Sahlqvist formula

In modal logic, Sahlqvist formulas are a certain kind of modal formula with remarkable properties. The Sahlqvist correspondence theorem states that every Sahlqvist formula is canonical, and corresponds to a first-order definable class of Kripke frames.

Sahlqvist's definition characterizes a decidable set of modal formulas with first-order correspondents. Since it is undecidable, by Chagrova's theorem, whether an arbitrary modal formula has a first-order correspondent, there are formulas with first-order frame conditions that are not Sahlqvist [Chagrova 1991] (see the examples below). Hence Sahlqvist formulas define only a (decidable) subset of modal formulas with first-order correspondents.

Definition

Sahlqvist formulas are built up from implications, where the consequent is positive and the antecedent is of a restricted form.

• A boxed atom is a propositional atom preceded by a number (possibly 0) of boxes, i.e. a formula of the form $\Box \cdots \Box p$ (often abbreviated as $\Box ^{i}p$ for $0\leq i<\omega$).
• A Sahlqvist antecedent is a formula constructed using ∧, ∨, and $\Diamond$ from boxed atoms, and negative formulas (including the constants ⊥, ⊤).
• A Sahlqvist implication is a formula A → B, where A is a Sahlqvist antecedent, and B is a positive formula.
• A Sahlqvist formula is constructed from Sahlqvist implications using ∧ and $\Box$ (unrestricted), and using ∨ on formulas with no common variables.

Examples of Sahlqvist formulas

$p\rightarrow \Diamond p$

Its first-order corresponding formula is $\forall x\;Rxx$, and it defines all reflexive frames

$p\rightarrow \Box \Diamond p$

Its first-order corresponding formula is $\forall x\forall y[Rxy\rightarrow Ryx]$, and it defines all symmetric frames

$\Diamond \Diamond p\rightarrow \Diamond p$ or $\Box p\rightarrow \Box \Box p$

Its first-order corresponding formula is $\forall x\forall y\forall z[(Rxy\land Ryz)\rightarrow Rxz]$, and it defines all transitive frames

$\Diamond p\rightarrow \Diamond \Diamond p$ or $\Box \Box p\rightarrow \Box p$

Its first-order corresponding formula is $\forall x\forall y[Rxy\rightarrow \exists z(Rxz\land Rzy)]$, and it defines all dense frames

$\Box p\rightarrow \Diamond p$

Its first-order corresponding formula is $\forall x\exists y\;Rxy$, and it defines all right-unbounded frames

$\Diamond \Box p\rightarrow \Box \Diamond p$

Its first-order corresponding formula is $\forall x\forall x_{1}\forall z_{0}[Rxx_{1}\land Rxz_{0}\rightarrow \exists z_{1}(Rx_{1}z_{1}\land Rz_{0}z_{1})]$, and it is the Church-Rosser property.

Examples of non-Sahlqvist formulas

$\Box \Diamond p\rightarrow \Diamond \Box p$

This is the McKinsey formula; it does not have a first-order frame condition.

$\Box (\Box p\rightarrow p)\rightarrow \Box p$

The Löb axiom is not Sahlqvist; again, it does not have a first-order frame condition.

$(\Box \Diamond p\rightarrow \Diamond \Box p)\land (\Diamond \Diamond q\rightarrow \Diamond q)$

The conjunction of the McKinsey formula and the (4) axiom has a first-order frame condition but is not equivalent to any Sahlqvist formula.

Kracht's theorem

When a Sahlqvist formula is used as an axiom in a normal modal logic, the logic is guaranteed to be complete with respect to the elementary class of frames the axiom defines. This result comes from the Sahlqvist completeness theorem [Modal Logic, Blackburn et al., Theorem 4.42]. But there is also a converse theorem, namely a theorem that states which first-order conditions are the correspondents of Sahlqvist formulas. Kracht's theorem states that any Sahlqvist formula locally corresponds to a Kracht formula; and conversely, every Kracht formula is a local first-order correspondent of some Sahlqvist formula which can be effectively obtained from the Kracht formula [Modal Logic, Blackburn et al., Theorem 3.59].

References

• L. A. Chagrova, 1991. An undecidable problem in correspondence theory. Journal of Symbolic Logic 56:1261-1272.
• Marcus Kracht, 1993. How completeness and correspondence theory got married. In de Rijke, editor, Diamonds and Defaults, pages 175-214. Kluwer.
• Henrik Sahlqvist, 1975. Correspondence and completeness in the first- and second-order semantics for modal logic. In Proceedings of the Third Scandinavian Logic Symposium. North-Holland, Amsterdam.