Publishers' note: The paper has been revoked on May 29, 2009, as it turned out that it plagiarises the article: Y. Pinchover, "On the maximum and anti-maximum principles", in: Differential equations and mathematical physics, Proceedings of an international conference, Birmingham, AL, USA, March 16-20, 1999, AMS/IP Stud. Adv. Math. 16, 323-338, 2000.