Full Idea
The four important logical equivalences in modal logic (the Modal Negation equivalences) are: ¬◊p↔□¬p, ◊¬p↔¬□p, □p↔¬◊¬p, and ◊p↔¬□¬p.
Gist of Idea
Modal logic has four basic modal negation equivalences
Source
Rod Girle (Modal Logics and Philosophy [2000], 1.2)
Book Reference
Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.3
A Reaction
[Possibly is written as a diamond, necessarily a square] These are parallel to a set of equivalences between quantifiers in predicate logic. They are called the four 'modal negation (MN) equivalences'.