Athanassios Tzouvaras

A complete list of Finsler, Scott and Boffa sets whose transitive closures contain 1, 2 and 3 elements is given. An algorithm for deciding the identity of hereditarily finite Scott sets is presented. Anti-well-founded sets, i. e., non-well-founded sets whose all maximal ∈-paths are circular, are studied. For example they form transitive inner models of ZFC minus foundation and empty set, and they include uncountably many hereditarily finite awf sets. A complete list of Finsler and Boffa awf sets with 2 and 3 elements in their transitive closure is given. Next the existence of infinite descending ∈-sequences in Aczel universes is shown. Finally a theorem of Ballard and Hrbáček concerning nonstandard Boffa universes of sets is considerably extended

Let be a countable first‐order language and be an ‐structure. “Definable set” means a subset of M which is ‐definable in with parameters. A set is said to be immune if it is infinite and does not contain any infinite definable subset. X is said to be partially immune if for some definable A, is immune. X is said to be totally non‐immune if for every definable A, and are not immune. Clearly every definable set is totally non‐immune. Here we ask whether the converse is true and prove that it is false for every countable structure whose class of definable sets satisfies a mild condition. We investigate further the possibility of an alternative construction of totally non‐immune non‐definable sets with the help of a subclass of immune sets, the class of cohesive sets, as well as with the help of a generalization of definable sets, the semi‐definable ones (the latter being naturally defined in models of arithmetic). Finally connections are found between totally non‐immune sets and generic classes in nonstandard models of arithmetic.

We argue that E. Wigner’s well-known claim that mathematics is unreasonably effective in physics is only one side of the hill. The other side is the surprising insufficiency of present-day mathematics to capture the uniformities that arise in science outside physics. We describe roughly what the situation is in the areas of everyday reasoning, theory of meaning and vagueness. We make also the point that mathematics, as we know it today, founded on the concept of set, need not be a conceptually final and closed system, but only a stage in a developing subject

Let ⊢ be the ordinary deduction relation of classical first-order logic. We provide an "analytic" subrelation ⊢a of ⊢ which for propositional logic is defined by the usual "containment" criterion Γ ⊢a φ iff Γ⊢φ and Atom ⊆ Atom, whereas for predicate logic, ⊢a is defined by the extended criterion Γ⊢aφ iff Γ⊢aφ and Atom ⊆' Atom, where Atom ⊆' Atom means that every atomic formula occurring in φ "essentially occurs" also in Γ. If Γ, φ are quantifier-free, then the notions "occurs" and "essentially occurs" for atoms between Γ and φ coincide. If ⊢ is formalized by Gentzen's calculus of sequents, then we show that ⊢a is axiomatizable by a proper fragment of analytic inference rules. This is mainly due to cut elimination. By "analytic inference rule " we understand here a rule r such that, if the sequent over the line is analytic, then so is the sequent under the line. We also discuss the notion of semantic relevance as contrasted to the previous syntactic one. We show that when introducing semantic sequents as axioms, i.e. when extending the pure logical axioms and rules by mathematical ones, the property of syntactic relevance is lost, since cut elimination no longer holds. We conclude that no purely syntactic notion of analytic deduction can ever replace successfully the complex semantico-syntactic deduction we already possess.

In the context of a distributive lattice we specify the sort of mappings that could be generally called ''negations'' and study their behavior under iteration. We show that there are periodic and nonperiodic ones. Natural periodic negations exist with periods 2, 3, and 4 and pace 2, as well as natural nonperiodic ones, arising from the interaction of interior and quasi interior mappings with the pseudocomplement. For any n and any even, negations of period n and pace s can also be constructed, but in a rather ad hoc and trivial way.

We present a formalization of collections that Cornelius Castoriadis calls “magmas”, especially the property which mainly characterizes them and distinguishes them from the usual cantorian sets. It is the property of their elements to _depend_ on other elements, either in a one-way or a two-way manner, so that one cannot occur in a collection without the occurrence of those dependent on it. Such a dependence relation on a set _A_ of atoms (or urelements) can be naturally represented by a pre-order relation \(\preccurlyeq \) of _A_ with the extra condition that it contains no minimal elements. Then, working in a mild strengthening of the theory \(\textrm{ZFA}\), where _A_ is an infinite set of atoms equipped with a primitive pre-ordering \(\preccurlyeq \), the class of magmas over _A_ is represented by the class \(LO(A,\preccurlyeq )\) of nonempty open subsets of _A_ with respect to the lower topology of \(\langle A,\preccurlyeq \rangle \). The non-minimality condition for \(\preccurlyeq \) implies that all sets of \(LO(A,\preccurlyeq )\) are infinite and none of them is \(\subseteq \) -minimal. Next the pre-ordering \(\preccurlyeq \) is shifted (by a kind of simulation) to a pre-ordering \(\preccurlyeq ^+\) on \({{\mathcal {P}}}(A)\), which turns out to satisfy the same non-minimality condition as well, and which, happily, when restricted to \(LO(A,\preccurlyeq )\) coincides with \(\subseteq \). This allows us to define a hierarchy \(M_\alpha (A)\), along all ordinals \(\alpha \ge 1\), the“magmatic hierarchy”, such that \(M_1(A)=LO(A,\preccurlyeq )\), \(M_{\alpha +1}(A)=LO(M_\alpha (A),\subseteq )\), and \(M_\alpha (A)=\bigcup _{\beta, for a limit ordinal \(\alpha \). For every \(\alpha \ge 1\), \(M_\alpha (A)\subseteq V_\alpha (A)\), where \(V_\alpha (A)\) are the levels of the universe _V_(_A_) of \(\textrm{ZFA}\). The class \(M(A)=\bigcup _{\alpha \ge 1}M_\alpha (A)\) is the “magmatic universe above _A_.” The axioms of Powerset and Union (the latter in a restricted version) turn out to be true in \(\langle M(A),\in \rangle \). Besides it is shown that three of the five principles about magmas that Castoriadis proposed in his writings (mildly modified and adapted to the needs of formalization), \(M2^*\), \(M3^*\) and \(M5^*\), are true of _M_(_A_). A selection of excerpts from these writings, in which the concept of magma was first introduced and elaborated, is presented in the Introduction.

We adapt C. Freiling's axioms of symmetry 190–200) to models of set theory with classes by identifying small classes with sets getting thus a sequence of principles An, for n2, of increasing strength. Several equivalents of A2 are given. A2 is incompatible both with the foundation axiom and the antifoundation axioms AFA considered in Aczel . A hierarchy of symmetry degrees of preorderings is introduced and compared with An. Models are presented in which this hierarchy is strict. The main result of the paper is that a class X satisfies ¬An iff it has symmetry degree n−2