3,100 research outputs found
Partial Horn logic and cartesian categories
A logic is developed in which function symbols are allowed to represent partial functions. It has the usual rules of logic (in the form of a sequent calculus) except that the substitution rule has to be modified. It is developed here in its minimal form, with equality and conjunction, as “partial Horn logic”.
Various kinds of logical theory are equivalent: partial Horn theories, “quasi-equational” theories (partial Horn theories without predicate symbols), cartesian theories and essentially algebraic theories.
The logic is sound and complete with respect to models in , and sound with respect to models in any cartesian (finite limit) category.
The simplicity of the quasi-equational form allows an easy predicative constructive proof of the free partial model theorem for cartesian theories: that if a theory morphism is given from one cartesian theory to another, then the forgetful (reduct) functor from one model category to the other has a left adjoint.
Various examples of quasi-equational theory are studied, including those of cartesian categories and of other classes of categories. For each quasi-equational theory another, , is constructed, whose models are cartesian categories equipped with models of . Its initial model, the “classifying category” for , has properties similar to those of the syntactic category, but more precise with respect to strict cartesian functors
W-types in Homotopy Type Theory
We will give a detailed account of why the simplicial sets model of the
univalence axiom due to Voevodsky also models W-types. In addition, we will
discuss W-types in categories of simplicial presheaves and an application to
models of set theory.Comment: We have corrected the statement of Theorem 3.4. We thank Christian
Sattler for alerting us to the error in the original versio
Reasoning about Unreliable Actions
We analyse the philosopher Davidson's semantics of actions, using a strongly
typed logic with contexts given by sets of partial equations between the
outcomes of actions. This provides a perspicuous and elegant treatment of
reasoning about action, analogous to Reiter's work on artificial intelligence.
We define a sequent calculus for this logic, prove cut elimination, and give a
semantics based on fibrations over partial cartesian categories: we give a
structure theory for such fibrations. The existence of lax comma objects is
necessary for the proof of cut elimination, and we give conditions on the
domain fibration of a partial cartesian category for such comma objects to
exist
A convenient category for directed homotopy
We propose a convenient category for directed homotopy consisting of
preordered topological spaces generated by cubes. Its main advantage is that,
like the category of topological spaces generated by simplices suggested by J.
H. Smith, it is locally presentable
Variations on Algebra: monadicity and generalisations of equational theories
Dedicated to Rod Burstal
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