Identity in Subcategory C': Does it Coincide with Identity in Category C?

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SUMMARY

The discussion centers on the relationship between identity in two categories, C' and C, under specific conditions. It establishes that if C' is a subcategory of C, where each object in C' belongs to C, the hom-sets of C' are contained within those of C, and the composition in C' is a restriction of that in C, then the identity of an object A in C' coincides with its identity in C. The example provided illustrates this with a single object A, demonstrating that the hom-sets and compositions align under the defined conditions.

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  • Understanding of category theory concepts, specifically categories and subcategories.
  • Familiarity with hom-sets and their properties in category theory.
  • Knowledge of composition laws in categories.
  • Basic grasp of identity morphisms in category theory.
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  • Study the definitions and properties of subcategories in category theory.
  • Explore the concept of hom-sets in greater detail, particularly in relation to morphisms.
  • Investigate examples of identity morphisms in various categories.
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This discussion is beneficial for mathematicians, specifically those studying category theory, as well as educators and students seeking to deepen their understanding of the relationships between identities in different categories.

SVD
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Using the definition given below, I wonder whether we can deduce that for each object A in C',
the identity for A in C' coincides with the identity for A in C.

Let C' and C be two categories which satisfies that
(i)each objects in C' belongs to C
(ii)each hom-set in C' is contained in the corresponding hom-set in C.
(iii)each composition in C' is the restriction of that in C .
 
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No. Let C and C' consist out of exactly one object A. Let

Hom_{C^\prime}(A,A)=\{a\}~\text{and}~Hom_C(A,A)=\{a,b\}

with

a\circ a = a\circ b=b\circ a=a~\text{and}~b\circ b=b
 

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