Blok, W. J.; Pigozzi, D. Algebraic semantics for universal Horn logic without equality. (English) Zbl 0768.03008 Universal algebra and quasigroup theory, Lect. Conf., Jadwisin/Pol. 1989, Res. Expo. Math. 19, 1-56 (1992). Summary: [For the entire collection see Zbl 0745.00017.]An algebraic model theory for universal Horn logic (UHL) is developed that encompasses both strict UHL with equality, in particular quasi- equational logic, and sentential logic, which can be naturally viewed as UHL without equality and with a single, unary predicate. The underlying formal mechanism is the \(k\)-deductive system; its propositions are \(k\)- tuples of sentences and its models are algebras with a designated set of “true” \(k\)-tuples called a filter. Inherent in every \(k\)-deductive system, in fact in every universal Horn theory, is a notion of equality in the sense of Leibniz that may have no representation in the formal language. It determines the Leibniz operator, a mapping between the lattices of filters and congruences of an algebra, in terms of which a hierarchy of \(k\)-deductive systems can be defined. Monotone Leibniz operators give rise to protoalgebraic \(k\)-deductive systems. They form the widest class of \(k\)-deductive systems with a reasonable algebraic semantics; a major part of the paper is devoted to their study.Two of the main results of the paper: (1) a metamathematical characterization of protoalgebraic \(k\)-deductive systems in the spirit of Mal’cev’s characterization of congruence-permutable equational theories; (2) an algebraic characterization of the reduced model classes of \(k\)- deductive systems analogous to Birkhoff’s well-known characterization of varieties. Similar characterizations are obtained for other classes of \(k\)-deductive systems arising from the Leibniz operator. Some applications to computer science and formal language theory are given. Cited in 3 ReviewsCited in 47 Documents MSC: 03C05 Equational classes, universal algebra in model theory 03D05 Automata and formal grammars in connection with logical questions 68Q45 Formal languages and automata 08A70 Applications of universal algebra in computer science 08C10 Axiomatic model classes Keywords:algebraic model theory for universal Horn logic; quasi-equational logic; sentential logic; \(k\)-deductive system; Leibniz operator Citations:Zbl 0745.00017 PDFBibTeX XMLCite \textit{W. J. Blok} and \textit{D. Pigozzi}, Res. Expo. Math. 19, 1--56 (1992; Zbl 0768.03008)