http://blog.renren.com/blog/282031576/756528999
虽然drop了这节课,但是在那里的最后一堂课听教授讲的冷笑话还是挺有意思的。
“In the last several minutes of the class, I want to talk about mathematical induction (数学归纳法), in case some of you don't know."
接下来,他在黑板上写了 "Prove that all positive integers are interesting."
是的,他写的真的是“求证所有的正整数都有趣”,我以为我看错了,特意看了下旁边同学的笔记,发现我们记得一模一样。
"The first step is to show the base case." 接着他写了 "Base case: 1, the smallest positive integer, is interesting." 教授解释说,因为1是最小的,于是它很interesting。
接下来他写了,"Induction step: Assume that all integers 1,2,3,...,n are interesting."
"The next step is to prove. Prove: Statement iis true for the next integer n+1."
"But how to prove?"他说,"We show by contradiction."
于是我开始期待他要用什么歪门逻辑来证明这个问题,难道这也能用反证法证吗?
他写下,"Show by contradiction: If n+1 is not interesting, n+1 would be the first positive integer that is not interesting. This makes it interesting."
写完他说,"Have a nice weekend!"
同学们先是一愣:这就证完了吗?三秒钟后第一个同学喊了一声"Oh",之后渐渐地同学们全都明白了,笑声充满了整个教室。
的确这是一个非常有意思的数学笑话,也很好地诠释了什么是数学归纳法。 |