Classification problems in continuum theory

Abstract

We study several natural classes and relations occurring in continuum theory from the viewpoint of descriptive set theory and infinite combinatorics. We provide useful characterizations for the relation of likeness among dendrites and show that it is a bqo with countably many equivalence classes. For dendrites with finitely many branch points the homeomorphism and quasi-homeomorphism classes coincide, and the minimal quasi-homeomorphism classes among dendrites with infinitely many branch points are identified. In contrast, we prove that the homeomorphism relation between dendrites is S ∞ S_\infty -universal. It is shown that the classes of trees and graphs are both D 2 ( Σ 3 0 ) \mathrm {D}_{2}({{\boldsymbol \Sigma _{3}^{0}}}) -complete, the class of dendrites is Π 3 0 {{\boldsymbol \Pi _{3}^{0}}} -complete, and the class of all continua homeomorphic to a graph or dendrite with finitely many branch points is Π 3 0 {{\boldsymbol \Pi _{3}^{0}}} -complete. We also show that if G G is a nondegenerate finitely triangulable continuum, then the class of G G -like continua is Π 2 0 {\boldsymbol \Pi _{2}^{0}} -complete.