  Let Q be an infinite set of positive integers. Denote by W(Q) the set of
n-tuples of real numbers simultaneously tau-well approximable by infinitely
many rationals with denominators in Q but only by finitely many rationals with
denominators in the complement of Q. The Hausdorff dimension of the liminf set
W(Q) is computed when tau > 2 + 1/n. A p-adic analogue of the problem is also
studied.
