Equations in oligomorphic clones and the Constraint Satisfaction Problem   for $\omega$-categorical structures
  There exist two conjectures for constraint satisfaction problems (CSPs) of reducts of finitely bounded homogeneous structures: the first one states that tractability of the CSP of such a structure is, when the structure is a model-complete core, equivalent to its polymorphism clone satisfying a certain non-trivial linear identity modulo outer embeddings. The second conjecture, challenging the approach via model-complete cores by reflections, states that tractability is equivalent to the linear identities (without outer embeddings) satisfied by its polymorphisms clone, together with the natural uniformity on it, being non-trivial.   We prove that the identities satisfied in the polymorphism clone of a structure allow for conclusions about the orbit growth of its automorphism group, and apply this to show that the two conjectures are equivalent. We contrast this with a counterexample showing that $\omega$-categoricity alone is insufficient to imply the equivalence of the two conditions above in a model-complete core.   Taking a different approach, we then show how the Ramsey property of a homogeneous structure can be utilized for obtaining a similar equivalence under different conditions.   We then prove that any polymorphism of sufficiently large arity which is totally symmetric modulo outer embeddings of a finitely bounded structure can be turned into a non-trivial system of linear identities, and obtain non-trivial linear identities for all tractable cases of reducts of the rational order, the random graph, and the random poset.   Finally, we provide a new and short proof, in the language of monoids, of the theorem stating that every $\omega$-categorical structure is homomorphically equivalent to a model-complete core. 
Libor Barto, Michael Kompatscher, Miroslav Ol\v{s}\'ak, Trung Van   Pham, Michael Pinsker
cs.LO math.LO