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Wellfounded Trees in Categories
, 1999
"... this paper, we give an abstract 2 categorical characterization of Wtypes. We calculate these Wtypes explicitly in some categories of presheaves and sheaves on a site, and in the gluing category or Freyd cover. (We also have an explicit description in the case of Hyland's realizability topos, whic ..."
Abstract

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this paper, we give an abstract 2 categorical characterization of Wtypes. We calculate these Wtypes explicitly in some categories of presheaves and sheaves on a site, and in the gluing category or Freyd cover. (We also have an explicit description in the case of Hyland's realizability topos, which will be presented in [17].) These explicit calculations can be formalized in a weak predicative metatheory, and lead to the result that if E is any suitably filtered pretopos with dependent products and Wtypes, then so is the category of internal sheaves on a site in E (Remark 5.9). Our paper is organized as follows. In Section 2 we review some standard definitions concerning pretoposes and dependent products. In Section 3 we present the categorical definition of the Wconstruction, and in Section 4 we prove some of its basic functoriality properties; e.g., that it turns coequalizers into equalizers. In Section 5, a construction is presented which to each map between (pre)sheaves of sets associates a sheaf of wellfounded trees, and it is proved that this is in fact the Wtype in the category (pre)sheaves of sets (Theorem 5.6). In Section 6, we discuss the Wconstruction for the Freyd cover. Finally, in Section 7 it is shown how these categorical constructions are not only analogous to but explicitly related to MartinLof type theory. 2 Pretoposes and dependent products