sentences:  A direct generalization of this problem is a "k"-NN search, where we need to find the "k" closest points.Most commonly "M" is a metric space and dissimilarity is expressed as a distance metric, which is symmetric and satisfies the triangle inequality.Even more common, "M" is taken to be the "d"-dimensional vector space where dissimilarity is measured using the Euclidean distance, Manhattan distance or other distance metric.