Published online by Cambridge University Press: 02 December 2021
The Gödel translation provides an embedding of the intuitionistic logic $\mathsf {IPC}$ into the modal logic
$\mathsf {Grz}$, which then embeds into the modal logic
$\mathsf {GL}$ via the splitting translation. Combined with Solovay’s theorem that
$\mathsf {GL}$ is the modal logic of the provability predicate of Peano Arithmetic
$\mathsf {PA}$, both
$\mathsf {IPC}$ and
$\mathsf {Grz}$ admit provability interpretations. When attempting to ‘lift’ these results to the monadic extensions
$\mathsf {MIPC}$,
$\mathsf {MGrz}$, and
$\mathsf {MGL}$ of these logics, the same techniques no longer work. Following a conjecture made by Esakia, we add an appropriate version of Casari’s formula to these monadic extensions (denoted by a ‘+’), obtaining that the Gödel translation embeds
$\mathsf {M^{+}IPC}$ into
$\mathsf {M^{+}Grz}$ and the splitting translation embeds
$\mathsf {M^{+}Grz}$ into
$\mathsf {MGL}$. As proven by Japaridze, Solovay’s result extends to the monadic system
$\mathsf {MGL}$, which leads us to a provability interpretation of both
$\mathsf {M^{+}IPC}$ and
$\mathsf {M^{+}Grz}$.