Counterexamples
[background - Iāve taken a bunch of grad math classes but Iāve never done serious mathematical research]
I was never explicitly taught how to doĀ math. I was taught lots of math. But I never had a proof writing class or the like.
And for a time, I never even thought about that, because, writing a proof isnāt anything special. Iāve read a bunch of proofs, now I have to solve a problem and write it down and those are things Iāve done separately before. Putting them together wasnāt even notable to me.Ā
And at some point, I was asked to generate a counterexample for something, and I didnāt have a problem with that either. Iāve seen counterexamples before too and all.
And yet thereās definitely something different about producing a counterexample. A proof, if itās simple enough, you can produce by just following the definitions, maybe using the theorems youāve seen in class. You read how itās a chain of reasoning, and you can go through your own chain of reasoning and make a proof.
A counterexample on the other hand... well Iāve seen them treated as these weird things which come out of nowhere because a really smart mathematician magically found it. In books and in class, often itās just presented as a completed object, with no hint as to how it was produced. Heck thereās books of standard counterexamples. Because these are mysteriousĀ āpathologicalā objects which can only be cataloged, not described and categorized and systematized themselves, presumably.
I disagree. Itās a shame we donāt talk more about how to produce counterexamples. Some of my favorite math moments have been making a counterexample. Itās a shame that a professor could present a non-trivial counterexample and decide that suffices, rather than describe how it was made, how to come up with it from scratch. How itās actually as simple a counterexample as you can get (not necessarily with proof, just intuitively would suffice.
Actually, sometimes we make a step in a proof thatās similarlyĀ āmagicalā. Itās easy to just sayĀ āwell, this clearly works. Why did we decide to multiply by the sum of all the x\_iās in this step? Well who knows, it was an inspired step, very creative and genius. Itās clearly a valid step though, and it gets what we want!ā (thatās not a great example, but hopefully clear. A toy real example would beĀ āwhy do we add bx/2a to both sides of a quadratic equation to complete the square, where did that magical coefficient come fromā.) Too often those are left unexplained. And more than that, itās not like a usual unexplained moment where the author saysĀ āclearlyā and leaves it the reader to fill in the details if they donāt agree itās clear. Itās not expected that the reader will be curious about how the author came up with the magic (correct me if Iām wrong?). Hints arenāt given. The expected reaction isĀ āyes this magic works, quite reasonable, congratulations wizard on your magic, very impressive, moving on with my life now.ā Maybe Iāve had a series of poor professors in that one sense. But I suspect itās just a common attitude in mathematics.

















