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arXiv 1504.05947math.LO
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一阶逻辑模态片断中的有限关系代数与省略类型

Finite relation algebras and omitting types in modal fragments of first order logic

Tarek Sayed Ahmed

摘要:

令2<n \leq l<m< ω。设L_n表示限制于前n个变量的一阶逻辑。我们发现,省略类型定理显着失败的n-变量片段的一阶逻辑相对于集团保护语义,其包装的n-变量片段。它们都是L_n的模态片断。作为一个例子,我们证明了:如果存在一个具有强l-模糊的有限关系代数,且不存在m维关系基,则存在一个可数的、原子的、完备的L_n理论T和类型Γ,使得Γ在T的每一个所谓的m-平方模型中都是可实现的,但任何隔离Γ的证明不能使用小于$l$的变量. T的一个$m$ -平方模型M给出了一种形式的集团保护语义,其中参数m度量了M的局部良好行为。每一个普通的模型都是k平方的,对于任意的n<k<ω,但反之则不成立。任何模型M都是ω-平方的,如果M是可数的,这两个概念是等价的。对于l和m的某些值,存在这样的关系代数,如对于n \leq l<ω和m=ω,以及对于l=n和m \geq n+3.当l=n,m=ω时,L_n的省略类型定理在(通常的)塔斯基语义下是不成立的:存在一个原子可数L_n理论T,对于T的任何模型M,由余原子组成的单个非主类型不能被省略。当n<ω时,通过对理论和/或被省略的类型附加条件,得到了L_n的省略类型的正结果.对于L_{ω, ω}的无穷变体和扩展,得到了关于省略类型的肯定和否定结果.

英文摘要:

Let 2<n\leq l<m< ω. Let L_n denote first order logic restricted to the first n variables. We show that the omitting types theorem fails dramatically for the n--variable fragments of first order logic with respect to clique guarded semantics, and for its packed n--variable fragments. Both are modal fragments of L_n. As a sample, we show that if there exists a finite relation algebra with a so--called strong l--blur, and no m--dimensional relational basis, then there exists a countable, atomic and complete L_n theory T and type Γ, such that Γis realizable in every so--called m--square model of T, but any witness isolating Γcannot use less than $l$ variables. An $m$--square model M of T gives a form of clique guarded semantics, where the parameter m, measures how locally well behaved M is. Every ordinary model is k--square for any n<k<ω, but the converse is not true. Any model M is ω--square, and the two notions are equivalent if M is countable. Such relation algebras are shown to exist for certain values of l and m like for n\leq l<ωand m=ω, and for l=n and m\geq n+3. The case l=n and m=ωgives that the omitting types theorem fails for L_n with respect to (usual) Tarskian semantics: There is an atomic countable L_n theory T for which the single non--principal type consisting of co--atoms cannot be omitted in any model M of T. For n<ω, positive results on omitting types are obained for L_n by imposing extra conditions on the theories and/or the types omitted. Positive and negative results on omitting types are obtained for infinitary variants and extensions of L_{ω, ω}.

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