[mlevental]

are you under the impression the people write books like that without referring to sources themselves? because they don't. I doubt any one knows all the proofs in a book like by heart or could reconstruct them without references for key facts.

[tangentspace]

No, I was never under that impression. I once had a discussion about the creation of the book with one of Federer's students who had also helped proofread it. The book was essentially an outgrowth of his personal and course notes, since of course he couldn't keep all this in his head.

What he likely did have in his head was an index into the contents, that's the crux of my observation. If I am very good at organizing my workshop, I can quickly grab the tools and materials needed for a particular task without breaking my flow of thought. Same basic principle applies to mathematicians and other intellectual workers, just as it does with physical trades.

[n4r9]

People of that calibre are quite rare but they do exist. I've seen final year undergraduate courses pulled off without the lecturer once having to refer to notes. All proofs were completed in exacting detail on the whiteboard.

[electricslpnsld]

Sure, but those math profs have probably taught the same course 20+ times. Going through material that many times will permanently burn it into your brain.

[tangentspace]

Absolutely. And in mathematics, advanced structures and theorems are built up layers by layer upon more elementary material. A professor who has mastered presentation of undergraduate material on a topic also likely teaches a graduate course on it, and mentors students on it, and does research on it.

They can talk about their chosen topic at many levels to many different audiences, from general audience (who may provide funding to them), high school students (outreach and recruiting), university students, and peers. This flexibility is an important part of being a very successful mathematician, and you have to burn it into your brain to reach that level of fluency.

[marcelluspye]

I took a philosophy class with a professor who had taught it about that many times. Talked to a guy who had taken it before me, he said that the prof has literally word-for-word memorized certain parts of the lectures because he's figured out and internalized the wording he thinks is best. Mathematicians can certainly do the same.

[mlevental]

also they review the lecture before hand just like anyone delivering a speech does