2000 Mathematics Subject Classification: 20E18, 12G05, 12F10, 12F99.Given a profinite group Γ acting continuously on a discrete quasi-cyclic group A, certain classes of closed subgroups of Γ (radical, hereditarily radical, Kneser, almost Kneser, and hereditarily Kneser) having natural field theoretic interpretations are defined and investigated. One proves that the hereditarily Kneser subgroups of Γ form a closed subspace of the irreducible spectral space of all closed subgroups of Γ, and a hereditarily...