On small analytic relations (2005.13212v1)

Abstract: We study the class of analytic binary relations on Polish spaces, compared with the notions of continuous reducibility or injective continuous reducibility. In particular, we characterize when a locally countable Borel relation is $\Sigma$ 0 $\xi$ (or $\Pi$ 0 $\xi$), when $\xi$ $\ge$ 3, by providing a concrete finite antichain basis. We give a similar characterization for arbitrary relations when $\xi$ = 1. When $\xi$ = 2, we provide a concrete antichain of size continuum made of locally countable Borel relations minimal among non-$\Sigma$ 0 2 (or non-$\Pi$ 0 2) relations. The proof of this last result allows us to strengthen a result due to Baumgartner in topological Ramsey theory on the space of rational numbers. We prove that positive results hold when $\xi$ = 2 in the acyclic case. We give a general positive result in the non-necessarily locally countable case, with another suitable acyclicity assumption. We provide a concrete finite antichain basis for the class of uncountable analytic relations. Finally, we deduce from our positive results some antichain basis for graphs, of small cardinality (most of the time 1 or 2).