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Ultrafilters of sets

Antonín Sochor, Petr Vopěnka (1981)

Commentationes Mathematicae Universitatis Carolinae

Uncountable cardinals have the same monadic ∀₁¹ positive theory over large sets

Athanassios Tzouvaras (2004)

Fundamenta Mathematicae

We show that uncountable cardinals are indistinguishable by sentences of the monadic second-order language of order of the form (∀X)ϕ(X) and (∃X)ϕ(X), for ϕ positive in X and containing no set-quantifiers, when the set variables range over large (= cofinal) subsets of the cardinals. This strengthens the result of Doner-Mostowski-Tarski [3] that (κ,∈), (λ,∈) are elementarily equivalent when κ, λ are uncountable. It follows that we can consistently postulate that the structures ( 2 κ , [ 2 κ ] > κ , < ) , ( 2 λ , [ 2 λ ] > λ , < ) are indistinguishable...

Valuations of lines

Josef Mlček (1992)

Commentationes Mathematicae Universitatis Carolinae

We enlarge the problem of valuations of triads on so called lines. A line in an e -structure 𝔸 = A , F , E (it means that A , F is a semigroup and E is an automorphism or an antiautomorphism on A , F such that E E = 𝐈𝐝 A ) is, generally, a sequence 𝔸 B , 𝔸 U c , c 𝐅𝐙 (where 𝐅𝐙 is the class of finite integers) of substructures of 𝔸 such that B U c U d holds for each c d . We denote this line as 𝔸 ( U c , B ) c 𝐅𝐙 and we say that a mapping H is a valuation of the line 𝔸 ( U c , B ) c 𝐅𝐙 in a line 𝔸 ^ ( U ^ c , B ^ ) c 𝐅𝐙 if it is, for each c 𝐅𝐙 , a valuation of the triad 𝔸 ( U c , B ) in 𝔸 ^ ( U ^ c , B ^ ) . Some theorems on an existence of...

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