Author: IMC

Credit: Getty Images

Better living through set-builder notation

As a publicly mathematical person, one of the matters upon which I am called to adjudicate is what I think of as “viral order of operations questions” with a math problem along the lines of “48÷2(9+3) = ?”

In the past, I used to tell people who asked me about one of those questions something like, “I think the correct answer is __, but it’s better to write the expression unambiguously.” But recently I decided I need to put my foot down. I will no longer give an answer to those questions. The only way to win is not to play.

An ambiguous sequence of digits and mathematical operation symbols is not interesting to mathematicians. Most of us learn something about the order of operations fairly early on in our mathematical educations. We might learn PEMDAS or BIDMAS, or “Please excuse my dear aunt Sally.” All of these expressions tell us that we’re supposed to take care of expressions inside parentheses or brackets first, followed by exponential expressions, followed by multiplication and division, then addition and subtraction. It’s good that we have a system, but mathematicians and other people who use mathematical expressions regularly would never write down something like “48÷2(9+3) = ?” because its potential for causing confusion is too great.

In general, mathematicians strive to reduce ambiguity when possible. A mathematician would write (48÷2)(9+3) or 48÷(2(9+3)), depending on which one they meant. Viral order of operations problems are unappealing. Just toss in a few more parentheses to clarify your meaning and move on. There are cat pictures to scroll through, for goodness’ sake!

In fact, I think if mathematicians had their way, they would get rid of easily-fixed ambiguous order of operations problems altogether, and I don’t think they’d stop there. The English language often leaves room for ambiguity, and I think mathematical notation could help us make some improvements.

I remember chuckling to myself when I saw the phrase “I like to play board games and read a book while taking a bath.” It conjures up an image of a very exciting game of Monopoly where Baltic Avenue is replaced by the Baltic Sea. Set-builder notation could resolve that titillating ambiguity there while simultaneously freeing me of a tiny shred of joy in this world of woe. Mathematicians use curly brackets to indicate things that are in one set and can be treated as one object. A person who enjoys two separate activities, one of which is playing board games and one of which is reading a book while taking a bath, could write “I like to {play board games} and {read a book while taking a bath}.” A person with a much more interesting bathtime routine than mine could write “I like to {play board games and read a book} while taking a bath.”

Curly brackets take care of written English, but we communicate through speech as well. The late Danish pianist and comedian Victor Borge had a routine about phonetic punctuation. He assigned sounds to some common punctuation marks and inserted them into sentences.

He didn’t include a sound for curly brackets, and I’m not sure the best option. Perhaps a “zzp” sound would work, but I’m open to other suggestions. In the meantime, there is a precedent for air quotes, and perhaps we could add extend that to air parentheses.

When I started thinking about using brackets and air-parentheses in English writing and speech, I wondered if I was just reinventing sentence diagramming. I don’t know how many people learned to diagram a sentence in school, but a sentence diagram is a graphical representation of a sentence that shows how each word and phrase functions in the sentence. The diagram for the sentence “I like to {play board games} and {read a book while taking a bath}” is different from the diagram for “I like to {play board games and read a book} while taking a bath.” It’s been a while since I did any sentence diagramming, so I beg for lenience from any grammar teachers reading, but these were the diagrams I came up with. The placement of the “while” clause is the only difference between the two diagrams.

Two possible diagrams for the sentence “I like to play board games and read a book while taking a bath.” Credit: Evelyn Lamb

Diagramming the sentence does remove the ambiguity, and it gives even more information than the set-builder notation I’m suggesting, but it comes at a cost of both space and effort. It’s much more practical to throw a few brackets into English prose than to draw every sentence as a complex, multi-storied building.

Set-builder brackets are just the tip of the iceberg when it comes to resolving linguistic ambiguity through mathematical notation. After we’ve mastered those, we can consider incorporating union and disjoint union symbols and the distinction between or and xor into our speech and writing. But that will have to wait until after we’ve relaxed with an extra-realistic game of Battleship in the bathtub.

For more insights like this, visit our website at www.international-maths-challenge.com.

Credit of the article given to Evelyn Lamb

The humble honeybee can use symbols to perform basic maths including addition and subtraction, shows new research published today in the journal Science Advances.

Despite having a brain containing less than one million neurons, the honeybee has recently shown it can manage complex problems – like understanding the concept of zero.

Honeybees are a high value model for exploring questions about neuroscience. In our latest study we decided to test if they could learn to perform simple arithmetical operations such as addition and subtraction.

Addition and subtraction operations

As children, we learn that a plus symbol (+) means we have to add two or more quantities, while a minus symbol (-) means we have to subtract quantities from each other.

To solve these problems, we need both long-term and short-term memory. We use working (short-term) memory to manage the numerical values while performing the operation, and we store the rules for adding or subtracting in long-term memory.

Although the ability to perform arithmetic like adding and subtracting is not simple, it is vital in human societies. The Egyptians and Babylonians show evidence of using arithmetic around 2000BCE, which would have been useful – for example – to count live stock and calculate new numbers when cattle were sold off.

This scene depicts a cattle count (copied by the Egyptologist Lepsius). In the middle register we see 835 horned cattle on the left, right behind them are some 220 animals and on the right 2,235 goats. In the bottom register we see 760 donkeys on the left and 974 goats on the right. Wikimedia commons, CC BY

But does the development of arithmetical thinking require a large primate brain, or do other animals face similar problems that enable them to process arithmetic operations? We explored this using the honeybee.

How to train a bee

Honeybees are central place foragers – which means that a forager bee will return to a place if the location provides a good source of food.

We provide bees with a high concentration of sugar water during experiments, so individual bees (all female) continue to return to the experiment to collect nutrition for the hive.

In our setup, when a bee chooses a correct number (see below) she receives a reward of sugar water. If she makes an incorrect choice, she will receive a bitter tasting quinine solution.

We use this method to teach individual bees to learn the task of addition or subtraction over four to seven hours. Each time the bee became full she returned to the hive, then came back to the experiment to continue learning.

Addition and subtraction in bees

Honeybees were individually trained to visit a Y-maze shaped apparatus.

The bee would fly into the entrance of the Y-maze and view an array of elements consisting of between one to five shapes. The shapes (for example: square shapes, but many shape options were employed in actual experiments) would be one of two colours. Blue meant the bee had to perform an addition operation (+ 1). If the shapes were yellow, the bee would have to perform a subtraction operation (- 1).

For the task of either plus or minus one, one side would contain an incorrect answer and the other side would contain the correct answer. The side of stimuli was changed randomly throughout the experiment, so that the bee would not learn to only visit one side of the Y-maze.

After viewing the initial number, each bee would fly through a hole into a decision chamber where it could either choose to fly to the left or right side of the Y-maze depending on operation to which she had been trained for.

The Y-maze apparatus used for training honeybees. Scarlett Howard

At the beginning of the experiment, bees made random choices until they could work out how to solve the problem. Eventually, over 100 learning trials, bees learnt that blue meant +1 while yellow meant -1. Bees could then apply the rules to new numbers.

During testing with a novel number, bees were correct in addition and subtraction of one element 64-72% of the time. The bee’s performance on tests was significantly different than what we would expect if bees were choosing randomly, called chance level performance (50% correct/incorrect)

Thus, our “bee school” within the Y-maze allowed the bees to learn how to use arithmetic operators to add or subtract.

Why is this a complex question for bees?

Numerical operations such as addition and subtraction are complex questions because they require two levels of processing. The first level requires a bee to comprehend the value of numerical attributes. The second level requires the bee to mentally manipulate numerical attributes in working memory.

In addition to these two processes, bees also had to perform the arithmetic operations in working memory – the number “one” to be added or subtracted was not visually present. Rather, the idea of plus one or minus “one” was an abstract concept which bees had to resolve over the course of the training.

Showing that a bee can combine simple arithmetic and symbolic learning has identified numerous areas of research to expand into, such as whether other animals can add and subtract.

For more insights like this, visit our website at www.international-maths-challenge.com.
Credit of the article given to Scarlett Howard, Adrian Dyer, Jair Garcia

Credit: If you know your height, can you predict how big your arm span is? What about the length of your femur? Or the circumference of your head? Try this simple activity and find out how you knowing these simple ratios can even make you a better artist!  George Retseck

A project that measures up

Introduction
Our bodies are amazing! They are full of mysteries and surprising facts such as this one: Did you know that you are about a centimeter taller in the morning, when you have just woken up after hours of lying down, than you are in the evening? You might never have noticed it. These interesting facts only reveal themselves when you look closely, measure and compare. That is what this activity is about: recording, comparing and discovering how the human body measures up!

Background
Did you know that human bodies come in all sizes and forms? When you start measuring them, however, you will find our bodies show surprising similarities—and even more surprisingly, we can express these with mathematical concepts.

For one thing, our bodies are quite symmetrical. When you draw a vertical line down the center of a body, the left and right sides are almost mirror images of each other. Human bodies also show interesting ratios. Ratios compare two quantities, like the size of one part of the body to the size of another part, or to the size of the whole. An example of a human body ratio is a person’s arm span—the distance from the middle fingertip of the left hand to that of the right hand when stretching out both arms horizontally—to their height. This ratio is approximately a one to one ratio, meaning that a person’s arm span is about equal to their height. There are many more human body ratios; some are independent of age, and others change as we grow from a baby to an adult.

Wondering who would be interested in these ratios? Artists are avid users of human body ratios, because it helps them draw realistic-looking figures. They are also used in the medical world; a sizable deviation from a human body ratio can indicate a body that does not develop according to expectations. In this science activity we will examine some human body ratios and, if you like, we can explore how they can help you draw more realistic-looking figures.

Materials

• Yarn
• Scissors
• A hardcover book
• A helper
• Pen and paper (optional)
• Measuring tape (optional)

Preparation

• To compare the length of different parts of your body with your height, we will first create a string the length of your height. Take off your shoes. The easiest way is to lie on the ground with your heels pressing against a wall. Look straight up and have your helper place a hardcover book flat against the top of your head, resting on the ground. Get out from under the book and, together, span the yarn across the floor from the wall to the book, cutting the yarn just where it reaches it. Now you have a piece of yarn that is as long as you are tall. (If lying on the ground is not possible, you can also stand flat on the floor against the wall and have the book rest on top of your head and against the wall.)

Procedure

• First, we examine your arm span to height ratio. Your arm span is the distance between the middle fingertips on each hand when you stretch your arms out as far as they can reach. How do you think your height compares with your arm span? Would it be similar, way longer or way shorter?
• Now stretch your arms out as far as they can reach. Your arms will be parallel to the ground. Hold one end of the piece of yarn you just cut off with the fingertips of your left hand. Let your helper span the yarn toward the tip of your right hand’s middle finger. Is piece long enough, way longer or way too short? What does this tell you about how your arm span compares to your height?
• For most people, their arm span is about equal to their height. Mathematicians say the arm span to height ratio is one to one: your arm span goes once into your height.
• Now let’s explore another ratio: the length of your femur bone to your height. The femur bone is the only bone in your thigh. To measure its length, sit down and span a new piece of yarn over your thigh from the hip joint to the edge of your knee and cut the yarn there.
• Make an estimate. How many times would this piece of yarn go into the piece that is as long as you are tall? Can you find a way to test your estimate?
• There are several ways to compare the length of the two pieces of yarn: You might cut several pieces of the length of your shorter string, lay them end to end next to your longer piece, and count how many you need. Another way is to fold the longer string into equal parts so the length of the folded string equals the length of the shorter string. The number of folds needed is exactly the number of times your shorter string goes into your longer string.
• Did you see that the length of your femur bone goes about four times into your height? You can also say that if you divide your height in four equal pieces, you have the length of your femur bone, or the length of your femur bone is one fourth of your height. Mathematicians call this a one to four ratio.
• Now let’s move on to a ratio that might help you make more realistic drawings: the head to body ratio. How many times would the length of your head fit into your height? Maybe four, six or eight times? To test six times, fold the yarn with length equaling your height into six equal pieces. Have your helper place a book flat on your head and hang the folded string from the side of the book. If the other end of the string is about level with your chin, your height would be about six times the length of your head, or your head to body ratio would be one to six. Which number of folds fits best for you?
• There are many more bodily ratios you can explore: the circumference of your head compared with your height, or the length ratios of your forearm and foot or thumb and hand. Use pieces of yarn to measure, compare and detect these and/or your other bodily ratios.
• Extra: You have explored some ratios in your body and might wonder if these hold for other people as well. Do you think they hold for most people of your age? What about adults or babies? Do you think these ratios hold for them or would some be different? Make a hypothesis, find some volunteers, measure and compare. Was your hypothesis correct?
• Extra: This activity uses pieces of yarn to compare lengths. You can also measure your height, arm span, femur bone, etcetera with measuring tape, round the values and write the ratios as fractions. Can you find a way to simplify these fractions?
• Extra: Draw some stick figures on a sheet of paper. Can you apply some of the bodily ratios you explored (like the arm span to height or the head to body ratio) to the figures?Which ones look most realistic to you?
• Extra: Ratios are all around us. Can you find other places where ratios play an important role? To get you started, think about a recipe and the ratio of the quantity of one ingredient to another. For avid bikers, can you find the ratios that correspond to the different gears on a bike?

Observations and results
You probably found your arm span to height ratio approximately to be one to one whereas the femur to height was approximately one to four. This is expected because on average and over a large age range the human body has an arm span that is roughly equal to its height and a femur bone roughly a quarter of its height.

The head to body ratio is a little more complex as it changes from a ratio of about one to four for a small child to about one to eight for an adult. A five-year-old is likely to have a head to body ratio of about one to six.

It is good to remember these ratios are averages over a large group of people. Individual variations occur; some might even be used to one’s advantage—for example, having exceptionally long arms can be advantageous when playing basketball.

For more insights like this, visit our website at www.international-maths-challenge.com.

Credit of the article given to Science Buddies & Sabine De Brabandere

In mathematics, no researcher works in true isolation. Even those who work alone use the theorems and methods of their colleagues and predecessors to develop new ideas.

But when a known technique is too difficult to use in practice, mathematicians may neglect important – and otherwise solvable – problems.

Recently, I joined several mathematicians on a project to make one such technique easier to use. We produced a computer package to solve a problem called the “S-unit equation,” with the hope that number theorists of all stripes can more easily attack a wide variety of unsolved problems in mathematics.

Diophantine equations

In his text “Arithmetica,” the mathematician Diophantus looked at algebraic equations whose solutions are required to be whole numbers. As it happens, these problems have a great deal to do with both number theory and geometry, and mathematicians have been studying them ever since.

Why add this restriction of only whole-number solutions? Sometimes, the reasons are practical; it doesn’t make sense to raise 13.7 sheep or buy -1.66 cars. Additionally, mathematicians are drawn to these problems, now called Diophantine equations. The allure comes from their surprising difficulty, and their ability to reveal fundamental truths about the nature of mathematics.

In fact, mathematicians are often uninterested in the specific solutions to any particular Diophantine problem. But when mathematicians develop new techniques, their power can be demonstrated by settling previously unsolved Diophantine equations.

Andrew Wiles, right, receives the Wolflskehl award for his solution of Fermat’s Last Theorem. Peter Mueller/REUTERS

Andrew Wiles’ proof of Fermat’s Last Theorem is a famous example. Pierre de Fermat claimed in 1637 – in the margin of a copy of “Arithmetica,” no less – to have solved the Diophantine equation xⁿ + yⁿ = zⁿ, but offered no justification. When Wiles proved it over 300 years later, mathematicians immediately took notice. If Wiles had developed a new idea that could solve Fermat, then what else could that idea do? Number theorists raced to understand Wiles’ methods, generalizing them and finding new consequences.

No single method exists that can solve all Diophantine equations. Instead, mathematicians cultivate various techniques, each suited for certain types of Diophantine problems but not others. So mathematicians classify these problems by their features or complexity, much like biologists might classify species by taxonomy.

Finer classification

This classification produces specialists, as different number theorists specialize in the techniques related to different families of Diophantine problems, such as elliptic curves, binary forms or Thue-Mahler equations.

Within each family, the finer classification gets customized. Mathematicians develop invariants – certain combinations of the coefficients appearing in the equation – that distinguish different equations in the same family. Computing these invariants for a specific equation is easy. However, the deeper connections to other areas of mathematics involve more ambitious questions, such as: “Are there any elliptic curves with invariant 13?” or “How many binary forms have invariant 27?”

The S-unit equation can be used to solve many of these bigger questions. The S refers to a list of primes, like {2, 3, 7}, related to the particular question. An S-unit is a fraction whose numerator and denominator are formed by multiplying only numbers from the list. So in this case, 3/7 and 14/9 are S-units, but 6/5 is not.

The S-unit equation is deceptively simple to state: Find all pairs of S-units which add to 1. Finding some solutions, like (3/7, 4/7), can be done with pen and paper. But the key word is “all,” and that is what makes the problem difficult, both theoretically and computationally. How can you ever be sure every solution has been found?

In principle, mathematicians have known how to solve the S-unit equation for several years. However, the process is so convoluted that no one could ever actually solve the equation by hand, and few cases have been solved. This is frustrating, because many interesting problems have already been reduced to “just” solving some particular S-unit equation.

The process of solving the S-unit equation is so convoluted that few have attempted to do it by hand. Jat306/shutterstock.com

How the solver works

Circumstances are changing, however. Since 2017, six number theorists across North America, myself included, have been building an S-unit equation solver for the open-source mathematics software SageMath. On March 3, we announced the completion of the project. To illustrate its application, we used the software to solve several open Diophantine problems.

The primary difficulty of the S-unit equation is that while only a handful of solutions will exist, there are infinitely many S-units that could be part of a solution. By combining a celebrated theorem of Alan Baker and a delicate algorithmic technique of Benne de Weger, the solver eliminates most S-units from consideration. Even at this point, there may be billions of S-units – or more – left to check; the program now tries to make the final search as efficient as possible.

This approach to the S-unit equation has been known for over 20 years, but has been used only sparingly, because the computations involved are complicated and time-consuming. Previously, if a mathematician encountered an S-unit equation that she wanted to solve, there was no automated way to solve it. She would have to carefully step through the work of Baker, de Weger and others, then write her own computer program to do the computations. Running the program could take hours, days or even weeks for the computations to finish.

Our hope is that the software will help mathematicians solve important problems in number theory and enhance their understanding of the nature, beauty and effectiveness of mathematics.

For more insights like this, visit our website at www.international-maths-challenge.com.
Credit of the article given to Christopher Rasmussen

Credit: Annalisa Crannell analyzes art with the help of chopsticks and projective geometry. Evelyn Lamb

To fully appreciate perspective art, mathematician Annalisa Crannell says both the artist and the art viewer need to do some math

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Annalisa Crannell goes to art museums with chopsticks. She is not unusually hungry or over-prepared; she uses them to figure out how to look at the art.

Crannell, a mathematician at Franklin and Marshal College in Lancaster, Pennsylvania, studies mathematical perspective and applies her work to the world of art. She writes not only about how artists use perspective but also about how viewers can use it to see art in different ways.

In a 2014 Math Horizons article (pdf, also available in The Best Writing on Mathematics 2015, edited by Mircea Pitici), she and coauthors Marc Frantz and Fumiko Futamura take on the case of the mysterious table in 15th- and 16th-century German artist Albrecht Dürer’s famous engraving St. Jerome in His Study.

Credit: St. Jerome in His Study, by Albrecht Dürer. Public domain, via Wikimedia Commons.

This work is an early example of mathematical perspective in art, but some critics have maligned Dürer’s technique. William Mills Ivins Jr., a former curator at the Metropolitan Museum of Art in New York, described it as “the oddest trapezoidal shape” and claimed it wasn’t even level with the floor. Crannell and her coauthors say it’s a matter of perspective. They write, “Surprisingly, the answers to these questions depend not only on what Dürer did 500 years ago, but also on what Ivins did in 1938. And, as we will show, it depends on what you, the reader, do when you look at St. Jerome today.”

Crannell and her coauthors describe how to use straight lines in St. Jerome in His Study to determine exactly where the viewer should stand to see the painting from the perspective Dürer probably intended, and therefore to see the table as a square instead of a trapezoid. The proper viewing location for that particular engraving turns out to be closer to the picture and farther to the right than most people would naturally stand. They write, “The oddness that Ivins saw in the table wasn’t because Dürer was in the wrong, but because Ivins was in the wrong, literally: he was looking from the wrong place!”

In February, I had the pleasure of going to the Brigham Young University Museum of Art with Crannell, and she shared some of her secrets with me. (The BYU Museum of Art does not normally allow photography, so I thank them for graciously making an exception for us.)

In perspective art, lines in the painting that represent parallel lines in the real world—say, train tracks or the opposite sides of a table—intersect on the canvas at so-called vanishing points. These vanishing points are the key to determining the optimal location from which to view a painting.

The most obvious way to find the vanishing points of a painting and thus to determine the optimal viewing location is to place rulers directly on “parallel” lines in the drawing, but shockingly, most museums frown on that practice. That’s where the chopsticks come in.

Standing in front of a piece of art, Crannell closes one eye and holds the chopsticks in front of her so they line up with lines in the artwork that represent parallel lines in the real world. The place the chopsticks appear to intersect is in front of the vanishing point of those lines. For art that has one vanishing point, the viewer should stand directly in front of that point. The viewing distance can be determined by trial and error or by some sneaky geometry with squares.

Credit: A diagram illustrating the orthocenter of a triangle. Each red line is one altitude of the triangle. Image: Public domain-self, via Wikimedia Commons.

For art with two vanishing points, the optimal viewing point is somewhere on the semicircle that connects the two vanishing points. For three vanishing points, determining the optimal vantage point is a bit more involved. It is at the intersection of three hemispheres, each one of which has two of the vanishing points as a diameter. Equivalently, it is somewhere in front of the orthocenter of the triangle whose vertices are the vanishing points. (The orthocenter of a triangle is the intersection point of the three altitidues of the triangle, as illustrated in the diagram on the right.) For a more complete description of how to find viewing points in art, check out Viewpoints: Mathematical Perspective and Fractal Geometry in Art by Crannell and Frantz.

In addition to giving me a way to look eccentric at the museum, Crannell’s technique helps me understand why some paintings seem to leap off the page, and some, even though they basically look realistic, don’t quite pop. In some pieces we looked at, lines that should have represented parallel lines in the real world didn’t end up determining a consistent vanishing point. Looking at any one part of the painting, nothing was clearly wrong, but the overall effect was slightly imperfect. When artists do manage to deploy perfect perspective—and viewers manage to find the correct vantage point—the effect can be startlingly realistic.

As Crannell and her coauthors describe it,  we can see the effect that the master geometer Albrecht Dürer intended. If you view St. Jerome in His Study [from the mathematically determined vantage point], you’ll see that the engraving takes on an amazing realism and depth. The gourd in the picture seems to hover over your head; you feel you could stick your hand in the space under the table; the bench off to the left invites you to come sit down and fluff up the pillows.

If you would like to know how to find those fluffy pillows for yourself, Crannell has information about projective geometry and perspective art on her website.

For more insights like this, visit our website at www.international-maths-challenge.com.

Credit of the article given to Evelyn Lamb |

Have you ever taken a walk through the rain on a warm spring day and seen that perfect puddle? You know, the one where the raindrops seem to touch down at just the right pace, causing a dance of vanishing circles?

Even before I entered the field of fluid flow research nearly 15 years ago, I was fascinated by the waves that appear after a raindrop hits a puddle.

As I became focused on the study of unstable waves in liquid sheets – geared toward mitigating undesirable waves in industrial coating and atomization processes – my fascination with puddle waves turned into an obsession. What is going on? Where does the pattern come from? Why does the impact of rain in a puddle look different than when rain falls elsewhere, like in a lake or the ocean?

It turns out that it all has to do with something called dispersion.

In the context of water waves, dispersion is the ability of waves of different wavelengths to each move at their own individual speeds. Looking down on a puddle, we see a collection of such waves moving together as one ripple in the water.

When a raindrop touches down, imagine it as a “ding” to the water surface. This ding can be idealized as a packet of waves of all different sizes. After the raindrop falls, the packet’s waves are ready to begin their new life in the puddle.

However, whether we see those waves as ripples depends on the body of water that the raindrop lands on. The number and spacing of rings that you see depends on the height of the puddle. This has been verified in some very cool ripple tank experiments, where a drop of the same velocity falls into a container with water at different depths.

Shallow puddles enable ripples, because they are much thinner than they are wide. The balance between the surface force – between the water puddle and the air above it – and the gravitational force tips in favor of surface force. This is key, since the surface force depends on the curvature of the water surface, whereas the gravitational force does not.

An initially still shallow puddle becomes curved at the surface after the raindrop hits. The surface force is different for long waves than for short ones, causing waves of different sizes to separate into ripples. For shallow puddles, the long waves move slowly away from the point of impact, while the short waves move fast, and the really short waves move really fast, becoming tightly packed at the perimeter. This creates the enchanting pattern that we see.

Raindrops may react differently in other situations. Imagine that rain is hitting a lake or ocean – or those deep pothole puddles that require galoshes. Here, the raindrop hits the water, but the force due to gravity becomes more important. It moves waves of all sizes at the same speed which may overpower the rippling effect due to the surface force.

The combination of teaching undergraduate partial differential equations while simultaneously continuing to research liquid sheets led to what I’ve been calling the “puddle equation.” When solved, the equation creates an animated simulation of what happens after a raindrop hits a puddle. It’s a simplified version of an equation in one of our group’s more recent research endeavors, but it’s also consistent with the classical description of ripples.

I use this approximate description of puddle waves as one way to get students excited about math by relating it to the world around them.

A model of waves in a dispersive puddle, after a raindrop hits. The top three figures show what happens after a drop hits the puddle, with arrows indicating the passage of time. The bottom figure shows the cross-sectional view through the puddle, highlighting that the initial wave bundle caused by the raindrop splits into waves of different sizes. Large waves in the center move more slowly than small waves at the perimeter. Nate Barlow

The study of surface-force-driven waves is important for applications such as coating processes involved in making batteries and solar cells.

Such waves also appear as a result of the leg stroke of a water strider insect, but research has found that the water strider isn’t specifically looking to make those waves to enable travel.

The beauty of puddle waves is no small thing by itself. By connecting nature with its primal language – mathematics – we gain access to its control panel, allowing us to observe every little detail, uncovering all the secrets.

For more insights like this, visit our website at www.international-maths-challenge.com.
Credit of the article given to Nate Barlow

Credit: Getty Images/iStockphoto Thinkstock Images/photoncatcher

An embellished account of a border crossing

“How many days will you be staying?” The immigration officer’s question made my blood run cold. I could easily tell him my origin city, nationality, flight number, and eventual destination, but this question was different. It was a fencepost question.

Fencepost questions, dealing as they do with the difference between count and duration, discrete and continuous, have always been difficult for me. May 1st and 8th were both Sundays. There are 7 days in a week, and 8-1 is 7. 9 am on May 8th is 7 days after 9 am on May 1st, but if I do something every day from the 1st to the 8th, I do it 8 times. If someone asks me about something that takes place from the 1st to the 8th, mere subtraction is not enough. I need to know the context of the question and what type of answer is expected.

For this trip, I left on the 14th and would return on the 24th. 24-14 is 10, but I would be there for portions of 11 calendar days and the entirety of only 9 of them. I arrived at 5 pm and would leave at 10 am, so I would be there for less than 240 hours, the length of 10 days. On the other hand, I would be away from my home for well over 240 hours. Would the officer just look at my arrival and departure dates, subtract, and be done with it? Would it confuse him if I said 11? Or did he know about fenceposts too? Was he trying to trap me? Luring me into the false certainty of subtraction, ready to pounce when I gave him a number that failed to reflect the nuances of my visit?

I felt like a cornered animal who doesn’t know whether, if I do something every other day, I do it 3 or 4 times in a week, and if I do it 4 times, whether that means a week has 8 days in it.

My mind was racing, my palms sweaty. The officer looked at me expectantly. Would he think I was flustered because I had something to hide? Would I be dragged away for more questioning, as another mathematician was last week?

Trying to keep my voice steady, I replied: “Ten.” I resisted the urge to explain the situation further.

I held my breath. As if in slow motion, he looked down, reached for the stamp, and flipped to a blank page in my passport. “Have a nice visit.”

For more insights like this, visit our website at www.international-maths-challenge.com.

Credit of the article given to Evelyn Lamb

Breathe. As your lungs expand, air fills 500 million tiny alveoli, each a fraction of a millimeter across. As you exhale, these millions of tiny breaths merge effortlessly through larger and larger airways into one ultimate breath.

These airways are fractal.

The branches within lungs are an example of self-similarity. Brockhaus and Efron Encyclopedic Dictionary/Wikimedia

Fractals are a mathematical tool for describing objects with detail at every scale. Mathematicians and physicists like me use fractals and related concepts to understand how things change going from small to big.

You and I translate between vastly different scales when we think about how our choices affect the world. Is this latte contributing to climate change? Should I vote in this election?

These conceptual tools apply to the body as well as landscapes, natural disasters and society.

Fractals everywhere

In 1967, mathematician Benoit Mandelbrot asked, “How long is the coast of Britain?”

It’s a trick question. The answer depends on how you measure it. If you trace the outline on a map, you get one answer, but if you walk the coastline with a meter stick, the result is quite different. Anyone who has tried to estimate the length of a rugged hiking trail from a map knows the treachery of the large-scale picture.

Satellite image of Great Britain and Northern Ireland. NASA

That’s because lungs, the British coastline and hiking trails all have fractality: their length, number of branches or some other quantity depends on the scale or resolution you use to measure them.

The coastline is also self-similar – it’s made out of smaller copies of itself. Fern fronds, trees, snail shells, landscapes, the silhouettes of mountains and river networks all look like smaller versions of themselves.

That’s why, when you’re looking at an aerial photograph of a landscape, it’s often hard to tell whether the scale bar should be 50 km or 500 m.

Your lungs are self-similar, because the body finely calibrates each branch in exact proportions, making each branch a smaller replica of the previous. This modular design makes lungs efficient at any size. Think of a child and an adult, or a mouse, a whale. The only difference between small and large is in how many times the airways branch.

Self-similarity and fractality appear in art and architecture, in the arches within arches of Roman aqueducts and the spires of Gothic cathedrals that mirror the forest canopy. Even ancient Chinese calligraphers Huai Su and Yan Zhenqing prized the fractality of summer clouds, cracks in a wall and water stains in a leaking house in 722.

Scale invariance

Self-similar objects have a scale invariance. In other words, some property holds regardless of how big they get, such as the efficiency of lungs.

In effect, scale invariance describes what changes between scales by saying what doesn’t change.

A sketch from Leonardo da Vinci’s notes on tree branches. Fractal Foundation

Leonardo da Vinci observed that, as trees branch, the total cross-sectional area of all branches is preserved. In other words, going from trunk to twigs, the number of branches and their diameter change with each branching, but the total thickness of all branches bundled together stays the same.

Da Vinci’s observation implies a scale invariance: For every branch of a certain radius, there are four downstream branches with half that radius.

Earthquake frequency has a similar scale invariance, which was observed in the 1940s. The big ones come to mind – Lisbon 1755, San Francisco 1989 – but many small earthquakes occur in California every day. The Gutenberg-Richter law says that earthquake frequency depends on the size of the earthquake. The answer is surprisingly simple. A tenfold bigger earthquake occurs roughly one-tenth as often.

Society and the power law

A 19th-century economist Vilifredo Pareto – famous in business school for the 80/20 rule – observed that the number of families with a certain wealth is inversely proportional to their wealth, raised to some exponent. Pareto measured the exponent for different years and different countries and found that it was usually around 1.5.

Patterns in an oak’s branches. Schlegelfotos/shutterstock.com

Pareto’s wealth distribution came to be known as the power law, ostensibly because of the exponent or “power.”

Anything self-similar has a corresponding power law. In an April paper, my colleague and I describe the corresponding power law for lungs, blood vessels and trees. It differs from Pareto’s power law only by taking into account specific ratios between branches.

The sizes of fortunes then are akin to the sizes of tree twigs or blood vessels – a few trunks or large branches and exponentially more tiny twigs.

Pareto thought of his distribution of wealth as a natural law, but many different models of social organization give rise to a Pareto distribution and societies do vary in wealth inequality. The higher Pareto’s exponent, the more egalitarian the society.

From understanding how humans are made up of tiny cells to how we affect the planet, self-similarity, fractality and scale invariance often help translate from one level of organization to another.

For more insights like this, visit our website at www.international-maths-challenge.com.
Credit of the article given to Mitchell Newberry

Credit: Getty Images

The right mix of people who already know one another, of boys and girls–Ramsey numbers may hold the answer

Let’s say you’re planning your next party and agonizing over the guest list. To whom should you send invitations? What combination of friends and strangers is the right mix?

It turns out mathematicians have been working on a version of this problem for nearly a century. Depending on what you want, the answer can be complicated.

Our book, “The Fascinating World of Graph Theory,” explores puzzles like these and shows how they can be solved through graphs. A question like this one might seem small, but it’s a beautiful demonstration of how graphs can be used to solve mathematical problems in such diverse fields as the sciences, communication and society.

A puzzle is born

While it’s well-known that Harvard is one of the top academic universities in the country, you might be surprised to learn that there was a time when Harvard had one of the nation’s best football teams. But in 1931, led by All–American quarterback Barry Wood, such was the case.

That season Harvard played Army. At halftime, unexpectedly, Army led Harvard 13–0. Clearly upset, Harvard’s president told Army’s commandant of cadets that while Army may be better than Harvard in football, Harvard was superior in a more scholarly competition.

Though Harvard came back to defeat Army 14-13, the commandant accepted the challenge to compete against Harvard in something more scholarly. It was agreed that the two would compete – in mathematics. This led to Army and Harvard selecting mathematics teams; the showdown occurred in West Point in 1933. To Harvard’s surprise, Army won.

The Harvard–Army competition eventually led to an annual mathematics competition for undergraduates in 1938, called the Putnam exam, named for William Lowell Putnam, a relative of Harvard’s president. This exam was designed to stimulate a healthy rivalry in mathematics in the United States and Canada. Over the years and continuing to this day, this exam has contained many interesting and often challenging problems – including the one we describe above.

Red and blue lines

The 1953 exam contained the following problem (reworded a bit): There are six points in the plane. Every point is connected to every other point by a line that’s either blue or red. Show that there are three of these points between which only lines of the same color are drawn.

In math, if there is a collection of points with lines drawn between some pairs of points, that structure is called a graph. The study of these graphs is called graph theory. In graph theory, however, the points are called vertices and the lines are called edges.

Graphs can be used to represent a wide variety of situations. For example, in this Putnam problem, a point can represent a person, a red line can mean the people are friends and a blue line means that they are strangers.

Show that there are three points connected by lines of the same color. Credit: richtom80 Wikimedia (CC BY-SA 3.0)

For example, let’s call the points A, B, C, D, E, F and select one of them, say A. Of the five lines drawn from A to the other five points, there must be three lines of the same color.

Say the lines from A to B, C, D are all red. If a line between any two of B, C, D is red, then there are three points with only red lines between them. If no line between any two of B, C, D is red, then they are all blue.

What if there were only five points? There may not be three points where all lines between them are colored the same. For example, the lines A–B, B–C, C–D, D–E, E–A may be red, with the others blue.

From what we saw, then, the smallest number of people who can be invited to a party (where every two people are either friends or strangers) such that there are three mutual friends or three mutual strangers is six.

What if we would like four people to be mutual friends or mutual strangers? What is the smallest number of people we must invite to a party to be certain of this? This question has been answered. It’s 18.

What if we would like five people to be mutual friends or mutual strangers? In this situation, the smallest number of people to invite to a party to be guaranteed of this is – unknown. Nobody knows. While this problem is easy to describe and perhaps sounds rather simple, it is notoriously difficult.

Ramsey numbers

What we have been discussing is a type of number in graph theory called a Ramsey number. These numbers are named for the British philosopher, economist and mathematician Frank Plumpton Ramsey.

Ramsey died at the age of 26 but obtained at his very early age a very curious theorem in mathematics, which gave rise to our question here. Say we have another plane full of points connected by red and blue lines. We pick two positive integers, named r and s. We want to have exactly r points where all lines between them are red or s points where all lines between them are blue. What’s the smallest number of points we can do this with? That’s called a Ramsey number.

For example, say we want our plane to have at least three points connected by all red lines and three points connected by all blue lines. The Ramsey number – the smallest number of points we need to make this happen – is six.

When mathematicians look at a problem, they often ask themselves: Does this suggest another question? This is what has happened with Ramsey numbers – and party problems.

For example, here’s one: Five girls are planning a party. They have decided to invite some boys to the party, whether they know the boys or not. How many boys do they need to invite to be certain that there will always be three boys among them such that three of the five girls are either friends with all three boys or are not acquainted with all three boys? It’s probably not easy to make a good guess at the answer. It’s 41!

Very few Ramsey numbers are known. However, this doesn’t stop mathematicians from trying to solve such problems. Often, failing to solve one problem can lead to an even more interesting problem. Such is the life of a mathematician.

This article was originally published on The Conversation. Read the original article.

For more insights like this, visit our website at www.international-maths-challenge.com.

Credit of the article given to Gary Chartrand, Arthur Benjamin, Ping Zhang & The Conversation US

Alice in Wonderland enthusiasts recently celebrated the story’s anniversary with creative events like playing with puzzles and time — and future Alice exhibits are in the works. The original 1865 children’s book Alice’s Adventures in Wonderland, sprung from a mathematician’s imagination, continues to inspire exploration and fun.

But is a connection between math and creativity captured in schools? Much discussion across the western world from both experts and the public has emphasized the need to revitalize high school mathematics: critics say the experience is boring or not meaningful to most students. Experts concerned with the public interest and decision-making say students need skills in critical thinking, creativity, communication and collaboration.

Mathematicians, philosophers and educators are also concerned with the excitement and energy of creative expression, with invention, with wonder and even with what might be called the romance of learning.

Mathematics has all the attributes of the paragraph above, and so it seems to me that what’s missing from high school math is mathematics itself.

I am now working with colleagues at Queen’s University and the University of Ottawa to develop RabbitMath, a senior level high-school math curriculum designed to enable students to work together creatively with a high level of personal engagement. My preparation for this has been 40 years of working with teachers in high-school classrooms.

In partnership with grades 11 and 12 math teachers, we will be piloting this curriculum over the next few years.

Mathematical novels

When students study literature, drama or the creative arts in high school, the curriculum centres on what can be called sophisticated works of art, created in response to life’s struggles and triumphs.

But currently in school mathematics, this is rarely the case: students are not connected to the larger imaginative projects through which professional mathematicians confront the world’s problems or explore the world’s mysteries.

The author, Peter Taylor, right, at a Lisgar Collegiate Institute Grade 11 math classroom in Ottawa, 2018. (Ann Arden), Author provided

Mathematician Jo Boaler from the Stanford Graduate School of Education says that a “wide gulf between real mathematics and school mathematics is at the heart of the math problems we face in school education.”

Of the subject of mathematics, Boaler notes that:

“Students will typically say it is a subject of calculations, procedures, or rules. But when we ask mathematicians what math is, they will say it is the study of patterns that is an aesthetic, creative, and beautiful subject. Why are these descriptions so different?”

She points out the same gulf isn’t seen if people ask students and English-literature professors what literature is about.

In the process of constructing the RabbitMath curriculum, problems or activities are included when team members find them engaging and a challenge to their intellect and imagination. Following the analogy with literature, we call the models we are working with mathematical novels.

For example, one project invites students to work with ocean tides. It would hard to find a dramatic cycle as majestic as the effect of that sublime distant moon on the powerful tidal action in the Bay of Fundy.

Student engagement

In the 1970s, the extraordinary mathematician and computer scientist at Massachusetts Institute of Technology, Seymour Papert, noticed that in art class, students, just as mature artists, are involved in personally meaningful work. Papert’s objective was to be able to say the same of a mathematics student.

I had a parallel experience in 2013 when I was the internal reviewer for the Drama program at Queen’s. I marvelled at students’ creative passion as they prepared to stage a performance. And they weren’t all actors: they were singers, musicians, writers, composers, directors and technicians.

In Papert’s curriculum model, students with diverse abilities and interests work together on projects, whereby they collaborate on problems, strategies and outcomes.

As a pioneering computer scientist, Papert understood that students could directly access the processes of design and construction through digital technology. Papert used his computer system LOGO for this technical interface. LOGO was limited in its scope, but Papert’s idea was way ahead of its time.

Students in the RabbitMath classroom will work together using the programming language Python to construct diagrams and animations to better understand their experiments with springs and tires, mirrors and music. They will produce videos that can explain to their classmates the workings of a sophisticated structure.

Today, technology, the internet, computer algebra systems and mathematical programming provide possibilities for immediate engagement in processes of design and construction — exactly what Papert wanted. The platform for RabbitMath is the Jupyter Notebook, a direct descendant of LOGO.

Technical skill

For too many years, real progress in school mathematics education has been hamstrung by a ridiculous confrontation between so-called “traditional” and “discovery” math. The former is concerned with technical facility and the latter is about skills of inquiry and investigation.

There is no conflict between the two; in fact they support each other rather well. Every sophisticated human endeavour, from conducting a symphony orchestra to putting a satellite into orbit, understands the complementary nature of technical facility and creative investigation.

Stanford University Graduate School of Education mathematician Keith Devlin advises parents to ensure their child has mastery of what he calls number sense, “fluidity and flexibility with numbers, a sense of what numbers mean, and an ability to use mental mathematics to negotiate the world and make comparisons.” But for students embarking on careers in science, technology or engineering, that is not enough, he says. They need a deep understanding of both those procedures and the concepts they rely on — the capacity to analyze and work with complex systems.

A high-school math class is a rich ecosystem of differing abilities, capacities, objectives and temperaments.

The educator’s goal must be to enable a diverse mix of students to work together in a math class as creatively and intensely as students in the drama program, or to bring the same personal passion as they might to writing fiction.

For more insights like this, visit our website at www.international-maths-challenge.com.
Credit of the article given to Peter Taylor