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Methods for proving nonexistence of valued minion homomorphisms

Develop techniques to prove nonexistence of valued minion homomorphisms between specific pairs of templates (for example, from Plu(A,B) to projection-like minions), enabling robust hardness transfers and separations.

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Background

Hardness results via the reduction theorem rely on demonstrating that certain homomorphisms do not exist. Systematic methods for such nonexistence proofs are underdeveloped in the valued setting, especially with plurimorphisms.

Creating general tools (invariants, obstructions, or algebraic certificates) would strengthen the framework’s ability to capture inapproximability.

References

There are however many basic questions and theory-building tasks left open already for finite-domain valued PCSPs: to incorporate the trivial reduction as in e.g. \cref{ex:crips-vs-valued}; to characterize gadget reductions (or versions of definability) in terms of polymorphisms or plurimorphisms; to characterize plurimorphism valued minions of templates; to clarify whether plurimorphisms are necessary to determine computational complexity or enough information is provided already by polymorphisms; to develop methods for proving nonexistence of homomorphisms; to revisit the valued CSP dichotomy without fixed threshold and Raghavendra's result on unique games hardness of approximation for all MaxCSPs; among others. An interesting special case for a full complexity classification is the valued non-promise CSPs with fixed threshold.

The Rise of Plurimorphisms: Algebraic Approach to Approximation (2401.15186 - Barto et al., 26 Jan 2024) in Conclusion