Citation for Published Item: Use Policy First-order Definable Retraction Problems for Posets and Reflexive Graphs *
نویسندگان
چکیده
A retraction from a structure P to its substructure Q is a homomorphism from P onto Q that is the identity on Q. We present an algebraic condition which completely characterises all posets and all reflexive graphs Q with the following property: the class of all posets or reflexive graphs, respectively, that admit a retraction onto Q is first-order definable.
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