The Medvedev lattice consists of degrees of so-called mass problems (subsets of $ω^ω)$. This paper emphasizes the lattice’s filter generated by the nonzero degrees that contain mass problems that are closed in the Baire topology. The authors use a Kleene-Post argument to show that this filter is not principal. They also examine the relationship between this filter and other filters that have previously appeared in the literature.
Reviewer: L.Harkleroad (Ithaca)