Some errors have been found in the proofs of the published paper by
Da Feng <jvfengda at yahoo dot com>
This is a fixed version of the paper, containing the following
differences with respect to the published version:
pg.5 left column near the bottom
1) IB U IR ...
entire item replaced with new text;
pg. 8, entire proof of theorem 1 replaced with new proof.
Last updated: February 18 2015.
Paolo Valente (paolo DOT valente AT unimore DOT it)