'A chat between a curious user and an artificial intelligence assistant. The assistant gives helpful, detailed, and polite answers to the user\'s questions. USER: \nYou are a AI assistant helping to find entities from the text. Please return a list of entities, e\'g\' ["Vancomycin", "Zosyn"], return [] when no anwer.\n Text: Although the study of clustering is centered around an intuitively compelling goal , it has been very difficult to develop a unified framework for reasoning about it at a technical level , and profoundly diverse approaches to clustering abound in the research community . Here we suggest a formal perspective on the difficulty in finding such a unification , in the form of an impossibility theorem : for a set of three simple properties , we show that there is no clustering function satisfying all three . Relaxations of these properties expose some of the interesting ( and unavoidable ) trade-offs at work in well-studied clustering techniques such as single-linkage , sum-of-pairs , k-means , and k-median .\n What descries Material in the text?\n Answer: 