Girard’s phase semantics is a way of building star-autonomous posets with exponential modalities, hence models of classical linear logic.
Let be a commutative monoid, and a specified subset. ( equipped with is sometimes called a phase space.) Then , the powerset of , is a commutative quantale, with tensor product
and internal-hom
Indeed, if we regard as a discrete poset, hence as a category enriched over , then is its -enriched presheaf category, and this is its Day convolution monoidal structure.
Now is an object of , hence induces a contravariant self-adjunction of , which is idempotent since is a poset. We define a fact to be a fixed point of this adjunction, i.e. a subset of the form , or equivalently such that .
The poset of facts is star-autonomous.
It is closed under , since . And it is reflective, with reflector . Thus, it is closed symmetric monoidal with tensor product . Since it also contains , as , it is therefore star-autonomous by construction.
The poset of facts is also, of course, a complete lattice, since it a reflective sub-poset of the complete lattice . In addition, it admits exponential modalities and . There are different ways to obtain these, but perhaps the simplest (see here) is to take
where is the set of idempotents in , and is the unit object of the monoidal category of facts.
Phase semantics is complete for provability in linear logic, i.e. if a sequent of linear logic is valid in the phase semantics for all choices of and , then it is provable. This follows from a construction of a particular and out of the syntax: let be the free commutative monoid on the formulas of linear logic subject to the relation that formulas of the form are idempotent, and let be the set of that are provable when regarded as one-sided sequents , and interpret a formula by the set of such that is provable. See Girard for details.
When phrased categorically as above, there is an obvious generalization to the case when is a commutative monoidal poset, with replaced by its -enriched presheaf category, i.e. the poset of downsets in . We can also generalize to other enrichments, although in that case the idempotence of the adjunction is no longer automatic but has to be assumed.
If is assumed only to be a promonoidal -enriched category, i.e. equipped with a suitable relation , then it is a ternary frame. This can be used to construct models of more general substructural logics.
Last revised on May 3, 2024 at 19:34:24. See the history of this page for a list of all contributions to it.