Singleton core in many-to-one matching problems

We explore two necessary and sufficient conditions for the singleton core in college admissions problems. One is a condition on the colleges' preference profiles, called acyclicity, and the other is a condition on their capacity vectors. We also study the implications of our acyclicity condition. The student-optimal stable matching is strongly efficient for the students, given an acyclic profile of the colleges' preference relations. Even when the colleges' true preference profile is acyclic, a college may be better off by misreporting its preference when the college-optimal stable mechanism is used.