<p>A <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathcal {C}-\text {set}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">C</mi> <mo>-</mo> <mtext>set</mtext> </mrow> </math></EquationSource> </InlineEquation> is a functor from a category <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\mathcal {C}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">C</mi> </math></EquationSource> </InlineEquation> to the category of finite sets, and <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\mathcal {C}-\text {set}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">C</mi> <mo>-</mo> <mtext>set</mtext> </mrow> </math></EquationSource> </InlineEquation>&#xa0;denotes the category of such functors with natural transformations as morphisms. In this work, we prove that <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\mathcal {C}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">C</mi> </math></EquationSource> </InlineEquation> is a groupoid if and only if <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\mathcal {C}-\text {set}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">C</mi> <mo>-</mo> <mtext>set</mtext> </mrow> </math></EquationSource> </InlineEquation> &#xa0;has finitely many indecomposable objects, which is the categorical analog of transitive <i>G</i>-sets in classical group actions.</p>

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A Characterization of Groupoids in Terms of Their Category of \(\mathcal {C}\)-Sets

  • J. Miguel Calderón,
  • Alberto G. Raggi-Cárdenas,
  • Itzel Rosas,
  • Ramón H. Ruiz-Medina

摘要

A \(\mathcal {C}-\text {set}\) C - set is a functor from a category \(\mathcal {C}\) C to the category of finite sets, and \(\mathcal {C}-\text {set}\) C - set  denotes the category of such functors with natural transformations as morphisms. In this work, we prove that \(\mathcal {C}\) C is a groupoid if and only if \(\mathcal {C}-\text {set}\) C - set  has finitely many indecomposable objects, which is the categorical analog of transitive G-sets in classical group actions.