I believe I’ve heard a variation of the same story told about Hardy (the Cambridge mathematician), which resulted in the concept of the “Hardy obvious” (i.e. something that takes a genius 15 minutes to work out that it’s obvious.)

This story – albeit about a different professor (http://www.tagamlitzki.com/en/folklore.html) – was part of student folklore at the Department of Mathematics and Informatics (University of Sofia) around 1990… :-)

Cheers,
Flado

Number #8 was my math professor’s favorite.

Number #3 is used extensively and it works just great until the contrary is proved by someone else ;-)

Flavio

Did you ever suffer the “proof by circular reference”, which is a two-step as follows:

Professor X: “… and Professor Y will be demonstrating the proof of this theorem next term (semester)”.
…
Professor Y: “… and Professor X will have demonstrated the proof of this theorem to you last term.”

And speaking of time and space, I also like #8, where if we move from time to space we have Fermat’s Last Theorem, and elegant proof of which won’t fit in this margin.

He was known for starting to prove a theorem, only to leave the proof unfinished and move on to the next theorem or to interesting results. We never found out if he forgot the end of the proof, or if he got bored or maybe just distracted.

One day, when he started another proof, only to trail off mumbling “… and from here it is trivial to see the rest…”. One of the students couldn’t take this anymore. He jumped off his chair and said “It doesn’t look trivial! Are you sure its trivial? Please show how its done!”

Prof. Shelah looked at the proof and thought a bit. After few minutes of deep thought, he dismissed the class for 15 minutes. When we came back from the break he was still in deep contemplation.

Few minutes later he brightened up, looked at the class, announced: “Yes! It is trivial!” and continued on to the next theorem.

… You can add this as another method.