We consider an extension of the popular matching problem: the popular condensation problem. An instance of the popular matching problem consists of a set of applicants A and a set of posts P. Each applicant has a strictly ordered preference list, which is a sequence of posts in order of his/her preference. A matching M mapping from A to P is popular if there is no other matching M' such that more applicants prefer M' to M than prefer M to M'. Although some efficient algorithms have been proposed for finding a popular matching, a popular matching may not exist for those instances where the competition of some applicants cannot be resolved. The popular condensation problem is to find a popular matching with the minimum number of applicants whose preferences are neglected, that is, to optimally condense the instance to admit a local popular matching. We show that the problem can be solved in O(n + m) time, where n is the number of applicants and posts, and m is the total length of the preference lists.
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