Set-theoretic aspects of periodic $FC$-groups --- extraspecial p-groups   and Kurepa trees
  Given a group G, we let Z(G) denote its center, G' its commutator subgroup, and Phi (G) its Frattini subgroup (the intersection of all maximal proper subgroups of G). Given U leq G, we let N_G (U) stand for the normalizer of U in G. A group G is FC iff every element g in G has finitely many conjugates. A p-group E is called extraspecial iff Phi (E) = E' = Z(E) cong Z_p, the cyclic group with p elements.   When generalizing a characterization of centre-by-finite groups due to B. H. Neumann, M. J. Tomkinson asked the following question. Is there an FC-group G with vert G / Z(G) vert = kappa but [G:N_G(U)] < kappa for all (abelian) subgroups U of G, where kappa is an uncountable cardinal. We consider this question for kappa = omega_1 and kappa = omega_2. It turns out that the answer is largely independent of ZFC, and that it differs greatly in the two cases.   More explicitly, for kappa = omega_1, it is consistent with, and independent of, ZFC that there is a group G with vert G / Z(G) vert = omega_1 and [G:N_G (A)] leq omega for all abelian A leq G. We do not know whether the same statement is still consistent if we drop abelian. On the other hand, for kappa = omega_2, the non-existence of groups G with vert G / Z(G) vert = omega_2 and [G : N_G (A) ] leq omega_1 for all (abelian) A leq G is equiconsistent with the existence of an inaccessible cardinal. In particular, there is an extraspecial p-group with this property if there is a Kurepa tree. 
J\"org Brendle