Problem A. 451. (March 2008)
A. 451. Let be an upset and let be a downset on a set S of n elements, i.e.
(a) for every we have for all XYS;
(b) for every and YX, .
Prove that .
Deadline expired on April 15, 2008.
4 students sent a solution. 5 points: Lovász László Miklós, Nagy 235 János, Tomon István, Wolosz János.