The fixed-point property for represented spaces Mathieu Hoyrup To cite this version: Mathieu Hoyrup. The fixed-point property for represented spaces. 2021. hal-03117745v1 HAL Id: hal-03117745 https://hal.inria.fr/hal-03117745v1 Preprint submitted on 21 Jan 2021 (v1), last revised 28 Jan 2021 (v2) HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. The fixed-point property for represented spaces Mathieu Hoyrup Universit´ede Lorraine, CNRS, Inria, LORIA, F-54000 Nancy, France [email protected] January 21, 2021 Abstract We investigate which represented spaces enjoy the fixed-point property, which is the property that every continuous multi-valued function has a fixed-point. We study the basic theory of this notion and of its uniform version. We provide a complete characterization of countable-based spaces with the fixed-point property, showing that they are exactly the pointed !-continuous dcpos. We prove that the spaces whose lattice of open sets enjoys the fixed-point property are exactly the countably-based spaces. While the role played by fixed-point free functions in the diagonal argument is well-known, we show how it can be adapted to fixed-point free multi-valued functions, and apply the technique to identify the base-complexity of the Kleene-Kreisel spaces, which was an open problem. 1 Introduction 1.1 The fixed-point property A mathematical space X has the fixed-point property if every function f : X ! X in a certain class has a fixed-point, i.e. some x 2 X such that f(x) = x. This property plays an important role in several branches of mathematics, witnessed by a number of fixed-point theorems asserting identifying spaces enjoying this property. Among the most celebrated results let us cite Brouwer's fixed-point theorem in topology, Schauder's fixed-point theorem in convex analysis, Knaster-Tarski's and Kleene's fixed-point theorems in order theory, Rogers's fixed- point theorem (also called the Recursion theorem) in computability theory. In this article we work in the setting of computable analysis [Wei00], where the main struc- ture is a represented space, and the associated functions are the multi-valued functions (also called multifunctions) that are computable, or more generally continuously realizable (contin- uous for short). If X is a set and h : X ⇒ X is a multifunction then a point x 2 X is a fixed-point of h if x 2 h(x). Therefore, the version of the fixed-point property for computable analysis is the following: Definition 1.1. A represented space X has the fixed-point property if every continuous multifunction h : X ⇒ X has a fixed-point. This paper is an investigation of the following question: Question 1.1. Which represented spaces X enjoy the fixed-point property? It turns out that most represented spaces satisfying the fixed-point property actually enjoy a stronger, uniform version of it, where a fixed-point can be continuously found from the multi- function. We therefore introduce a uniform version of the fixed-point property, which will prove useful. 1 We give a first characterization of the uniform fixed-point property, which implies in par- ticular that its computable version is equivalent to the notion of multi-retraceable represented space recently introduced by Brattka and Gherardi in [BG21]. Theorem (Theorem 4.1). Let X be a represented space. The following conditions are equivalent: • X has the (computable) uniform fixed-point property, • Every (computable) continuous function f :⊆ N ! X has a total (computable) continuous extension. We relate the uniform fixed-point property to the notion of effective discontinuity introduced by Brattka in [Bra20] where it is proved, assuming the Axiom of Determinacy, that discontinuity is equivalent to effective discontinuity. It translates into the following result, explaining why in practice, spaces enjoying the fixed-point property actually enjoy its uniform version. Theorem (Theorem 4.2). Let X be a represented space. Assuming the Axiom of Determinacy, the following conditions are equivalent: • X has the fixed-point property, • X has the uniform fixed-point property. We next give a simple criterion which can be particularly helpful to disprove the fixed- point property, without having to explicitly build a fixed-point free continuous multifunction (here, [Γ](X) is the class of subsets of X whose preimage under the representation has complex- ity Γ, see Definition 2.1). Theorem (Theorem 4.3). Let Γ be a class of the Borel, Hausdorff-Kuratowski or hyperprojective hierarchy on N which is not self-dual, and assume that equality on X has complexity [Γ](X×X). For A ⊆ X: • If A has the fixed-point property then A is not Γˇ-hard, • If A has the uniform fixed-point property then A 2 [Γ](X). We then turn our attention to restricted classes of represented spaces, for which we can give an elegant characterization of the fixed-point property and at the same time show its equivalence with the uniform fixed-point property, without assuming the Axiom of Determinacy. We focus on the class of countably-based T0-spaces, on which a subsequent part of com- putable analysis was first developed (they are central in the book of Weihrauch [Wei00]). We show that among these spaces, the ones with the fixed-point property are exactly the pointed !- continuous dcpos, a very important class of partial orders studied in domain theory (the state- ment involves the notion of t-continuity, which will be explained in due time). Theorem (Theorem 5.1). Let X be a countably-based T0-space. The following statements are equivalent: 1. X has the fixed-point property, 2. X has the uniform fixed-point property, 3. X is a multivalued retract of P(!), 4. X is quasiPolish, has a least element and is t-continuous, 2 5. X is homeomorphic to a pointed !-continuous dcpo endowed with the Scott topology. We also focus on spaces of open sets, on which one can take advantage of the lattice structure to give a very neat characterization of the fixed-point property and again prove its equivalence with its uniform version, without assuming the Axiom of Determinacy. Theorem (Theorem 6.1). Let X be admissibly represented. The following conditions are equiv- alent: • X is countably-based, •O (X) has the uniform fixed-point property, •O (X) has the fixed-point property. 1.2 Diagonal argument In a second part of the article, we give applications of the failure of the fixed-point property. It is well-known that the diagonal argument is related to the fixed-point property, let us give two famous examples: • Cantor's diagonal argument uses the boolean negation : : f0; 1g ! f0; 1g which has no fixed-point to prove that there is no surjection from X to f0; 1gX , • By contraposition, and letting P be the class of partial computable functions, Rogers fixed- point theorem uses the existence of a computable surjection from N to the computable elements of PN to prove that every computable multifunction f : P ⇒ P has a fixed-point. This very general argument has been abstracted by Lawvere in [Law69] using the language of categories. However, applying the diagonal argument using fixed-point free multifunctions is not always straightforward, because it produces a multifunction rather than a function. We show how to overcome this problem and give a few applications of the technique, in particular we identify the base-complexity of Kleene-Kreisel spaces, which we explain now. For each k 2 N (actually, for each countable ordinal), one can define the Kleene-Kreisel hki space Nhki of continuous functionals of order k as follows: Nh0i = N and Nhk + 1i = NN , so that Nh1i = N and Nh2i = (NN)N for instance. It is known that the topology on the Kleene-Kreisel spaces is difficult to understand. It is strictly stronger than the compact-open topology [Hyl79]. It was conjectured for a long time that it is zero-dimensional, but Schr¨odereventually proved in [Sch09b] that it is neither zero-dimensional nor regular. The definition of the topology is implicit, and no explicit char- acterization is currently known. A way to tackle this problem is to understand the intrinsic difficulty of describing the open sets. This is what the base-complexity of a topological space, introduced in [dBSS16], is about: given a topological space, it measures the minimal descriptive complexity of a subset of N from which a basis of the topology can be continuously obtained. 1 The complexity is naturally measured in terms of the (hyper)projective hierarchy (Σα)α over the countable ordinals α. e The base-complexity of Nhαi was essentially unknown, except for the natural upper bound: Nhαi 1 is Σα+1-based, but no lower bound was known. A related problem is about the structure of 1 thee base-complexity hierarchy. It is proved in [dBSS16] that the Σα-based spaces are a proper 1 subclass of the Σα+3-based spaces (Corollary 6.12 in [dBSS16]),e but whether the gap can be reduced is left opene (Problem 6.2 in [dBSS16]). 3 Applying our version of the diagonal argument and the existence of a fixed-point free contin- uous multifunction on O(Nhαi), we prove that for α ≥ 2 the upper bound on the base-complexity 1 1 of Nhαi is optimal, giving an instance of a space that is Σα+1-based but not Σα-based, which shows that the base-complexity hierarchy does not collapse.e e 1 Theorem (Theorem 7.1).