let S = {1,2,..,n}
define a relation R on S,
such that xRy if and only if x=(2^k)y for some integer k
(a) show that the relation R is an equivalence relation
(c) show that if 下高斯[(n+1)/2] + 1 numbers are chosen from the set S
then there must be two numbers a and b such that a is divisible by b
http://www.lib.nsysu.edu.tw/exam/master/eng/infoe/infoe_89.pdf
第7頁的第2題..
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