B. Bordbar et J. Pym, The set of idempotents in the weakly almost periodic compactification of the integers is not closed, T AM MATH S, 352(2), 2000, pp. 823-842
This paper answers negatively the question of whether the sets of idempoten
ts in the weakly almost periodic compactifications of (N, +) and (Z, +) are
closed.