[SOUND] So first, we will introduce basic concepts of partitioning algorithms. The partitioning method is essentially to discover the groupings in the data. That means you get K groups if you want to partition, I mean, 2K groups by optimizing a specific object function, for example, sum of the square distance. And then we iteratively improve the quality of such partitioning. So the K-partitioning method, we partition a dataset D of n objects into a set of K clusters. Then we definitely can iteratively improve it, so that an object function is optimized, for example, the object function could be the sum of the square distance is minimized, where C sub k is the centroid or medoid of cluster capital C sub k. So a typical objective function is Sum of Squared Errors is often written as SSE. So a clustering, okay, for the SSE, is sum of K clusters, k is from 1 to K, okay. Then for each such cluster if an object's i is in this cluster, then the square error that simply says the distance of sum of these squares. These such distance, the whole function, is a K clusters, and each cluster get its points to its center the sum of such a square, distance or square errors. We want to minimize this objective function. Then in general what we can think, conceptual we can think of this, if you get a very nice K cluster, then, all the objects to the center is somehow shorter, okay. It is shorter than the sum of the square distance or the square errors. Actually, it will be pretty small. That's why we can't get within the cluster center, the sum of the square error is pretty small. Then if we get a K such thing, we add them together, the whole thing will be smaller. For the K-partitioning method, essentially the problem is given a K, the number of clusters, we want to find a partition of these K clusters that optimize the chosen partitioning criterion, for example, the sum of square distance. However, to find a global optimal, actually we need to exhaustively enumerate all partitioning because the potential number of partitioning actually is exponential. So more realistically, we should get a heuristic method, that means the greedy algorithms. For example, the typically used like the K-Means, K-Medians, K-Medoids and all the other methods are all greedy algorithms or heuristic methods to find such a nice partition. [MUSIC]