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A projection property

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Abstract

E. Corominas introduced recently this notion for posets:P is projective if every mapf fromP 2 toP which is order preserving and idempotent (i.e.f(x, x)=x for allx ε P) is a projection. We consider extensions of this notion to other structures, as well as to maps withn variables. We prove thatn-projectivity forn≥2 is equivalent to 2-projectivity, with a single exception: the structure has the same morphisms as the collection of congruences associated with a vector space over ℤ/2, of dimension at least two. Focusing on relational structures, Arrow's Theorem is introduced as an example. We consider particular types of relational structures: posets, graphs and metric spaces, and discuss for these the specific examples of crowns, cycles and circles.

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Author supported by the French group PRC Math-Info, and NSERC of Canada.

Author supported by NSERC Grant OGP 5407 and FCAR Québec grant Eq 0537.

Author supported by NSERC Grant 69-3039.

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Pouzet, M., Rosenberg, I.G. & Stone, M.G. A projection property. Algebra Universalis 36, 159–184 (1996). https://doi.org/10.1007/BF01234102

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  • DOI: https://doi.org/10.1007/BF01234102

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