Solution

ID: past-exam-of-the-graph-theory-and-probabilistic-combinatorics-course-of-the-university-of-oxford/2009/a/1/b/solution

Write the bipartition as . Counting edges by their left and right ends gives , so the parts have equal size. Put . If some violated Hall's marriage theorem in , let , so .
There are edges of incident with , of which at most can end in . Thus at least end in . All those edges must have been deleted, by the definition of . This contradicts . Hence has a matching covering , and equal part sizes make it a perfect matching avoiding every edge of .

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