Fibred sites and existential toposes

I have just uploaded to the ArXiv a new paper, entitled “Fibred sites and existential toposes“.

The paper introduces, the context of relative topos theory via stacks, the new notions of existential fibred site and of existential topos of such a site. These concepts allow us to develop relative topos theory in a way which naturally generalizes the construction of toposes of sheaves on locales and also provides a framework for investigating the connections between Grothendieck toposes as built from sites and elementary toposes as built from triposes.

The paper also contains a fibred generalisation of the ideal-completion of a preorder site, a construction which has played a key role in the development of formal topology since the eighties:

We expect this construction to find several applications, in particular in connection with the generation of dualities from multiple representations of toposes in terms of fibred preorder sites (in the spirit of this paper), but also in Logic; indeed, applications to the construction of completions of doctrines will be provided in a forthcoming paper by my doctoral student Joshua Wrigley.

Lastly, the paper provides an explicit description of the hyperconnected-localic factorization of a geometric morphism in terms of internal locales, with applications to the construction of alternative syntactic sites for the classifying topos of a theory.

I take this opportunity to send to all of you my Season’s Greetins and very best wishes for the New Year!

UPDATE: Slides presenting the contents of the paper are available here.

On Copernicus and Grothendieck

The slides of my recent talk at the conference in honor of Jean-Jacques Szceciniarz are available here:

The talk presents an analogy between the role of the Sun in Copernicus’ vision and that of Grothendieck toposes as ‘bridges’ between different mathematical theories, by building on the general notion of a ‘bridge’ object:

The Sun as a ‘bridge’ object

Interview for Radio3 Scienza

Today’s episode of the Italian national radio Radio3 Scienza was entirely devoted to the newly inaugurated Grothendieck Institute. I was interviewed by Roberta Fulci together with Laurent Lafforgue and Johanna Grothendieck, who both participated in the inaugural event on Saturday.

You may listen to the recording here.

Visit to Rome

Tomorrow I shall leave for a short mission in Rome.

I will be a Jury member for the Ph.D. thesis “Dynamical topoi” by Jacopo Garofali (supervised by Michael McQuillan) at Tor Vergata University, for which I have also acted as a referee. The thesis contains interesting applications of toposes to dynamical systems. The defense will take place in the afternoon of the 28th.

I will also visit Sergio Barbarossa’s research group at “La Sapienza” University, giving a talk at their Department on the 27th.

Domoschool 2022

Next week I will be in Domodossola giving a research course on topos theory for the 2022 edition of the International Alpine School of Mathematics and Physics.

The title of my course is Grothendieck toposes, invariants and ‘bridges’. The course will be an introduction to the theory of Grothendieck toposes, with a specific emphasis on the invariants that one can define on them. The point of view that we shall take is the one provided by the theory of toposes as ‘bridges’, which we shall present and illustrate through a selection of notable examples. We shall also discuss the application of these techniques to the investigation and discovery of dualities, equivalences and correspondences in mathematics and beyond.

Grothendieck: la moisson

The podcast of today’s broadcast at France Culture “Grothendieck: la moisson” is already available on the Radio website, which also provides several references for the general public to learn more about toposes and Grothendieck’s vision.

The associated Twitter account can be accessed here.

Thanks again to Nicolas Martin and all the staff of “La méthode Scientifique” for their invitation! I look forward to join you again in a few weeks, always with Alain Connes and Laurent Lafforgue, for the second part of the broadcast.

An update: a transcription of the emission by Denise Chemla is available here.

An interview for France Culture

On Thursday 17 February, from 16 to 17 French time, I will be a guest, together with Alain Connes and Laurent Lafforgue, at the scientific radio broadcast La Méthode Scientifique of France Culture, to talk about Grothendieck and crucial themes addressed in his recently published text Récoltes et Semailles, such as toposes and ethical issues in mathematics.

Looking forward to it!