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obstruction (489B)

      1 # Obstruction
      2 
      3 From [morvan2025homomorphism]: an *obstruction* of a finite structure B is a finite structure D that does not have a [homomorphism] to B
      4 
      5 Whenever an obstruction D of B has a [homomorphism] to a structure A, then it witnesses that A does not have a homomorphism to B
      6 
      7 Via [homomorphism_duality], this is a [query_match] of the dual of B
      8 
      9 [obstruction_critical]
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     11 Variant: [obstruction_set] for [minors]
     12 - [obstruction_treewidth_pathwidth]
     13 
     14 Up: [graph]
     15 
     16 Aliases: obstructions