# Logic of steps in proofs

Whenever I read a textbook or get help with a proof, I'm always suggested a very obscure and tricky step which usually helps me immediately solve the rest of the proof because everything else seems to becomes obvious after that one single trick. I believe this isn't helping me because it's just that one step I cannot think of, and I have no idea how other people are thinking of that idea. Do they work backwards, recall something from memory that is very similar to the problem, just trial-and-error various ideas very quickly in their head, etc.?

As a short example, page 7 on Rudin's "Principles of Mathematical Analysis," 3rd edition, on the proof for proposition 1.16(b) which says that if $x \neq 0$ and $y \neq 0$, then $xy \neq 0$. I would not have thought to try and use $1 = (\frac{1}{y})(\frac{1}{x})xy$ and then use contraction to show it can't be true. How would you know to use contradiction in this case? Is it something to recognize? And after you determine this, how do you know to use something like $1 = (\frac{1}{y})(\frac{1}{x})xy$? Or maybe it's the other way around; trying to prove something and accidentally finding a contradiction? I don't think I would have thought of this unless I spent considerable time, particularly on such an "easy" (I assume easy because it's on page 7 and pretty elementary) proof.

I know it's very subjective/situational, but I sometimes wish authors of texts and answers on this site would also include the thinking steps on how they arrived or thought of the lines in their proofs instead of just showing all the detailed work without the explanations.

• In many cases what may seem like tricks are basic mathematical principles, like your example a field is an integral domain. Further, some of these tricks are pervasive. – Rene Schipperus Jul 13 '14 at 1:15
• What you call tricks others call techniques, and reading these books teaches you how to use them yourself. – vadim123 Jul 13 '14 at 1:54
• There's a saying among mathematicians: "A technique is a trick you've seen more than once." What happens is that as you get experience, you keep seeing the same tricks used in different contexts; and you develop a bag of tools you can use as you need them. – user4894 Jul 13 '14 at 4:35