Some observations on the FGH theorem

We investigate the Friedman--Goldfarb--Harrington theorem from two perspectives. Firstly, in the frameworks of classical and modal propositional logics, we study the forms of sentences whose existence is guaranteed by the FGH theorem. Secondly, we prove some variations of the FGH theorem with respect to Rosser provability predicates.Comment: 28 page

The provability logic of all provability predicates

We prove that the provability logic of all provability predicates is exactly Fitting, Marek, and Truszczy\'nski's pure logic of necessitation $\mathsf{N}$. Moreover, we introduce three extensions $\mathsf{N4}$, $\mathsf{NR}$, and $\mathsf{NR4}$ of $\mathsf{N}$ and investigate the arithmetical semantics of these logics. In fact, we prove that $\mathsf{N4}$, $\mathsf{NR}$, and $\mathsf{NR4}$ are the provability logics of all provability predicates satisfying the third condition $\mathbf{D3}$ of the derivabiity conditions, all Rosser's provability predicates, and all Rosser's provability predicates satisfying $\mathbf{D3}$, respectively.Comment: 34 page

Incompleteness and undecidability of theories consistent with R\mathsf{R}

We prove the following version of the first incompleteness theorem that simultaneously strengthens Mostowski's theorem and Vaught's theorem: For any c.e. family $\{ T_i \}_{i \in \omega}$ of consistent extensions of Tarski, Mostowski and Robinson's arithmetic $\mathsf{R}$, there exists a sentence $\varphi$ of arithmetic such that $\varphi \vdash \mathsf{R}$ and for all $i \in \omega$, $T_i \nvdash \varphi$ and $T_i \nvdash \neg \varphi$.Comment: 14 page

Prenex normalization and the hierarchical classification of formulas

Akama et al. [1] introduced a hierarchical classification of first-order formulas for a hierarchical prenex normal form theorem in semi-classical arithmetic. In this paper, we give a justification for the hierarchical classification in a general context of first-order theories. To this end, we first formalize the standard transformation procedure for prenex normalization. Then we show that the classes $\mathrm{E}_k$ and $\mathrm{U}_k$ introduced in [1] are exactly the classes induced by $\Sigma_k$ and $\Pi_k$ respectively via the transformation procedure in any first-order theory.Comment: 15 page