## The wonderland of reflections.(English)Zbl 1397.08002

The paper is devoted to the study of the well-known notion of CSPs (constraint satisfaction problems). For a comparison of the complexity of relational structures $$\mathbb A$$, $$\mathbb B$$, the following reductions expressing that $$\mathrm{CSP}(\mathbb B)$$ is at most hard as $$\mathrm{CSP}(\mathbb A)$$ are used:
(a)
$$\mathbb B$$ is pp-interpretable in $$\mathbb A$$,
(b)
$$\mathbb B$$ is homomorphically equivalent to $$\mathbb A$$,
(c)
$$\mathbb A$$ is a core and $$\mathbb B$$ is obtained from $$\mathbb A$$ by adding a singleton unary relation.
The authors prove a theorem giving a criterion on the structure of the polymorphism clone $$\mathcal A$$ of $$\mathbb A$$, rather than $$\mathcal B$$, which divides NP-hard from polynomial-time solvable CPSs for all finite structures $$\mathbb A$$, without necessity to consider their cores. For this purpose, an operator $$\operatorname{E}$$ referred to as reflection acting on on function clones is defined: if $$\mathcal A$$ is a function clone then $$\operatorname{E}(\mathcal A)$$ yields all function clones obtained from $$\mathcal A$$ by adding functions to it.
Furthermore, a generalization for infinite (countable) categorical structures is dealt with, together with some topological considerations.

### MSC:

 08A40 Operations and polynomials in algebraic structures, primal algebras 08A70 Applications of universal algebra in computer science 03C05 Equational classes, universal algebra in model theory 03C15 Model theory of denumerable and separable structures 03C35 Categoricity and completeness of theories 68Q17 Computational difficulty of problems (lower bounds, completeness, difficulty of approximation, etc.) 68Q25 Analysis of algorithms and problem complexity
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