Confused about Cartesian closed categories.
This is all very simple stuff, which I should have worked out a long time ago. What is the correct definition of a Cartesian closed category? Some books seem to say that a CCC has a finite limits plus the relevant conditions to give you exponitails. While other books seem to define it as finite products plus exponitails. Am I reading all these books wrong (I always thought it was all finite limits so I might of systimacitally misread lots of books since). Or can you get finite limits from finite products (but where do you get the equalisers form?) Regards Justin Pearson
What is the correct definition of a Cartesian closed category?
MacLane ("Categories for the Working Mathematician") and many other standard texts do not assume equalizers in a CCC, nor in a Cartesian category. If your interest in CCCs is that they provide categorical models for the typed lambda calculus, then you don't need equalizers (see Lambek and Scott, "Intro. to Higher Order Categorical Logic"), and many of the standard examples in computer science don't have them. I can recall only one case of the other usage, where CCCs have all finite limits, and even there it was explained as a nonstandard convention. Freyd and Scedrov ("Categories - Allegories") define "Cartesian" to mean having all finite limits. Obviously one associates Cartesian coordinates with finite products, but they point out that Descartes was also concerned with equationally defined subsets, and those are equalizers. They avoid the phrase "Cartesian closed category". If you want my personal opinion, it's that neither CCCs nor - despite the force of Freyd and Scedrov's comment - Cartesian categories are exected to have limits other than finite products. Steve Vickers.
(Correction to an earlier posting - sorry) I wrongly stated that MacLane in Categories for Working Mathematicians defines Cartesian categories to be those that have finite products. He doesn't define the term at all. However, his Cartesian closed categories are indeed categories with finite products and exponentials. Thanks to Roy Crole for pointing this out. Steve Vickers.
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Justin K Pearson -
Steven Vickers