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Douglas, thank you very much. Your edit is close to my original problem and main applications. When k=2, I've already got its bound. Also, in my published paper on ICC 2012 (you can see it in my website, page 5), I gave a loose bound for one transform of this problem. Also before I submitted this question, I discussed it with many classmates and several famous guys such as Beigel, Richard, Richard Karp and Adi Shamir. I got to know this website yesterday and decided to post this question. And I will learn how to write my problem and paper in research level since I am not a student in Math.
David, thank you. Before I submitted the question, I read through mathoverflow.net/howtoask. I just wanted to make this question very clear, specific and precise. Actually, my original question is an optimization problem as you stated with many constraints.