Preprint
Sep 2026
Schrijver-Delsarte rigidity in association schemes and undecidability of quantum graph homomorphism
We prove RE-completeness of the quantum homomorphism problem parameterised by families of graphs derived from the classic metric association schemes. These include Kneser graphs, $q$-Kneser graphs, and the complements of Johnson, Grassmann, and Hamming graphs. Our proof develops a spectral method for establishing non-c...
Lorenzo Ciardo, Iris Hebbeker, Gideo Joubert et al.
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