?v=ytJMnHbG4D8

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar
    1. Statements

    This is probably the last recording by Max Roach, a 2002 duet album with trumpet player Clark Terry.
    It seems like these two musicians have blues flowing in their veins.
    (Only that track is available on YT, alas…)

    https://www.youtube.com/watch?v=3So8MoRpi9g

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar
    1. (Untitled performance)

    “New drum music is made in the culture? Of course, Max Roach is here.”
    In 1983, Max Roach met hip hop pioneer Fab 5 Freddy at The kitchen.
    From be bop to hip hop, Max Roach was there to create groove.

    https://www.youtube.com/watch?v=rDwLmMkxqGk

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar
    1. Fallen Feathers

    I wanted to find a copy a of a tune, “Fallen Petals”, that Max Roach recorded in 1999 with his “Beijing Trio”. Since that album does not seem to be available anywhere, let's go back to 1955 and listen to these Fallen Feathers, composed and arranged by Quincy Jones for the Cannonball Adderley octet.

    A gorgeous while simple arrangement of a beautiful ballad.

    https://www.youtube.com/watch?v=NzXWc19kUEg

    antoinechambertloir,
    @antoinechambertloir@mathstodon.xyz avatar
    1. Joy Spring

    A wonderful Clifford Brown composition from the 1954 album led by him and Max Roach.

    Each of the choruses is a marvel of musicality.

    https://www.youtube.com/watch?v=dnK6OHPQZbA

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