linx@buster.cps.msu.edu (Xiao-la Lin) (06/14/91)
I post the following question for a friend in china. He has proved the following probelm is NP-complete and he'd like to know if it was proved by the others or not. Instance: A collection of finite sets {s_1, s_2, ..., s_n}, an integer k, k>0. Question: Is there a collection of sets {t_1, t_2, ..., t_m} such that n m (1) U s_i = U t_j ; i=1 j=1 (2) m <= k ; (3) For any i, j, | s_i /\ t_j | <= 1. /* /\ intersection */ If you know it was proved somewhere, would you please send me a mail? Thanks much! linx@frith.egr.msu.edu