Mathematical Proofs

1. Dec 9, 2007

Oerg

What does QED stand for behind every mathematical proof?

i cant seem to find out what it stands for thanks.

2. Dec 9, 2007

arildno

Quod Erat Demonstrandum (Which was to be proven)

3. Dec 9, 2007

Oerg

LOL
my math teacher used to exclaim <quite easily done> whenever he completed a proof and then wrote QED on the board. I refused to believe it and thought QED was something he came up with lol.

4. Dec 9, 2007

arildno

I regret we don't use Q in Norwegian, so we can't crack jokes about QED's meaning like your teacher did..

5. Dec 9, 2007

HallsofIvy

Staff Emeritus
That reminds me of the professor who, in the middle of a long, complicated proof, said "Now, it is obvious that ...". The suddenly stopped and stared at the blackboard. He then sat down at his desk and, completely ignoring the class, wrote furiously on paper. After 10 minutes of that, he stood up and said, "Yes, it is obvious!"

6. Dec 10, 2007

morphism

There is another similar story about a professor who wrote down a lemma on the blackboard whose proof he said was obvious, only to have a student interrupt him and say that it was not obvious to him. The professor then attempted to prove it but failed, so he told the class he would show them the proof next time. After the lecture ended, he went to his office and tried to come up with a proof, but still couldn't. He then tried to track down the lemma in the literature, and after a long search he managed to find it in a paper, but alas the author had left the proof of the lemma as an exercise to the reader! It also didn't help that the author of the paper was the professor himself.

[I read this in the book Mathematical Apocrypha by Steven Krantz.]

7. Dec 10, 2007

LukeD

By the way, I just wanted to let you know that I decided the first time I saw you post this story that I would quote it whenever someone says that their math book says something is obvious but they can't see it.

I haven't yet had a chance to, but I promise I will.

8. Dec 10, 2007

Xevarion

That story is supposedly about Hardy.

9. Dec 10, 2007

HallsofIvy

Staff Emeritus
OMIGOD, how many times have I posted this story?

10. Dec 10, 2007

wildman

Hmmm... Hardy has an interesting commentary on the use of the word obvious in math in his book "A Course of Pure Mathematics" (on page 130 in the 9th edition at least). I wonder if this incident had something to do with that....

11. Dec 10, 2007

LukeD

No idea. This is only the second I've seen you post it, but I haven't been here very long. Where did the story originate?

Edit: Who is Hardy?

12. Dec 10, 2007

rock.freak667

Ah...for most mathematical proofs...my F.math teacher just writes like 2 lines of it...draws some dots and writes the word "clearly" and then just writes the last line for the proof then puts QED and all the students are like..."how is that so clear?"

13. Dec 11, 2007

Coin

G. H. Hardy

14. Dec 11, 2007

Invictious

This reminds of of last month, when our class was doing mathematical induction as a unit.
During one test, there were hilarious comments by our math teacher, since induction was such a 'unique' topic.

'really? i don't think so.'

'so what? what does this show?'

'NO.'

'NO.'

'There is nothing worse than saying QED when it is not'

'NO.'

'Try again, bummer'

Yes, our math teacher is great!

15. Dec 11, 2007

Gib Z

If I ever become a Professor (which I hope with all my might) that exact thing has a good chance with happening with me =] I've made up theorems that I thought existed because they were 'so obvious' to me, and when a friend said they weren't obvious to him, I tried to proof it in front of him, and instead found a counter example :(

16. Dec 11, 2007

trambolin

I like "Quod Errat Demonstrator" more, not a one-to-one traslation but it is something like "Where the author fails". I don't know why but it sounds so mean.