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retraction (313B)

      1 # Retraction
      2 
      3 A *retraction* is a [homomorphism] from a [structure] A to a [substructure] A' which is the [identity_function] on A'
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      5 The [core] of a [structure] is a retraction on a [substructure] whose [domain] has minimum [cardinality]
      6 
      7 Discussed in [morvan2025homomorphism] p40-41
      8 
      9 Up: [homomorphism_variants]