4. Assume that A is a friend of B if and only if B is a friend of A. Show that for any N ≥ 2, there are two people in the group of N persons having the same number of friends. (15%)
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4. Assume that A is a friend of B if and only if B is a friend of A. Show that for any N ≥ 2, there are two people in the group of N persons having the same number of friends. (15%)