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

Certified Σ1\Sigma_1-sentences

In this paper, we study the employment of $\Sigma_1$-sentences with certificate, i.e., $\Sigma_1$-sentences where a number of principles is added to ensure that the witness is sufficiently number-like. We develop certificates in some detail and illustrate their use by reproving some classical results and proving some new ones. An example of such a classical result is Vaught's theorem of the strong effective inseparability of ${\sf R}_0$. We also develop the new idea of a theory being ${\sf R}_{0{\sf p}}$-sourced. Using this notion we can transfer a number of salient results from ${\sf R}_0$ to a variety of other theories.Comment: 31 page