Palmgren, E and Vickers, Steven (2006) Partial Horn logic and cartesian categories. Annals of Pure and Applied Logic, 143 (3). pp. 314353. ISSN 01680072
  URL of Published Version: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TYB4MCWM891&_user=122868&_coverDate=03%2F31%2F2007&_rdoc=7&_fmt=high&_orig=browse&_srch=docinfo(%23toc%235614%232007%23998549996%23640819%23FLP%23display%23Volume)&_cdi=5614&_sort=d&_docanchor= Identification Number/DOI: doi:10.1016/j.apal.2006.10.001 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, “quasiequational” 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 quasiequational 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 quasiequational theory are studied, including those of cartesian categories and of other classes of categories. For each quasiequational 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.

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