Hypergraph Acyclicity and Extension Preservation Theorems

A class of structures satisfies the extension preservation theorem if, on this class, every first order sentence is preserved under extension iff it is equivalent to an existential sentence. We consider different acyclicity notions for hypergraphs (\gamma, \beta and \alpha-acyclicity and also acyclicity on hypergraph quotients) and estimate their influence on the validity of the extension preservation theorem on classes of finite structures. More precisely, we prove that \gamma-acyclic classes satisfy the extension preservation theorem, whereas \beta-acyclic classes do not. We also extend the validity of the extension preservation theorem for a generalization of \gamma-acyclicity that we call \gamma-acyclic k-quotient. To achieve this, we make a reduction from finite structures to their k-quotients and we use combinatorial arguments on \gamma-acyclic hypergraphs.