# For current students

**Meeting times, materials, and resources:**

**Future meeting times:**

**Future meeting times will resume in the Fall. Watch this website for updates later this summer.**

**Past meeting times:**

April 15, 2019:

In the middle school group, we completed several hands-on experiments. We first learned about Möbius bands, doughnut shapes, and “Möbius bands” with multiple twists, and then build these out of paper and tape. We then asked questions like: what happens if you cut these shapes in half, and what would it be like to play tic-tac-toe on a Möbius band? We found some surprising results, and then figured out general patterns, such as how many pieces would you get if you cut a Möbius band with 100, or 101, twists, in half.

Here is a fun video showing the best way to cut a bagel with Möbius strips:

After the break, we made soap bubbles, and build different geometrical shapes like cubes and pyramids and learn about the mathematics of weird-shaped (such as cube shaped!) bubbles. Students made their own shapes to experiment with, and we found a bunch of interesting examples of “minimal surfaces”.

One last experiment that we didn’t have the time to do, but which can be carried out at home, is how to “compute” the number π=3.14… by dropping toothpicks. This is called the “Buffon’s Needle” problem. A fun simulation of this, which allows you to drop hundreds of thousands of sticks at once to see what the result would be, is here: https://mste.illinois.edu/activity/buffon/

In the high school group, we also studied doughnuts and Möbius bands. and continued our questions about coloring graphs on shapes like these. We then learned about Ramsey theory, which tells you that in many situations, if you have a large enough set of possibilities in some setup, then patterns *must* emerge. For instance, if you have a very very large set of text (such as a long novel) and go out looking for secret messages or predictions of the future in it, then you are guaranteed to find some “surprising” patterns. We studied a problem which many residents of the Prussian city of Königsberg tried to solve. Namely, they often on evening strolls tried to walk along each of seven bridges in the city exactly one time. After a while, they started to feel that it was impossible. We learned why it is impossible, and how to *prove* when similar puzzles are possible or not possible. Finally, we studied the problem of determining winners of a tournament with many different games among the players, and in different situations whether it even makes sense to declare one person as the “winner”.

April 1, 2019:

In the middle school group, we learned about codes and ciphers. We learned a bunch of ways to send secret messages, as well as techniques to crack codes. We also saw some funny ways of adding and multiplying numbers, “clock arithmetic”, which gave us the tools to study some examples.

Slides from the lesson, which includes some further puzzles to solve than we had time for in the session, are here: Cyrptography

In the high school group, we learned about properties of coloring maps. We started with a map of the US, and found by trial and error that 4 colors are enough to color each state so that no two states have the same color. Then we discussed what kinds of maps need at least 4 colors. We then tried to come up with maps that need at least 5 colors, but didn’t find any, as long as we followed certain rules, such as saying that countries to color on a map consist of only one piece and that countries coming together only at one point doesn’t count as a border. It turns out that there is a very very difficult theorem, the *4-color theorem*, saying that this is indeed impossible to find a map which requires 5 colors. We then generalized the problem to coloring graphs, and saw that although the map coloring problem can be represented as a graph coloring problem, some graphs can’t be drawn in the plane without intersections, and so they don’t correspond to maps of countries on a flat surface.

We then started looking at the following take home problems, which, appropriately, include an April Fools’ puzzle of Martin Gardner: Take-Home Problems

February 25, 2019:

In the middle school group, first we reviewed the main topics from last class, to catch up the new students and looked at a game of Nim and determined which player would win and how that player would win using binary numbers. We covered some more combinatorial games including Gaskill Nim which is won using Fibonacci numbers, Thai 21 uses “mod 4” arithmetic, a game of 9-5, a game played with cards. We then looked at a Card Switching game and talked about using permutations, even and odd, to determine which player wins the game.

In the high school group, we continued learning about partitions. In particular, we learned about Ferrers diagrams, a pictorial way to play with partitions and to prove some useful facts about them. Then we learned about Euler’s Pentagonal Number Theorem, and how it helps us compute values of the partition function without actually having to find all the partitions.

The slides from the talk can be found here: PtnsPt2

Take home problems can be found here: problems2.25

February 11, 2019:

In the middle school group, the topic was how to use mathematics to improve a player’s chances of winning games. We talked about three characteristics of games- unknown information, chance and the moves available to the players. We focused on 4 purely combinatorial games, games with no chance and no unknown information. In particular we showed how a perfectly played game of Tic-Tac-Toe ended in a draw and then we played Daisy and Pennies on a Round and talked about how the player that created symmetry won. We then played Nim and talked about how we could use binary numbers to tell which player could create symmetry and how that player could create symmetry.

In the high school group, we introduced the idea of a generating function, and saw that it is a powerful tool for studying a sequence of numbers. For instance, we saw that some infinite sums really do make sense as finite numbers, and that 0.99999999…. really does equal 1. We also discovered an exact formula for the Fibonacci numbers, which allowed us to see why the Golden Ratio is closely connected to them. Finally, we introduced partitions, and started observing some strange properties of them, and asked the question as to how we would count the number of partitions of a large number without actually having to find them all.

Slides from this lecture are here: Lesson1

We also gave several take home problems, which can be found here: Problems2.11

January 28, 2019:

Topics:

In the middle school group, we introduced games of logic. We visited a mysterious island full of knights, liars, and occasionally tourists. Knights always tell the truth, liars always lie no matter what, and tourists, like regular people, lie sometimes and tell the truth sometimes. We solved various puzzles to determine who was what by their responses to different sorts of questions. We also played with craft “match sticks” and solved puzzles asking how to move only a given number of sticks to make certain shapes.

In the high school group, we covered a number of “proofs by picture”. We studied a number of summation formula, and gave proofs for the formulas for triangular numbers, the sum of the first n squares, and a geometric proof that the binomial coefficient with a 2 in the bottom is always a triangular number. We saw how geometric reasoning can be powerful and can lead to insightful proofs. We also worked on other puzzles, such as how to dissect an octagon into 5 pieces to obtain a square, and how to make a timer that measures out a very specific amount of time using unreliably burning fuses.

Here is a copy of the take home problems from the meeting: Take_home_problems_1.28

January 14, 2019:

Topic: Solving problems through coloring.

In this lesson, students had first introductions to problem solving and strategies about proving that certain things seem impossible in a completely convincing way. After introductions and several (mostly pun-based) warm-up problems, we asked if it was possible for “Sir Rick Cast-ley (OBE)” to run through rooms in his castle that starts in room “S” and ends up in room “E,” with the restriction that each room is visited exactly once (only left, right, up, down moves are allowed). Different possible positions of the start and end points were explored.

For example, our first problem considered:

There are many ways to draw such a path (try for yourself!). As one student asked, a very nice question then is *how many* ways are there to solve it?

We then looked at more examples of these problems on the same grid but with different S and E points. By inspection, we found that sometimes it is possible, since you can just do it after a little experimentation, and sometimes after several attempts a solution was not found. As the students suspected, sometimes, like in the figure below, its simply not possible.

The question is *why. *There are only so many possible paths to draw, so certainly drawing out **every** possibility does show that, but this would be very difficult to write out, and it would be easy to miss a possible path. Also, if I looked at a 600×400 grid instead of a 6×4 grid, I could ask the same question, but clearly it wouldn’t be very fun to list all possible paths then! There must be a better way. Mathematics is, above all else, the study of *patterns*, and there must be a simple pattern with makes the first layout above different than the second one. As several student groups discovered by direct exploration, the pattern seems to depend on the following. If you walk the shortest possible way from the first to the last room and you end up passing through an *even *number of rooms, it seems solvable. If you passed through an *odd* number of rooms, on the other hand, it doesn’t look possible.

Try this yourself! After doing enough examples, you can quickly convince yourself that this seems true. We are on the right track, but observing the pattern is not enough; we need to show *why* the pattern works.

The key is to **color** the grid. For example, the two layouts above then look like the following. Suppose now that Sir Cast-ley has to go from his (V)ault to his (U)ndercroft.

Now notice that every time you move between two rooms, the color switches. The pattern above can now be thought of in another way: it is the same as saying that the puzzle is solvable if the starting and end points are in rooms of different colors, and impossible to solve otherwise. How can we *prove* this new color version of our observation? Well, first note that there are 24 total squares, which is an even number. Thus, any path which hits each room exactly once is 24 moves long. By the color-alternating property, if you label the rooms as you go along the path by 1,2,…,24, then room 1 is the color of the starting square, let’s say gold for the sake of argument, and the color of a square is always the opposite of the next numbered one. For example, if 1 is gold, then the sequence of squares 1,2,3,… has colors G,B,G,… and so on. Thus, the color of the last room, room 24 with an even numbering, must be black, or the opposite color from the start point. This explains why if the colors are the same, then it is not possible.

This is a very powerful idea. It is our first example of an *invariant*: a path with an even number of steps on a checkerboard always must start and end in rooms of different colors.

After working through these examples, students worked through several further examples which can all be solved by coloring pictures with two colors, or by just “finding” the answer in some of the cases that were possible.

Students were given several take home problems to think about to reinforce the material. In addition, here are a few extra bonus problems to try out.

1). In the game Sudoku, players must fill a 9×9 grid such that the numbers 1-9 appear in each row, column, and in 9 special smaller 3×3 grids inside. Another type of problem like this is the following. Given an mxn grid, you want to fill it up with positive whole numbers so that every single row and column each adds up to 15.

Questions:

a) Show that m=n, i.e., that the grid must really be a square.

b) Find as many examples as you can of square grids which have this property and for which the diagonals also add up to the same number (they are called *magic squares*).

c) Can you find other nice examples with “15” replaced by other numbers?

2). Suppose that n is a positive odd number. Write down the list of numbers 1,2,…,n. Now pick two of them at random, say a and b, and replace them with the absolute value |a-b|. Keep doing this operation over and over until there is only one number left. Is the last number even or odd?

3). A map of a bunch of cities, represented as blobs, is below. There are various roads connecting cities. Is it possible to do a road trip which visits each city exactly one time?

[If you need a hint, consider coloring the cities blue and red. If you need a stronger hint, then consider the coloring in the picture here.]

**Resources: **

**Sites:**

The site Art of Problem Solving is **the** ultimate resource for problem solving, info on competitions at a number of levels and other programs, and discussion forums about problem solving.

**Videos:**

The web is full of great learning resources. A few classics are here and here. In particular, these show you, in beautifully animated fashion, how sometimes going to a higher dimension makes complicated things simple, and how to visualize the very strange fact that its possible to turn a sphere “inside out.”

The channel here also regularly posts interesting new content explaining big math ideas in fun and broken-down ways.

**Fun math books:**

The Man Who Knew Infinity This tells the inspiring story of Srinivasa Ramanujan, who flunked out of college twice and with almost no formal mathematical training re-discovered much of modern mathematics. He was truly a man who loved numbers and formulas, and wrote down so many shocking formulas that leading mathematicians of the day almost could not believe that he was real. In a short period of a few years after his chance “discovery” by a leading mathematician of the day, he went on to revolutionize entire fields of mathematics; today, his formulas have been shown to contain secrets behind the theory of black holes, and they are being implemented into signal processing methods in cell phone computer chips. A truly breathtaking read and proof that mathematical ideas have the power to change the world, even from the humblest of origins.

Also, this book was very recently turned into a Hollywood movie of the same name with several A-list actors like Dev Patel and Stephen Fry, which is also highly recommended!

The Pea and the Sun: a mathematical paradox

This brilliant book is one of the first exposures Dr. Rolen had to deep mathematics, and is a fun ride through various riddles, geometric puzzles, and paradoxes, all leading to a demonstration of the perplexing Banach-Tarski paradox. This paradox states that it is possible to cut, with very *very* precise scissors, a ball into just a few pieces, then to rearrange these pieces without stretching or adding new material, only using rotations and translations, into two balls of the same volume as the original! The title refers to the resulting fact that you can therefore cut a ball the size of a pea into finitely many pieces, rearrange, and turn it into a ball the size of the sun! In this book, you will be introduced in a very friendly and approachable way to different ideas about “infinity.” For example, you will see why some infinities are bigger than other infinities, learn how to fill a hotel with no vacancies with more guests without kicking any guests out, and see a complete explanation of the paradox as depicted by a big machine with various tubes and hoppers.