MODELS OF LINEAR LOGIC BASED ON THE SCHWARTZ ε-PRODUCT. YOANN DABROWSKI AND MARIE KERJEAN Abstract. From the interpretation of Linear Logic multiplicative disjunction as the ε-product de- fined by Laurent Schwartz, we construct several models of Differential Linear Logic based on usual mathematical notions of smooth maps. This improves on previous results in [BET] based on con- venient smoothness where only intuitionist models were built. We isolate a completeness condition, called k-quasi-completeness, and an associated notion stable by duality called k-reflexivity, allow- ing for a ∗-autonomous category of k-reflexive spaces in which the dual of the tensor product is the reflexive version of the ε product. We adapt Meise’s definition of Smooth maps into a first model of Differential Linear Logic, made of k-reflexive spaces. We also build two new models of Linear Logic with conveniently smooth maps, on categories made respectively of Mackey-complete Schwartz spaces and Mackey-complete Nuclear Spaces (with extra reflexivity conditions). Varying slightly the notion of smoothness, one also recovers models of DiLL on the same ∗-autonomous cat- egories. Throughout the article, we work within the setting of Dialogue categories where the tensor product is exactly the ε-product (without reflexivization). Contents 1. Introduction 2 1.1. A first look at the interpretation of Linear Logic constructions 6 Part 1. Three Models of MALL 7 2. Preliminaries 7 2.1. Reminder on topological vector spaces 7 2.2. Reminder on tensor products and duals of locally convex spaces. 8 2.3. Dialogue and ∗-autonomous categories 10 2.4. A model of MALL making appear the Arens dual and the Schwartz ε-product 11 3. Mackey-complete spaces and a first interpretation for ` 16 arXiv:1712.07344v1 [cs.LO] 20 Dec 2017 3.1. AMackey-Completionwithcontinuouscanonicalmap 17 3.2. A ` for Mackey-complete spaces 18 4. Original setting for the Schwartz ε-productandsmoothmaps. 25 4.1. ∗-autonomous category of k-reflexive spaces. 25 4.2. A strong notion of smooth maps 34 5. Schwartz locally convex spaces, Mackey-completeness and ρ-dual. 37 5.1. Preliminaries in the Schwartz Mackey-complete setting 37 5.2. ρ-reflexive spaces and their Arens-Mackey duals 38 5.3. Relation to projective limits and direct sums 42 5.4. The Dialogue category McSch. 43 5.5. Commutation of the double negation monad on McSch 45 5.6. The ∗-autonomous category ρ − Ref. 48 1 2 YOANNDABROWSKIANDMARIEKERJEAN Part 2. Models of LL and DiLL 48 6. Smoothmapsandinducedtopologies.NewmodelsofLL 49 6.1. C -Smooth maps and C -completeness 49 6.2. Induced topologies on linear maps 52 6.3. A general construction for LL models 55 6.4. A class of examples of LL models 58 6.5. A model of LL : a Seely category 60 6.6. Comparison with the convenient setting of Global analysis and Blute-Ehrhard-Tasson 62 7. ModelsofDiLL 63 7.1. An intermediate notion : models of differential λ-Tensorlogic. 63 7.2. A general construction for DiLL models 68 7.3. ρ-smooth maps as model of DiLL 74 7.4. k-smooth maps as model of DiLL 76 8. Conclusion 77 9. Appendix 78 References 79 1. Introduction Since the discovery of linear logic by Girard [Gir87], thirty years ago, many attempts have been made to obtain denotational models of linear logic in the context of some classes of vector spaces with linear proofs interpreted as linear maps [Blu96, Ehr02, Gir04, Ehr05, BET]. Models of linear logic are often inspired by coherent spaces, or by the relational model of linear logic. Coherent Banach spaces [Gir99], coherent probabilistic or coherent quantum spaces [Gir04] are Girard’s attempts to extend the first model, as finiteness spaces [Ehr05] or K¨othe spaces [Ehr02] were designed by Ehrhard as a vectorial version of the relational model. Three difficulties appear in this semantical study of linear logic. The equivalence between a formula and its double negation in linear logic asks for the considered vector spaces to be isomor- phic to their double duals. This is constraining in infinite dimension. This infinite dimensionality is strongly needed to interpret exponential connectives. Moreover, extension to natural models of Differential linear logic would aim at getting models of classical proofs by some classes of smooth maps, which should give a Cartesian closed category. Finally, imposing a reflexivity condition to respect the first requirement usually implies issues of stability by natural tensor products of this condition, needed to model multiplicative connectives. This corresponds to the hard task of finding ∗-autonomous categories [Ba79]. As pointed out in [Ehr16], the only model of differential Linear logic using smooth maps [BET] misses annoyingly the ∗-autonomous property for classical linear logic. Our aim in this paper is to solve all these issues simultaneously and produce several denotational models of classical linear logic with some classes of smooth maps as morphism in the Kleisli category of the monad. We will show that the constraint of finding a ∗-autonomous category in a compatible way with a Cartesian closed category of smooth maps is even relevant to find better mathematical notions of smooth maps in locally convex spaces. Let us explain this mathematical motivation first. MODELS OF LINEAR LOGIC BASED ON THE SCHWARTZ ε-PRODUCT. 3 It seems that, historically, the development of differential calculus beyond normed spaces suf- fered from the lack of interplay between analytic considerations and categorical, synthetic or logic ones. Partially as a consequence, analysts often forgot looking for good stability properties by duality and focused on one side of the topological or bornological viewpoint. Take one of the analytic summary of the early theory in the form of Keller’s book [Kel]. It already gives a unified and simplified approach based on continuity conditions of derivatives in various senses. But it is well-known that in order to look for good categorical properties such as Cartesian closedness, the category of continuous maps is not a good starting point, the category of maps continuous on compact sets would be better. This appears strongly in all the developments made to recover continuity of evaluation on the topological product (instead of considering the product of a Cartesian closed category), which is unavoidable for full continuity of composition of derivatives in the chain rule. This leads to considering convergence notions beyond topologi- cal spaces on spaces of linear maps, but then, no abstract duality theory of those vector conver- gence spaces or abstract tensor product theory is developed. Either one remains with spaces of smooth maps that have tricky composition (of module type) between different notions of smooth- ness or composition within the classes involving convergence vector spaces whose general theory remained underdeveloped with respect to locally convex spaces. At the end, everything goes well only on restricted classes of spaces that lack almost any categorical stability properties, and nobody understands half of the notions introduced. The situation became slightly better when [Me] con- sidered k-space conditions and obtained what analysts call kernel representation theorems (Seely isomorphisms for linear logicians), but still the class of spaces considered and the k-space condi- tions on products limited having a good categorical framework for the hugest classes of spaces: the only classes stable by products were Fr´echet spaces and (DFM)-spaces, which are by their very nature not stable by duality. The general lesson here is that, if one wants to stay within better studied and commonly used locally convex spaces, one should better not stick to functions continuous on products, and the corresponding projective topological tensor product, but always take tensor products that come from a ∗-autonomous category, since one also needs duality, or at least a closed category, to con- trol the spaces of linear maps in which the derivatives take values. ∗-autonomous categories are the better behaved categories having all those data. Ideally, following the development of polar- ization of Linear logic in [MT] inspired by game semantics, we are able to get more flexibility and allow larger dialogue categories containing such ∗-autonomous categories as their category of continuation. We will get slightly better categorical properties on those larger categories. A better categorical framework was later found and summarized in [FK, KM] the so-called con- venient smoothness. A posteriori, as seen [Ko], the notion is closely related to synthetic differential geometry as diffeological spaces are. It chooses a very liberal notion of smoothness, that does not imply continuity except on very special compact sets, images of finite dimensional compact sets by smooth maps. It gives a nice Cartesian closed category and this enabled [BET] to obtain a model of intuitionistic differential linear logic. As we will see, this may give the wrong idea that this very liberal notion of smoothness is the only way of getting Cartesian closedness and it also takes the viewpoint of focusing on bornological properties. This is the main reason why, in our view, they don’t obtain ∗-autonomous categories since bornological locally convex spaces have com- plete duals which gives an asymmetric requirement on duals since they only need a much weaker Mackey-completeness on their spaces to work with their notion of smooth maps. We will obtain in this paper several models of linear logic using conveniently smooth maps, and we will explain 4 YOANNDABROWSKIANDMARIEKERJEAN logically this Mackey-completeness condition in section 6.2. It is exactly a compatibility condition on F enabling to force our models to satisfy !E ⊸ F = (!E ⊸ 1) ` F. Of course, as usual for vector spaces, our models will satisfy the mix rule making the unit for multiplicative connectives self-dual and this formula is interpreted mathematically as saying that smooth maps with value in some complete enough space are never a big deal and reduced by duality to the scalar case.