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Adjoint functors on digraphs
Jan Foniok, Ecole Polytechnique
After explaining what they are, I will discuss how adjoint functors on digraphs can be used
as a tool for proving the non-existence of homomorphisms. Applications include polynomial
testing of the existence of a homomorphism (constraint satisfaction), a rich common structure
of non-3-colourable digraphs, and some observations related to Hedetniemi's conjecture about the chromatic number of products.
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