Possible cardinalities of maximal abelian subgroups of quotients of permutation groups of the integers

Abstract

If G is a group then the abelian subgroup spectrum of G is defined to be the set of allsuch that there is a maximal abelian subgroup of G of size �. The cardinal invariant A(G) is defined to be the least uncountable cardinal in the abelian subgroup spectrum of G. The value of A(G) is examined for various groups G which are quotients of certain permutation groups on the integers. An important special case, to which much of the paper is devoted, is the quotient of the full symmetric group by the normal subgroup of permutations with finite support. It is shown that, if we use G to denote this group, then A(G) ≤ a. Moreover, it is consistent that A(G) 6 a. Related results are obtained for other quotients using Borel ideals.