Weakly Free Multialgebras

In abstract algebraic logic, many systems, such as those paraconsistent logics taking inspiration from da Costa's hierarchy, are not algebraizable by even the broadest standard methodologies, as that of Blok and Pigozzi. However, these logics can be semantically characterized by means of non-deterministic algebraic structures such as Nmatrices, RNmatrices and swap structures. These structures are based on multialgebras, which generalize algebras by allowing the result of an operation to assume a non-empty set of values. This leads to an interest in exploring the foundations of multialgebras applied to the study of logic systems.It is well known from universal algebra that, for every signature $\Sigma$, there exist algebras over $\Sigma$ which are absolutely free, meaning that they do not satisfy any identities or, alternatively, satisfy the universal mapping property for the class of $\Sigma$-algebras. Furthermore, once we fix a cardinality of the generating set, they are, up to isomorphisms, unique, and equal to algebras of terms (or propositional formulas, in the context of logic). Equivalently, the forgetful functor, from the category of $\Sigma$-algebras to Set, has a left adjoint. This result does not extend to multialgebras. Not only multialgebras satisfying the universal mapping property do not exist, but the forgetful functor $\mathcal{U}$, from the category of $\Sigma$-multialgebras to Set, does not have a left adjoint.In this paper we generalize, in a natural way, algebras of terms to multialgebras of terms, whose family of submultialgebras enjoys many properties of the former. One example is that, to every pair consisting of a function, from a submultialgebra of a multialgebra of terms to another multialgebra, and a collection of choices (which selects how a homomorphism approaches indeterminacies), there corresponds a unique homomorphism, what resembles the universal mapping property. Another example is that the multialgebras of terms are generated by a set that may be viewed as a strong basis, which we call the ground of the multialgebra. Submultialgebras of multialgebras of terms are what we call weakly free multialgebras. Finally, with these definitions at hand, we offer a simple proof that multialgebras with the universal mapping property for the class of all multialgebras do not exist and that $\mathcal{U}$ does not have a left adjoint

A Category of Ordered Algebras Equivalent to the Category of Multialgebras

It is well known that there is a correspondence between sets and complete, atomic Boolean algebras ($\textit{CABA}$s) taking a set to its power-set and, conversely, a complete, atomic Boolean algebra to its set of atomic elements. Of course, such a correspondence induces an equivalence between the opposite category of $\textbf{Set}$ and the category of $\textit{CABA}$s. We modify this result by taking multialgebras over a signature $\Sigma$, specifically those whose non-deterministic operations cannot return the empty-set, to $\textit{CABA}$s with their zero element removed (which we call a $\textit{bottomless Boolean algebra}$) equipped with a structure of $\Sigma$-algebra compatible with its order (that we call $\textit{ord-algebras}$). Conversely, an ord-algebra over $\Sigma$ is taken to its set of atomic elements equipped with a structure of multialgebra over $\Sigma$. This leads to an equivalence between the category of $\Sigma$-multialgebras and the category of ord-algebras over $\Sigma$. The intuition, here, is that if one wishes to do so, non-determinism may be replaced by a sufficiently rich ordering of the underlying structures

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Neste trabalho, apresentamos uma lógica de primeira ordem com tipos para as categorias Psh(Q) e Sh(Q) dos prefeixes e feixes sobre quantais (right-sided e idempotentes). São estudadas as propriedades das operações lógicas entre sub-objetos em Psh(Q) com relação ao cálculo da imagem inversa por morfismos (interpretando a substituição de uma variável por um termo numa fórmula), estabelecendo condições suficientes, expressáveis na linguagem de primeira ordem, que garantem a preservação das operações. Em particular, é discutida a noção de extensão de sub-prefeixes -construídos a partir de sub-prefeixes elementares- por novos prefeixes, inerente ao processo de combinar fórmulas com variáveis de tipos diferentes. Portanto, as regras de lógica possuem cláusulas que prescrevem as condições de extensão, o que garante a corretude da lógica. São analizadas propriedades de primeira ordem das relações binárias em Psh(Q), assim algumas estruturas algébricas, tais como anéis, módulos e corpos. É provado na lógica um teorema de I. Kaplansky, que diz serem livre os módulos projetivos finitamente gerados sobre um anel localnot availabl

An alternative approach for quasi-truth

In 1986, Mikenberg et al. introduced the semantic notion of quasi-truth defined by means of partial structures. In such structures, the predicates are seen as triples of pairwise disjoint sets: the set of tuples which satisfies, does not satisfy and can satisfy or not the predicate, respectively. The syntactical counterpart of the logic of partial truth is a rather complicated first-order modal logic. In the present article, the notion of predicates as triples is recursively extended, in a natural way, to any complex formula of the first-order object language. From this, a new definition of quasi-truth is obtained. The proof-theoretic counterpart of the new semantics is a first-order paraconsistent logic whose propositional base is a 3-valued logic belonging to hierarchy of paraconsistent logics known as Logics of Formal Inconsistency, which was proposed by Carnielli and Marcos in 2002.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES