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            <title xml:lang="en">Free-Algebra Functors from a Coalgebraic Perspective</title>
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                <title xml:lang="en">Free-Algebra Functors from a Coalgebraic Perspective</title>
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                  <title>15th International Workshop on Coalgebraic Methods in Computer Science (CMCS)</title>
                  <date type="start">2020-04-25</date>
                  <date type="end">2020-04-26</date>
                  <settlement>Dublin</settlement>
                  <country key="IE">Ireland</country>
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                <editor>Daniela Petrişan</editor>
                <editor>Jurriaan Rot</editor>
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              <p>We continue our study of free-algebra functors from a coalgebraic perspective as begun in [8]. Given a set $$\varSigma $$ of equations and a set X of variables, let $$F_{\varSigma }(X)$$ be the free $$\varSigma -$$algebra over X and $$\mathcal {V}(\varSigma )$$ the variety of all algebras satisfying $$\varSigma .$$ We consider the question, under which conditions the Set-functor $$F_{\varSigma }$$ weakly preserves pullbacks, kernel pairs, or preimages [9].We first generalize a joint result with our former student Ch. Henkel, asserting that an arbitrary $$Set-$$endofunctor F weakly preserves kernel pairs if and only if it weakly preserves pullbacks of epis.By slightly extending the notion of derivative $$\varSigma '$$ of a set of equations $$\varSigma $$ as defined by Dent, Kearnes and Szendrei in [3], we show that a functor $$F_{\varSigma }$$ (weakly) preserves preimages if and only if $$\varSigma $$ implies its own derivative, i.e. $$\varSigma \vdash \varSigma '$$, which amounts to saying that weak independence implies independence for each variable occurrence in a term of $$\mathcal {V}(\varSigma )$$. As a corollary, we obtain that the free-algebra functor will never preserve preimages when $$\mathcal {V}(\varSigma )$$ is congruence modular.Regarding preservation of kernel pairs, we show that for n-permutable varieties $$\mathcal {V}(\varSigma ),$$ the functor $$F_{\varSigma }$$ weakly preserves kernel pairs if and only if $$\mathcal {V}(\varSigma )$$ is a Mal’cev variety, i.e. 2-permutable.</p>
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