Let D be the set of decreasing functions f: N -> N. (N 是自然數集)
The relation < on D is defined as follows: f < g iff there is an natural
number n such that f(n) < g(n) and f(i)=g(i) for all i < n.
Let S be a nonempty subset of D. Show that S has a least element under <.
想了一些方法可是好像都不太對
不知道有沒有熟悉集合論的高手幫忙解此題
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