Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6426042 | Advances in Mathematics | 2011 | 20 Pages |
Let N be the set of non-negative integer numbers, T the circle group and c the cardinality of the continuum. Given an abelian group G of size at most 2c and a countable family E of infinite subsets of G, we construct “Baire many” monomorphisms Ï:GâTc such that Ï(E) is dense in {yâTc:ny=0} whenever nâN, EâE, nE={0} and {xâE:mx=g} is finite for all gâG and mâNâ{0} such that n=mk for some kâNâ{1}. We apply this result to obtain an algebraic description of countable potentially dense subsets of abelian groups, thereby making a significant progress towards a solution of a problem of Markov going back to 1944. A particular case of our result yields a positive answer to a problem of Tkachenko and Yaschenko (2002) [22, Problem 6.5]. Applications to group actions and discrete flows on Tc, Diophantine approximation, Bohr topologies and Bohr compactifications are also provided.