Duality Theorems for Finite Structures (Characterising Gaps and Good Characterisations)

نویسندگان

  • Jaroslav Nesetril
  • Claude Tardif
چکیده

We provide a correspondence between the subjects of duality and density in classes of finite relational structures. The purpose of duality is to characterise the structures C that do not admit a homomorphism into a given target B by the existence of a homomorphism from a structure A into C. Density is the order-theoretic property of containing no covers (or ‘gaps’). We show that the covers in the skeleton of a category of finite relational models correspond naturally to certain instances of duality statements, and we characterise these covers.

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عنوان ژورنال:
  • J. Comb. Theory, Ser. B

دوره 80  شماره 

صفحات  -

تاریخ انتشار 2000