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China Northern MO
2022 China Northern MO
4
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Part of
2022 China Northern MO
Problems
(1)
friendships among 22 mathematicians
Source: China Northern MO 2022 p4 CNMO
4/6/2024
22
22
22
mathematicians are meeting together. Each mathematician has at least
3
3
3
friends (friends are mutual). And each mathematician can pass his or her information to any mathematician through the transfer between friends. Is it possible to divide these
22
22
22
mathematicians into
2
2
2
-person groups (that is, two people in each group, a total of
11
11
11
groups), so that the mathematicians in each group are friends?[hide=original wording in Chinese]仃22位数学家一起开会.每位数学家都至少有3个朋友(朋友是相互的).而且每 位数学家都可以通过朋友之间的传递.将门已的资料传给任意一位数学家.问:是否一定可 以将这22位数学家两两分组(即每组两人,共11组),使得每组的数学家都是朋友?
combinatorics