Concept

Interior algebra

Résumé
In abstract algebra, an interior algebra is a certain type of algebraic structure that encodes the idea of the topological interior of a set. Interior algebras are to topology and the modal logic S4 what Boolean algebras are to set theory and ordinary propositional logic. Interior algebras form a variety of modal algebras. Definition An interior algebra is an algebraic structure with the signature :⟨S, ·, +, ′, 0, 1, I⟩ where :⟨S, ·, +, ′, 0, 1⟩ is a Boolean algebra and postfix I designates a unary operator, the interior operator, satisfying the identities:

xI ≤ x

xII = xI

(xy)I = xIyI

1I = 1

xI is called the interior of x. The dual of the interior operator is the closure operator C defined by xC = ((x′)I)′. xC is called the closure of x. By the principle of duality, the closure operator satisfies the identities:

xC ≥ x

xCC = xC

(x + y)C = xC + yC

0C = 0

If the closure operator is taken as primitive, the interior operator can be defined as xI = ((x′)
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