The professor questions the proof

I do part time teaching and one of the reasons why I enjoy it very much is because I get to learn how young people think. They are very creative and I am fortunate to encounter some of these smart kids in class.

This is the end of the semester and I happen to do some marking for one of the professors in one of the universities where I work. The subject is Theory of Computing and one of the questions the professor asked in exam was…

Q. Prove that the Halting Problem is undecidable.

Here are a couple of proving techniques employed by some of our young people.

Proof:

Alan Turning says so. Q.E.D.

Proof:

Our lecture notes state it is true.

Our lecture notes are assumed to be true.

$\therefore$ the Halting Problem is undecidable? Q.E.D.

Proof:

If a program knew it was going to crash, it would never have run it. Q.E.D.

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I mean, how could you fault these answers, they have a point! LOL. 🙂

Do share if you have some examples of this type of proving technique, I will only be too happy to read them.