ABSTRACT
As we know, this is a classical combinatorial optimization problem and solving it is exactly NP-hard, even with just two clusters [13]. According to computation complexity theory [36], no complete algorithm can get the overall optimal solutions in a polynomial time, unless P = NP. Iterative refi nement method, a popular approximate algorithm, is widely adopted by various unsupervised learning algorithms. A general iterative refi nement clustering process can be summarized as Algorithm 7.1 [6].
