Browsing by Author "Janelidze, George"

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    Algebraic aspects of propositional logic
    (2024) Leisegang, Nicholas; Janelidze-Gray, Tamar; Janelidze, George
    In this dissertation, we seek to examine the connection between abstract algebra and propositional logic. We start by considering the category Bool of Boolean algebras, the algebraic counterpart of classical propositional logic. We provide an algebraic definition of theories and models of classical logic and provide algebraic algorithms to determine whether a chosen formula is a theorem of a given theory of classical logic. In order to generalize this approach, we then describe varieties of universal algebra and some of their properties. Using this framework, we show in a general setting how a formal theory of propositional logic induces a variety of universal algebra in which logical connectives become algebraic operations and logical formulae are considered equal when they are logically equivalent. We then discuss algebraic varieties corresponding to various non-classical propositional logics. In particular, we consider the variety of Heyting algebras Heyt which corresponds to intuitionistic logic, and certain subvarieties of Heyt which correspond to intermediate logics. We then describe several algebraic varieties which correspond to theories of normal modal logic. Moreover, by considering free algebras and completeness in Heyt, we establish that we are unable to use the same methods used in Bool to construct algorithms to determine theorems of intuitionistic logic. Lastly, we construct an adjunction between Heyt and the category of topological Boolean algebras, and through this show that we again cannot construct similar algebraic algorithms to determine theorems in the modal logic S4.
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    The annihilation graphs of commutator posets and lattices
    (2015) Mehdinezhad, Elham; Janelidze, George
    We propose a new, widely generalized context for the study of the zero-divisor/ annihilating-ideal graphs, where the vertices of graphs are not elements/ideals of a commutative ring, but elements of an abstract ordered set (imitating the lattice of ideals), equipped with a binary operation (imitating products of ideals). The intermediate level of congruences of any algebraic structure admitting a "good" theory of commutators is also considered.
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    Categorical semi-direct products in varieties of groups with multiple operators
    (2010) Inyangala, Edward Buhuru; Janelidze, George
    The notion of a categorical semidirect product was introduced by Bourn and Janelidze as a generalization of the classical semidirect product in the category of groups. The main aim of this work is to study the general properties of semidirect products of groups with operators, describe them in various classical varieties of such algebraic structures and apply the results to homological algebra and related areas of modern algebra. The context in which the study is done is a semiabelian category (that is, a pointed, Barr-exact and Bourn-protomodular category). The main result in the thesis is the construction of the semidirect product in a variety -RLoop of right -loops as the product of underlying sets equipped with the -algebra structure. A variety of right -loops is a variety that is pointed, has a binary + (not necessarily associative or commutative) and a binary satisfying the identities 0 + x = x, x + 0 = x, (x + y) y = x and (x - y) + y = x. Thus, -RLoop is a generalization of the variety of -groups introduced by Higgins and the results obtained are valid for varieties of -loops. We also describe precrossed and crossed modules in the variety -RLoop. The theory of crossed modules developed is independent of that developed by Janelidze for crossed modules in an arbitrary semiabelian category and gives simplified explicit formulae for crossed modules in -RLoop. Finally, we mention that our constructions agree with the known ones in the familiar algebraic categories, specifically the categories of groups, rings and Lie algebras.
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    Coverings and Descent Theory of Finite Spaces
    (2022) Mbewu, Thomas; Janelidze, George
    This thesis presents the categorical Galois theory of the reflection of the category of finite topological spaces into the category of discrete finite topological spaces. This turns out to be nothing but the equivalence between the category of coverings of a connected finite topological space and the actions of the fundamental group of that space. Since some descent theory is necessary for categorical Galois theory, this thesis also contains an account of some of the descent theory of finite topological spaces. The reader is assumed to know the basics of category theory, but no descent theory or categorical Galois theory, or even internal category theory, but to be somewhat familiar with coverings and fundamental groups, and the notion of “locally” for topological spaces.
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    Extensive categories, commutative semirings and Galois theory
    (2020) Poklewski-Koziell, Rowan; Janelidze, George
    We describe the Galois theory of commutative semirings as a Boolean Galois theory in the sense of Carboni and Janelidze. Such a Galois structure then naturally suggests an extension to commutative semirings of the classical theory of quadratic equations over commutative rings. We show, however, that our proposed generalization is impossible for connected commutative semirings which are not rings, leading to the conclusion that for the theory of quadratic equations, “minus is needed”. Finally, by considering semirings B which have no non-trivial additive inverses and no non-trivial zero divisors, we present an example of a normal extension of commutative semirings which has an underlying B-semimodule structure isomorphic to B×B.
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    Fractoids
    (2022) Brey, Khadija; Janelidze, George
    This dissertation will examine the properties of the algebraic structure herein named the fractoid. This structure will be defined and its properties closely examined. In this dissertation we will first provide context for this structure, by looking at both category theory and universal algebra. We present some first basic concepts of category theory and consider F-algebras (= algebras over an endofunctor F). We will then look at algebras in the sense of universal algebra. We will examine F-algebras and their properties in this context and compare them to the definitions used in some of the standard textbooks in universal algebra. Once fractoids are defined and examined, they will be compared to a similar existing algebraic structure, the wheel.
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    Internal factorisation systems
    (2023) Ranchod, Sanjiv; Janelidze, George; Janelidze Tamar
    We introduce internal factorisation systems for internal categories. We recall the definitions and theory of internal categories and factorisation systems. We develop a diagrammatic calculus of pullbacks for ease of internal calculation. To define an internal factorisation system we define and study the subobjects of isomorphisms, an internalisation of the class of isomorphisms of a category. We provide an abstract example of an internal factorisation system. We then internalise various properties of factorisation systems, such as the two components determining each other, the cancellation properties and the essential uniqueness of factorisations, and show that an internal factorisation system satisfies these internal conditions.
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    Internal monoid actions in a cartesian closed category and higher-dimensional group automorphisms
    (2015) Ramasu, Pako; Janelidze, George
    The notion of cat¹-group which was introduced by Loday is equivalent to the notions of crossed module and of internal category in the category of groups. This notion of cat¹-groups and their morphisms admits natural generalization to catⁿ-groups, which give rise to n-fold categories in the category of groups. There is also a characterization of catⁿ-groups in terms of crossed n-cubes which was given by Ellis and Steiner. The category Catⁿ (Groups) of internal n-fold categories in the category of groups is a cartesian closed category, however given an object X in Catⁿ (Groups), calculating corresponding action representing object Aut (X) directly would require an enormous calculations. The main purpose of the thesis is to describe that object avoiding such calculations as much as possible. The main tool used in the thesis, apart from the theory of cartesian closed categories, is Loday's theory of catⁿ-groups. We de ne a catⁿ-group X as an additive Mₙ-group X , and then construct the corresponding Aut (X), where Mₙ is a monoid. Since the category of catⁿ-groups is equivalent to Catⁿ (Groups) and since the cartesian closed category Sets Mₙ of Mₙ-sets is much easier to handle than the cartesian closed category of n-fold categories, we shall work just with catⁿ-groups. To assert that, Aut (X) is an action representing object in Sets Mₙ , is to as- sert that, there is a canonical bijection between B-actions of catⁿ-group B on X and the internal group homomorphism B --> Aut (X). Thus, we confirm the construction of Aut (X) by establishing that bijection. Finally, as one of the results of this work, we give the comparison between our cat¹-group Aut (X) and Norrie's actor crossed module (D (G;Z) ;Aut (Z;G;p) w) of a crossed module (Z;G;p) in dimension one.
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    Topics in 2-categorical Algebra
    (2023) Sutton, Matthew; Janelidze, George
    In this thesis we will examine 2-categories and higher categorical structures and formulate 1-categorical theorems in the language of higher categories as well as formulate some internal definitions of these base structures in finitely complete categories. We will begin by defining the relevant 2- categorical structures, such as 2-categories, double categories, bicategories and enriched categories, as well as examples of all. Following this, we will show first how these structures relate to each other (for instance, a 2-category is a special case of a double category) and then demonstrate that the category of V-enriched categories forms a 2-category. Chapter 2 begins with the definition of internal categories in a category C with pullbacks, as well as internal functors and internal natural transformations, after which we will demonstrate that the category of internal categories forms a 2-category. We will then show that in C with pullbacks and terminal object, one can define an internal 2-category and an internal bicategory , and show that these are the same as small 2-categories and small bicategories in the case of C = Set. In the final chapter, we demonstrate that some of the familiar constructions of 1-category theory can actually be defined in a 2-category, and certain theorems about these structures proven using only 2-categorical methods.
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    Topics in categorical algebra and Galois theory
    (2017) Fourie, Jason; Janelidze, George
    We provide an overview of the construction of categorical semidirect products and discuss their form in particular semi-abelian varieties. We then give a thorough description of categorical Galois theory, which yields an analogue for the fundamental theorem of Galois theory in an abstract category by making use of the notions of admissibility and effective descent. We show that the admissibility of a functor can be extended to the admissibility of the canonically induced functor on the associated category of pointed objects, and that an analogous extension can be made for effective descent morphisms. We use the extended notions of admissibility and effective descent to describe a new, pointed version of the categorical Galois theorem, and use this result to move the categorical formulation of the Galois theory of finite, separable field extensions into a non-unital context.