Abstract
Dating back to Birkhoff, pointwise ergodic theorems for probability
measure preserving (pmp) actions of countable groups are bridges
between the global condition of ergodicity (measure-transitivity) and
the local combinatorics of the actions. Each such action induces a
Borel equivalence relation with countable classes and the study of
these equivalence relations is a flourishing subject in modern
descriptive set theory. Such an equivalence relation can also be
viewed as the connectedness relation of a locally countable Borel
graph. These strong connections between equivalence relations, group
actions, and graphs create an extremely fruitful interplay between
descriptive set theory, ergodic theory, measured group theory,
probability theory, and descriptive graph combinatorics. I will
discuss how descriptive set theoretic thinking combined with
combinatorial and measure theoretic arguments yields a pointwise
ergodic theorem for quasi-pmp locally countable graphs. This theorem
is a vastly general random version of pointwise ergodic theorems for
group actions and is provably the best possible pointwise ergodic
result for some of these actions.