I came across this neat pdf by Troy Jones about using salt to do geometry. So, over Thanksgiving break I got my kids and their cousins together to do a little mathematics. We cut various shapes out of paper, propped them up on glasses, and poured salt over them. The salt is a natural bisector. The ridges can be used to bisect angles and to find the locus of points equidistant from two curves. We had fun making triangle centers, Voronoi diagrams, and conic sections. I had a good time thinking about why this worked (it is a fun exercise to see why these ridges form the various conic sections).

In a recent paper by John Sharp I learned about tying a strip of paper into a regular pentagon. It goes back to ‘Tom Tit,’ which was the pen-name of Arthur Good (1853–1928).

The American Institute of Physics gave templates to make physics-related snowflakes. I used their template to make this Isaac Newton snowflake. (As cool as it is, it doesn’t have six-fold symmetry like a true snowflake.) They also have a crystallography and a Nikola Tesla snowflake.

I have been wanting to make a mathematical flip book for a long time. Yesterday was the day that I made it happen. My 13-year old son and I figured out how to do it using the Adobe suite (my son is becoming a self-taught Adobe wiz). I started by creating the first four stages of the Koch snowflake using Illustrator. My son imported them into After Effects and had them morph from one to the other. He used Media Encoder to export them as an animated gif:

Then we exported the frames of the gif as 90 separate images. I printed them on card stock with nine per page (here’s a pdf). I cut them out and used a binder clip to hold them together. It seemed like 90 frames was perhaps too many, so I took every other frame and made two books of 45 frames. Those worked well. Lastly, I put the two books back to back and took following video of the triangle turning into a snowflake turning back into a triangle. I’m excited to make more flip books.

I found this blog post, which has instructions on how to make a fractal Christmas tree. I made one and thought it looked great. In fact, it folds up nicely into a card. So my daughter made one to give to her teacher.

After that, I tried making one of my own design. Rather than cutting the paper in half at each stage, I cut it into thirds. The result is below. It looks cool, but when it is folded as a card, part of the inside sticks out.

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As you can see, the tweet was widely “liked” and retweeted.

I was wondering: Is it true?

I don’t know much about the history of the division symbol ÷, although I do know that it is called an *obelus*. So I cracked open a copy of the famous 1928 text written by the math historian Florian Cajori, *A History of Mathematical Notations* (Volume 1). OK, I didn’t crack it open as much as open the web page. (All of the information in the blog post comes from pages 239–245 and 268–275 of Cajori’s book.)

From Cajori I learned that ÷ was first used for subtraction, not division—and it was not just used once in a while. It had a long life in that role. The first known such use was by Adam Riese in 1525. It was used for subtraction in a variety of countries by a number of different mathematicians in the 16th, 17th, and 18th centuries. In fact, the last known use of ÷ for subtraction was in 1915!

The first known use of ÷ to represent division was in the 1659 book *Teutsche Algebra* by the Swiss mathematician Johann Heinrich Rahn. His book was translated into English shortly afterward. However, once the notation started to catch on in England, it became known as *Pell’s symbol*, in reference to John Pell. This would not be the first instance of a mathematical object having the wrong name attached. But in this case it isn’t clear that it is misnamed; Pell had visited Rahn in Switzerland, so he may have been the originator of the symbol. (Neither mathematician claimed to be the one to introduce the ÷ symbol.)

Apparently ÷ was not the only symbol for division. Leibniz’s notation of : had its proponents too. And according to Cajori, which symbol people used depended on where they lived. He wrote

There are perhaps no symbols which are as observant of political boundaries as are ÷ and : as symbols for division. The former belongs to Great Britain, the British dominions, and the United States. The latter belongs to Continental Europe and the Latin-American countries. There are occasional authors whose practices present exceptions to this general statement of boundaries, but their number is surprisingly small.

As a final neat fact, apparently the Mathematical Association of America tried to rid mathematics of the symbols ÷ and :. In a 1923 report they wrote

Since neither ÷ nor :, as signs of division plays any part in business life, it seems proper to consider only the needs of algebra, and to make more use of the fractional form and (where meaning is clear) of the symbol /, and to drop the symbol ÷ in writing algebraic expressions.

So, what about the original question? Expressing rational numbers as fractions is old. According to Cajori, the use of the horizontal line for fractions dates back at least to al-Hassâr in the 12th century. And Leonardo of Pisa (Fibionacci) used the fractional line in his 1202 *Liber Abaci. *So in principle, fractions could be the source of the ÷ symbol. But it seems unlikely given the history that Cajori presented. And Cajori does not say that the symbol came from fractions (as far as I could tell), as @Advil asserts.

In fact, I see that the math historian Thony Christie calls @Advil’s claim “folk etymology.

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I will also be the academic adviser to the students in my class until they declare a major. With this last role in mind, I decided to write up some advice for these new students. Here’s my list. [It is a slight modification of the one I gave the last time I taught a FYS.]

**Advice for new college students**

**Get to class on time.****Read your email, but not during class.****Spend a summer on campus.**Work for a professor, be a tour guide, do research, get an internship, etc.**Use proper grammar and capitalization in the email messages to your professors.**The email shorthand that may be appropriate between friends is not appropriate when corresponding with your professor (e.g., “hey, prof. when r u going 2 b in yr office?”).**Call your teachers “Professor —” not “Mr. —” or “Mrs. —.”**Almost all of your professors have the highest degree in their field (usually a PhD). (Addressing them as “Dr. —” is appropriate too, although it isn’t common at our school.)**Get to know your professors and let them get to know you.**They’re nice people. Ask your professors about their research, their family, their schooling, etc. Tell them about your summer research projects, your internships, etc. Down the road you may want to ask them for a letter of recommendation and they will be able to write you a much better letter if they know you. Besides, they are human beings, if you are rude to them, they will be less enthusiastic about helping you.**Don’t skip class.**Either you won’t be able to learn the material that you missed or the “free hour” that you gained will be lost several times over trying to catch up. If you do skip class, DON’T ask the professor what you missed—get notes from a classmate.**Take classes outside of your comfort zone.****Be protective of your online identity.**Don’t post photos online that you wouldn’t want your parents, your professors, your future inlaws, or your future employers to see.**Don’t sell your books back, especially for classes in your major.****Don’t be a member of a clique.**For many of you college will be the most diverse living experience of your life. Get to know as many people as possible and not just those with the same background as you.**Be organized, use a calendar, and pay attention to due dates.****Find a good distraction-free place to study.****Learn to write well.**I’ve seen far too many mathematics and science students avoid writing courses. They are under the impression that it won’t be relevant to them. Writing is an extremely important skill that is a prerequisite for almost all careers. You will be amazed at how much you will need to write.**Learn from your mistakes.**Look over your assignments when you get them back. The professor put those comments on there for your benefit. If you don’t understand the comments, ask.**Do the assigned work.**And the related…**Don’t ask for extra credit.**I don’t give extra credit and neither do most other college professors; if they do, they would give it to the entire class not just to you individually. Extra credit is great for the strong students—it can boost their grades from an A to an A+. Weaker students who*need*a grade booster should spend their time doing the assigned work (which they often haven’t done—that’s why their grade is in trouble in the first place). Doing the assigned work is the best preparation for the exams in the class—it gives the best “bang for the buck.”**Start assignments early and start studying early.**Related: don’t email the professor late the night before (or worse, the day of) an exam or the due date for an assignment asking for help.**Admit when you are wrong.**It may be difficult, painful, or embarrassing, but it is liberating. Living with a lie or a guilty conscience is worse than coming clean.**If you choose to drink alcohol, do so in moderation.**Not all college students drink alcohol.**Stay healthy: eat well, exercise, and get enough sleep.****Take the courses you want to take, not the ones your parents want you to take.****Beware of technology such as video games, movies, social media, etc.**They can be unhealthy, addictive time sinks.**Don’t read or send text messages in class.****Try new things (clubs, sports, volunteering, etc.), but don’t spread yourself too thin.****Call home, but not too often.****Get off campus and explore the area.**Eat in restaurants, go for a hike, see a movie, visit a museum, etc.**Study abroad.****Do not beg your professor for more points on your graded work.**If you have doubts about the grading, ask the professor to explain his or her reasoning. Most likely, if there was a grading error, your professor will fix it.**Show up for appointments and be punctual.****Don’t let your parents fight your battles.**Professors cannot speak with your parents anyway (without a FERPA release).**If things start going wrong, see a counselor.**Each year the counseling center is used by 15–20% of the student body. The service is completely confidential; they won’t notify your parents, your professors, your friends, or your insurance company.**Let go of your high school anxieties.**Your classmates didn’t know you in high school. Make new friends, wear new clothes, listen to different music, and try new things.**Don’t lie to your professors; they’ve heard them all (otherwise known as the “dead grandmother rule”).**(A retired professor I know used to send a condolence card to the student’s parents every time a student informed him of a death in the family.)**Be considerate of the neighbors.**Not everyone in town is a college student. Keep this in mind when you are returning from a party at 2:00 AM.**Be a good roommate.****Don’t cheat.**The penalties are steep if you are caught. If you are not caught you will have to contend with a guilty conscience. Cheating will produce a short-term gain and a long-term loss. Besides, it is a slippery slope—this is not the way you want to conduct the rest of your life.**Become a novice.**You’ll learn more and get more out of college if you don’t hold onto the attitude that you know everything already.**Go on a road trip.****Look at your final exam schedule before scheduling your flight home.****Have a growth mindset**(not a fixed mindset)—you can improve at anything if you put in the time and effort.

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I know that if you point your arm directly away from the sun (so in this case, you’ll be pointing down into the water), then the primary rainbow will be in a circle 42° from that line. In other words, move your arm off the line 42° and sweep it in a cone about the line. You will be pointing at the rainbow. For a primary rainbow, red is on the outside of the circle and violet is on the inside. This primary rainbow occurs when the light bounces once off the back of each drop of water.

The secondary rainbow occurs when light bounces off the back of the raindrop twice. The secondary rainbow sits on a circle with the same center as the primary, but at an angle of approximately 52°. And the colors are reversed—red on the inside, violet on the outside.

Putting this together, I concluded that the left rainbow is the primary rainbow and the right one is the secondary rainbow. And the sun is behind Albert on his right (which seems to agree with the shadows on the shore).

What about the “reflected rainbows”? First of all, they aren’t reflected rainbows, they are rainbows made from the reflected sunlight. Here, the sun is coming down, reflecting off the water, and heading upward. Because the lake is horizontal, it will give the impression that the sun is below the lake, behind Albert on the right. In other words, if my arm was pointing downward at an angle θ from the horizon before, to find the center of the reflected rainbow I would point it up at an angle θ. The upshot is that this means the concentric circles would be reflected over the horizon.

So, here’s my best guess of the locations of the four rainbows. The red circles are the original rainbows; the more opaque one is the primary and the more transparent is the secondary. And the blue circles are the rainbows from the reflected light.

That’s what I think is going on. If you disagree, let me know in the comments.

Thanks for the beautiful photo Albert!

Update: after I posted this on Facebook, Albert (who is an Assistant Professor of Geospatial Technology & Director of Harrisburg University’s Geospatial Technology Center) shared the screenshot below and commented: “The green arrow represents the direction of the photography contrasted to the location of sunrise. This was taken about 15-20 minutes after sunrise so only a few degrees above the horizon over my right shoulder.”

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Interestingly, when the legs are folded, the underside of the table encodes a famous proof of the Pythagorean theorem.

As we see in the photo below, the edge of the table and two adjacent legs form a right triangle with legs *a *and *b *and hypotenuse *c*. The table is square, so it has area But, the table is divided up into four right triangles each having area and a smaller square of area Thus,

Ta da!

After I posted this on Twitter I found out that Debra Borkovitz made the same observation about her card table.

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- is a Fermat prime.
- The teenage Gauss proved that the regular 17-gon is constructible by compass and straightedge (which is related to the previous bullet).
- There are 17 wallpaper groups.
- A haiku has 17 syllabes.
- A Sudoku needs at least 17 clues to have a unique solution.
- Theodorus proved that each of …, is an integer or is irrational. (Actually, the wording in Plato’s
*Theaetetus*is ambiguous; it could have been that is the first one that Theodorus was unable to show was irrational.) - According to hacker lore, 17 is the “least (or most) random number.”
- To the nearest order of magnitude, the universe is seconds old (approximately seconds).
- It is the smallest number that is the sum of two distinct positive integers raised to the fourth power:
- It is the smallest number that can be written as the sum of a square and a cube in two different ways
- Some cicadas have a 17-year life cycle.
- There are 17 ways to write 17 as the sum of primes.
- The Italians think 17 is unlucky (apparently because XVII can be rearranged to be VIXI, which means “my life is over”).
- Plutarch wrote “The Pythagoreans also have a horror for the number 17, for 17 lies exactly halfway between 16, which is a square, and the number 18, which is the double of a square, these two, 16 and 18, being the only two numbers representing areas for which the perimeter equals the area.”
- There are 17 nonabelian groups of order at most 17.
- 17 is the smallest whole number whose reciprocal contains all ten digits:
- In Ramsey theory In other words, when it is possible to color the edges of any graph with
*n*vertices using three colors so that there are no monochromatic triangles. But this is impossible for (the complete graph with 17 vertices can’t be colored in this way). I don’t think this was what Stevie Nicks was signing about in her song “Edge[s] of [K_]Seventeen.”

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Yesterday I was looking at a few methods of angle trisection.

For instance, I made this applet showing how to use the “cycloid of Ceva” to trisect an angle. (It is based on Archimedes’s *neusis* [marked straightedge] construction.)

I also found David Alan Brook’s *College Mathematics Journal *article “A new method of trisection.” He shows how you can use the squared-off end of a straightedge (or equivalently, a carpenter’s square) to trisect an angle. (This is a different carpenter’s square construction than the one I wrote about recently.)

To perform the trisection of (see figure below), bisect the segment at Then draw the segment perpendicular to . Draw a circle with center and radius Next, arrange the carpenter’s square so that one edge goes through , one edge is tangent to the circle, and the vertex, , sits on . Then .

I made an applet to illustrate this trisection.

Brooks’s proof used trigonometry. My question is: Is there a *geometric proof* that I spent a little while working on it yesterday and couldn’t find one. If you can, let me know!

UPDATE: We have a proof! Thank you Marius Buliga!

In the proof we are referring to the figure below. Let and Then it suffices to show that Let be the point of tangency. Draw segment and extend to on Then draw segments and Because lines and are parallel, and hence is a parallelogram. This implies that is the midpoint of the diagonal so Moreover, Because is the hypotenuse of the right triangle and is the median, We see that so Finally, because is a chord of the circle, that is,

Update 2: Andrew Stacey just sent me an another proof:

In the proof we are referring to the figure below. Let and Then it suffices to show that Let be the point of tangency of the carpenter’s square and the circle. Because Construct Because is a tangent line and is a radius of the circle, Because and are parallel. Thus Construct a point so that is a parallelogram. Notice that and are collinear. Let be the midpoint of . Construct the line segment and the circle with center and radius . Note that the circle passes through and —the first three because and the fourth because and is a diameter of the circle. Because is parallel to Finally, is a central angle and is an inscribed angle and both cut off the same arc of the circle; thus

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The example I gave on my blog and in the article was a cylinder that looked like a circular cylinder but like a square cylinder in the mirror. You can download a printable pdf template to make your own, and you can watch this video that I made.

Today I designed two more impossible cylinders. The first looks like a square cylinder and like a triangular cylinder in the mirror. Download a printable pdf template to make your own.

I also designed one that looks like a heart-shaped cylinder and like a diamond-shaped cylinder in the mirror. (The spade/club one is an exercise for the reader.) Here’s a template to make your own.

Update: I just created another cylinder. I’m not as pleased with this one. The shape of the cylinder is complicated enough that it is difficult to get it to look right. Here’s my best attempt at a star/moon cylinder. (Pdf template.)

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Today I created a printable template from which you can make your own version of Sugihara’s object.

Click the following image to download the pdf.

Making the shape and seeing the illusion is easy.

- Cut out the figure at the top of the page.
- Fold a sharp crease along the dotted line.
- Tape the left and right edges together.
- Fold a sharp crease along the taped seam.
- Lightly squeeze together the creased sides so that the shape opens. Looking down on the shape it should have the shape at the bottom of the printout.
- Close one eye. Look down on the shape at a 45 degree angle so that the two creases line up with each other.
- Then turn it around 180 degrees and look again.

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Kokichi Sugihara created a video called Ambiguous Optical Illusion: Rectangles and Circles. In it he shows a variety of 3-dimensional objects that look like one shape when viewed from the front but look like a different shape in the mirror behind it.

In this blog post we show how he achieved the effect. For simplicity, we will show how he made a shape that looks like a circular cylinder from the front and a square cylinder in the mirror.

The following applet shows our final product (clicking the image links to the GeoGebra applet). It is a closed curve that represents the top rim of Sugihara’s shape. You can rotate the axes with your mouse. If you view the coordinate system with the positive green and blue axes lined up (1 with 1, 2 with 2, and so on), the curve will look like the unit circle in the green-red plane. If you drag the image so that the positive blue axis lines up with the negative green axis (1 with -1, 2 with -2, and so on), it will look like you are viewing a square (oriented as a diamond) in the green-red plane.

Here are screenshots showing the two views.

How does it work? It is all about perspective.

To set this up mathematically, we imagine two viewers in 3-dimensional space. One viewer is at and the other is at (in the video this second viewer is you, in the mirror). They are looking down on a curve However, from their vantage points it looks like they are seeing two different curves in the -plane: and respectively.

In our example the two observed curves are the unit circle and the square passing through the points , as shown in the -plane below. We will have to break each of these shapes into two different curves, so we’ll have and Also, we could choose to be some suitably large number greater than 1, but in fact, as we will see, taking the limit as tends to infinity produces a lovely final expression. For now we will continue to work in generalities and will wait to insert these specifics later.

Our aim is now to define Let’s fix , and let and be two points on the curves in the -plane. In order for the person at to view her shape, must lie on the line (see figure below). Likewise, for the person at to see his shape, must lie on the line Thus, must be the point of intersection of lines and (We know that the lines are not skew because they lie in the plane containing the points and and for appropriate choices of and the lines intersect and the point of intersection is below

It is straightforward to show that is a parametrization of the line and is a parametrization of A little algebra shows shows that their point of intersection is

and thus our desired curve is

Because is a large value, we can take the limit as goes to infinity. This yields the elegant expression

We may now plug in our functions. The portion of our curve with nonnegative -coordinates is given by

for and the other half by

for This is the curve shown in the applet.

[Update: See the comment by Joshua and my reply for a simpler way of obtaining the parametrization.]

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