At the beginning of my Ph.D. studies, I had the intuition that Grothendieck toposes could serve as sorts of 'bridges' for effectively transferring information between distinct mathematical theories.
The results obtained in my Ph.D. thesis "The Duality between Grothendieck Toposes and Geometric Theories" actually provided compelling technical evidence for the validity of such a view, in the form of non-trivial applications pertaining to different fields of Mathematics, and stimulated new developments of the original idea, which eventually led to the general methodologies described in the programmatic paper "The unification of Mathematics via Topos Theory".
The principles introduced in this work are abstract and transversal to various mathematical fields, and the application of them can lead to surprising and non-trivial results in several areas of Mathematics; in fact, these methods have already generated ramifications into distinct branches including Algebra, Topology, Algebraic Geometry, Functional Analysis, Model Theory and Proof Theory, and the potential of this theory has just started to be explored.
My technical contributions, arising from an implementation of this philosophy in a variety of different mathematical contexts, can be described as follows.
My Ph.D thesis has addressed the long-standing problem of understanding the nature of the relatioship between Grothendieck toposes and geometric theories given by the theory of classifying toposes. The dissertation contains several new theorems on the relationship between geometric theories and their classifying toposes, in the form of criteria characterizing the geometric theories classified by relevant classes of toposes, but, most importantly, provides results and methods which justify the view of classifying toposes as 'bridges' mentioned above.
My Ph.D. work resulted in a series of eight papers, which contain theoretical results as well as 'concrete' applications of the philosophy 'toposes as bridges' in a variety of different mathematical contexts.
I provide a brief description of the contents of these papers as well as of the later ones below.
In the paper "Yoneda representations of flat functors and classifying toposes" I established a fundamental connection between topos theory and model theory leading to a number of new developments and applications. The main result of this paper is an explicit model-theoretic description of the theory classified by a given topos of sheaves on a site in terms of the models of the theory classified by the corresponding presheaf topos.
In the paper "Fraïssé's construction from a topos-theoretic perspective" I produced a topos-theoretic interpretation of Fraïssé’s construction in model theory, leading to new results in logic, in particular on countably categorical theories. Specifically, the three concepts involved in Fraisse’s constructions are seen to correspond precisely to three different ways for looking at the same classifying topos according to the philosophy 'toposes as bridges'.
In the paper "De Morgan classifying toposes" I introduced a general method for deciding whether a Grothendieck topos satisfies De Morgan’s law (resp. the law of excluded middle) or not, with various applications to the theory of classifying toposes. In particular, I provided complete characterizations of the classes of theories classified by a De Morgan (resp. a Boolean) topos and introduced the notions of Booleanization and DeMorganization of a geometric theory. In this context I also exhibited equivalences of well-known logical notions (those of De Morgan and Boolean algebra) with topological ones.
In my paper with Peter Johnstone "De Morgan’s law and the theory of fields" we provided applications in algebra of a new notion (that of DeMorganization of a topos) introduced by myself in the paper "De Morgan classifying toposes"; specifically, we have shown that the DeMorganization of the classifying topos for the theory of fields classifies the theory of fields of nonzero characteristic which are algebraic over their prime fields.
In the paper "Atomic toposes and countable categoricity" I established several results on atomic toposes and gave a model-theoretic characterization of the class of geometric theories classified by an atomic topos having enough points; in particular, I showed that every complete geometric theory classified by an atomic topos is countably categorical.
The central result of the paper "Lattices of theories" is a duality theorem providing a central connection between geometric logic and the theory of Grothendieck toposes and paving the way for a transfer of ideas and results from the theories of elementary and Grothendieck toposes to logic. Specifically, the theorem provides, for any geometric theory, a bijection between the subtoposes of its classifying topos and the geometric extensions of the theory (considered up to a natural notion of syntactic equivalence). The paper gives two different proofs of the theorem and investigates the logical counterparts of many constructions and results in elementary topos theory which apply to subtoposes; the resulting notions are seen to be of natural logical and mathematical interest and several applications of the general theory are given. The results of this paper have also led to a solution to the third of the "Open problems in Topos Theory" by F. W. Lawvere.
The paper "Universal models and definability" consists in a systematic investigation of universal models in Topos Theory with a particular emphasis on their applications to definability by geometric formulae and to the investigation of the law of excluded middle and De Morgan’s law on Grothendieck toposes.
The paper "Syntactic characterizations of properties of classifying toposes" provides characterizations, for various fragments of geometric logic, of the class of theories classified by a locally connected (respectively connected and locally connected, atomic, compact, presheaf) topos, and exploited the existence of multiple sites of definition for a given topos to establish various results on quotients of theories of presheaf type.
In the paper "A characterization theorem for geometric logic", I gave a semantic characterization of the classes of structures arising as the classes of models of a geometric theory inside Grothendieck toposes, solving in particular a twenty-year old problem of Ieke Moerdijk.
unification of Mathematics via Topos Theory"
a sort of
of my research programme, in which I advocate that Grothendieck toposes
can serve as unifying spaces being able to act as
for effectively transferring knowledge between distinct mathematical
theories. A general overview
of the ideas in this paper can be found in
of the ideas in this paper can be found in this section.
The paper "A topos-theoretic approach to Stone-type dualities" provides a general topos-theoretic perspective on Stone-type dualities. The theory described in this paper arises from a faithful implementation of the philosophy ‘toposes as bridges’, and consists in a general framework for analyzing the known Stone-type dualities, as well as for introducing new ones: several known dualities are seen to be specializations of just one topos-theoretic phenomenon, and new dualities and equivalences between categories of preorders and categories of posets, locales or topological spaces are established. The paper also contains a variety of other applications of the philosophy ‘toposes as bridges’ in Algebra, Algebraic Geometry, Topology and Logic.
In the paper "Site
characterizations for geometric invariants of toposes" we establish criteria for deciding whether a Grothendieck topos satisfies
invariant property (such as for example the property of a topos to be
atomic, locally connected, equivalent to a presheaf topos); these
criteria are based on a reformulation of the property of a topos to
satisfy the given invariant as a property of its sites of definition.
In the paper "Site characterizations for geometric invariants of toposes" we establish criteria for deciding whether a Grothendieck topos satisfies a given ‘geometric’ invariant property (such as for example the property of a topos to be atomic, locally connected, equivalent to a presheaf topos); these criteria are based on a reformulation of the property of a topos to satisfy the given invariant as a property of its sites of definition.
In the paper "A general method for building reflections" we introduce a general technique for generating reflections between categories. The method can be profitably applied to generate adjunctions starting from geometric morphisms between Grothendieck toposes; as particular cases, we recover various well-known Stone-type adjunctions and establish several new ones.
The paper "Priestley-type dualities for partially ordered structures" provides a general framework for generating dualities between categories of partial orders and categories of ordered Stone spaces; the classical Priestley duality for distributive lattices is recovered as a particular case and several other dualities for different kinds of partially ordered structures are established.
In the paper "Gelfand spectra and Wallman compactifications" we carry out a systematic, topos-theoretically inspired, investigation of Wallman compactifications with a particular emphasis on their relations with Gelfand spectra and Stone-Cech compactifications. In addition to proving several specific results about Wallman bases and maximal spectra of distributive lattices, we establish a general framework for functorializing the representation of a topological space as the maximal spectrum of a Wallman base for it, which allows to generate different dualities between categories of topological spaces and subcategories of the category of distributive lattices; in particular, this leads to a categorical equivalence between the category of commutative C*-algebras and a natural category of distributive lattices. We also establish a general theorem concerning the representation of the Stone-Cech compactification of a locale as a Wallman compactification, which subsumes all the previous results obtained on this problem.
In the paper "Topologies for intermediate logics" we investigate the problem of characterizing the classes of Grothendieck toposes whose internal logic satisfies a given assertion in the theory of Heyting algebras, and introduce natural analogues of the double negation and De Morgan topologies on an elementary topos for a wide class of intermediate logics.
In the paper “Topological Galois Theory” we introduce an abstract topos-theoretic framework for building Galois-type theories in a variety of different mathematical contexts; such theories are obtained from representations of certain atomic two-valued toposes as toposes of continuous actions of a topological group. Our framework subsumes in particular Grothendieck's Galois theory and allows to build Galois-type equivalences in new contexts, such as for example graph theory and finite group theory.
In the paper "The Morita-equivalence between MV-algebras and abelian l-groups with strong unit" (joint with A. C. Russo), we show that the theory of MV-algebras is Morita-equivalent to that of abelian l-groups with strong unit. This generalizes the well-known equivalence between the categories of set-based models of the two theories established by D. Mundici in 1986, and allows to transfer properties and results across them by using the methods of topos theory. We discuss several applications, including a sheaf-theoretic version of Mundici's equivalence and a bijective correspondence between the geometric theory extensions of the two theories.
In the paper "Extensions of flat functors and theories of presheaf type", we develop a general theory of extensions of flat functors along geometric morphisms of toposes, and apply it to the study of the class of theories whose classifying topos is equivalent to a presheaf topos. As a result, we obtain a characterization theorem providing necessary and sufficient semantic conditions for a theory to be of presheaf type. This theorem subsumes all the previous partial results obtained on the subject and has several corollaries which can be used in practice for testing whether a given theory is of presheaf type as well as for generating new examples of theories belonging to this class. Along the way, we establish a number of other results of independent interest, including developments about colimits in the context of indexed categories, expansions of geometric theories and methods for constructing theories classified by a given presheaf topos.
A research project based on the technique 'toposes as bridges' has recently been funded by the European Commission in the form of a Marie Curie Fellowship enabling Dr. Marco Benini (University of Insubria) to spend one and a half year working on the project at the University of Leeds under the supervision of Dr. Peter Schuster.
The Mathematical Logic group of the University of Padua has recently obtained funding for a post-doctoral project investigating, amongst other things, aspects of my work relevant for formal topology.
Laurent Lafforgue has advocated in this document the importance of investigating the technique 'toposes as bridges' in connection to the Langlands programme.