# College Math Teaching

## June 22, 2013

### About teaching continuity: a math ed talk

Filed under: calculus, editorial, elementary mathematics, mathematics education, pedagogy — collegemathteaching @ 12:11 pm

There was a talk about students and how they understand the concept of “continuity” of a function. That is a good topic.
One of the examples that was brought up was someone in a graduate program who didn’t understand why the function
$f(x) = 2x$ if $x$ is rational and $f(x) = x^2$ otherwise”
is continuous at $x = 0$. The graduate student said that she couldn’t draw the graph without “lifting the pencil”.

I don’t think that this is a problem with calculus teaching; this person shouldn’t have made it through analysis.

But yes, I agree; sometimes students have trouble with the concept of continuity. So we went on; the idea is that when asked about “what it means for a function to be continuous” students often struggled. Fair enough. The answer that was looked for was: “the function $f$ is continuous at $x = a$ if $lim_{x \rightarrow a} f(x) = f(a)$ which, of course, means that $f$ is defined at $x = a$ to begin with.

Instead, students responded with “keep the pencil on the paper”, “has the same formula (as opposed to a conditional formula)”, “connected graph”, etc.

So I asked “how is the concept of limit defined to begin with” and….see the previous sentence! Such nonsense.

Seriously, if you are going to wave your hands at “limit” (and it may be appropriate to do so) then what is the problem to doing that with continuity?

There is more. Consider the frequently quoted idea that a function can be defined by “formula, text, or a *table of values*”, etc.

We were given something like:

 x | 1.98 | 1.9908 | 2.001 | 2.051 y | 8.94 | 8.9671 | 9.003 | 9.023

And the first row is considered to be in the domain. The question: “it is reasonable to expect $f(2) =$“. You know what the expected answer was, but my question was immediately: “why is it reasonable to expect $f$ to take the integers to the integers?”.

Then “good point”.

My larger point: a “table of values” only defines a function IF the domain is restricted to the entries in the appropriate row (or column) OR if there is an associated interpolation scheme to go with the table.

Then we moved on.

Example: students were given two examples:
1. Example one: say the temperature at 6 am was 60.0 F and the temperature at noon was 75.0 F. So, was there a time between 6 and noon when the temperature was, say, 68.5 F? Ok, that is reasonable, though students might be confused by digital readouts and maybe a physics student might talk about quantum effects.

2. Example two: The winning team in a basketball game scored 81 points. Does it mean that, at some point in the game, that team had 45 points? Ok, “no” is the correct answer but THIS HAS NOTHING TO DO WITH CONTINUITY, at least as defined by the topology that the calculus students have seen. Example: in a volleyball game (new rally scoring), it takes 25 points to win a game. So the winning team must have had 1, 2, 3, 4,….24 points at one time or another, and that is because in volleyball, scores can only be made in 1 point increments and that is NOT true in basketball.

No wonder students are often confused!

Note: this is not necessarily an attack on the intellect of the person giving a talk. For example, there was a research mathematician at a division I research university who gave the following problem on a calculus exam: $f(x) = x + 1, x \le 1$, $f(x) = x^2 -x +2$ elsewhere. The question: “is $f$ differentiable at $x = 1$? She told TAs to mark the problem “wrong” if the students said yes, because the function changed formula at $x =1$!!! Note: the question asked “differentiable” and not “smooth”.

Vent over…

## June 12, 2013

### Just a 3 page paper…

Filed under: academia, advanced mathematics, editorial, research, topology — Tags: — collegemathteaching @ 9:17 pm

I just sent off the final (I think) revisions of a 3 page paper that has been accepted for publication.
Now, as I get ready to start writing another paper (different area entirely), I picked up an old notebook: 65+ pages of notes of work are related to this paper!
I submitted it; had a rejection (due to writing form), re did it, resubmitted it, had to do more revisions, etc. I talked about this in a seminar, had a colleague show me that a similar result had appeared (but mine was different enough to warrant publication)

Of course, I did things a different way and I created a “new to me” technique for “smoothing” a piecewise linear half-line in such a way that the resulting curve is $C^1$ (has a continuous first derivative) and remains convex.

And the result: 3 pages in a journal. In terms of time, that is one page per year!

So what was in all of these notes?

Some of these notes were about the idea itself (some elementary point set topology of the plane was involved); the idea: if one has a collection of points in the plane that has a limit point, can one run a locally piecewise linear arc though a selected convergent subset of points to the limit point, while keeping the resulting arc convex? (Yes, you can)

Now is there a way to “smooth” this arc (round off the corners) so as to pass though an infinite subset of this convergent sub-sequence of points so as to produce a $C^1$ curve? (Yes, there is; that is where the spline construction came in).

Can this curve be made into a $C^2$ curve? NO!!! The counter example is rather indirect.

Anyway, the details of the above is what fills up 65+ pages of my little notebook.

My point: expect research to take a while; there are starts, false starts, dead ends, revisions and revisions to the revisions, BEFORE you send off the first draft to the journal for consideration!

What comes next
I have one result ready to be proofed and another that I am writing up; hopefully these will be sent to a journal in a month’s time. But I won’t be surprised if these papers also take quite a bit of time.

So, for my academic year: what do I do as far as research?

1. Work on a specific problem?
2. Learn something “new to me” but related to my research?
3. Explore new topics (new to me) at a shallow level?
4. Work on a lower division book?

We shall see.

### A couple of instances of math in action

Filed under: advanced mathematics, applied mathematics, Fourier Series, physics, popular mathematics — Tags: — collegemathteaching @ 9:02 pm

Via Jerry Coyne’s website; you’ll see some great comments there.

Watch standing waves in action:

Here is what is going on; the particles collect at the “stationary” points.
This is an excellent reason to take a course that deals with Fourier Series!

Here is an example of a projection, and what happens when you take the image and move it a little.