Sahlqvist Formula - Examples of Non-Sahlqvist Formulas

Examples of Non-Sahlqvist Formulas

This is the McKinsey formula; it does not have a first-order frame condition.
The Löb axiom is not Sahlqvist; again, it does not have a first-order frame condition.
The conjunction of the McKinsey formula and the (4) axiom has a first-order frame condition but is not equivalent to any Sahlqvist formula.

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