# Forest Fires, Scrubbing Calculator, and Bongard Problems

Welcome to this week’s Math Munch!

Last summer where I live, in California, there were a lot of forest fires. We’re having a big drought, and that made fires started for lots accidental of reasons– lightning, downed power lines, things like that– get much bigger than usual. I thought I’d learn a little about forest fires so that I can be a more responsible resident of my state.

And I found this great website with an awesome computer simulation that you can manipulate to experiment with the factors that lead to forest fires!

My forest fire, just starting to burn. Click on this picture to play with the simulation! (Note: It doesn’t work as well in certain browsers. I recommend Firefox.)

This site was built by Nicky Case, who studies things that mathematicians and scientists call complex systems. Basically, a complex system is some phenomenon that has a simple set of causes but unpredictable results. An environment with forest fires is a good example– a simple lightning strike in a drought-ridden forest can cause a wildfire to spread in patterns that firefighters struggle to predict. It turns out that simulations are perfect for modeling complex systems. With just a few simple program rules we can create a huge number of situations to study. Even better, we can change the rules to see what will happen if, say, the weather changes or people are more careful about where and when they set fires.

What do you think? Click the picture to see Nicky’s simulation.

You can use Nicky’s program to change the probability that trees grow, a burning tree sets its neighbors on fire, and many other factors. You can even invent your own and model them with emojis. What if there were two kinds of trees and one was more flammable than the other? What if trees grew quickly but lightning was common? As Nicky shows, simulations are useful for exploring what-if questions in complex systems. Use your imagination and explore!

Next up, have you ever heard of a scrubbing calculator? No, it’s not a calculator that doubles as a sponge. It’s a calculator that helps you solve for unknowns in equations by “scrubbing,” or approximating, the answer until you find a number that works.

Here’s how it works: Say you’re trying to solve for an unknown, like the x in the equation above (maybe for some practical reason or just because you’re doing your homework). You could do some pretty complicated algebraic manipulations so that the x alone equals some number. But what if you could make a guess and change it until the equation worked?

Click on this image to see a scrubbing calculator in action!

If your guess was too big, you’d know because the expression wouldn’t equal 768 anymore– it would equal something larger. And if you had a calculator that instantly told you the solution based on your guess, you could do this guessing and checking pretty quickly.

Well, lucky for you I found a scrubbing calculator that you can use online! It’s very simple– just type in your equation and your guess, and click on the number you want to change (most likely the guess) to make it larger or smaller. It’s useful for solving equations, like I said. But I actually find it most interesting to watch how the whole expression changes as you change one of the numbers in it. For instance, check out the Pythagorean triple calculator I built. What do you notice as you gradually change one of the numbers in the expression?

https://www.cruncher.io/?/embed/xiY1IjwUJS

Finally, I’m excited to share with you one of my favorite kinds of puzzles– Bongard problems!
A Bongard problem has two sets of pictures, with six pictures in each set. All of the pictures on the left have something in common that the pictures on the right do not. The challenge is to figure out what distinguishes the two groups of pictures.

This problem was made by Douglas Hofstadter, who introduced Bongard problems to the U.S. I haven’t figured it out yet. Can you?

I got the Bongard problems shown above from a collection of problems put together by cognitive scientist Harry Foundalis. He has almost 300 of them, some made by Mikhail Bongard himself, who developed these problems while studying how to train computers to recognize patterns.

Harry also has guidelines for how to develop your own Bongard problems. He encourages people to send their problems to him and says he might even put them up on his site!

Bon appetit!

# Mobiles, Mathematical Objects, and Math Magazine

Welcome to this week’s Math Munch!

Before we start, a little business. You may have noticed that posts have been few and far between lately. Those of you who know us, the members of the Math Munch Team, know that we’ve all made a lot of life changes in the past year or two. We started out together teaching in the same school in New York– but now we live on far corners of the country and spend our time doing very different things. In case you’re curious, here are some pictures of the things we’ve been up to!

 Justin’s genus 19, rotationally symmetric surface

But even though we’ve moved apart physically, we’ve decided that we really want to keep the Math Munch Team together. We LOVE sharing our love of math with you– and we love hearing from you about the amazing things you make and do with math, too.

So, we’ve decided to revamp our posting process and came up with a schedule for when you can expect posts. There will be a new post every Thursday. (Though if Anna is posting from the West Coast, it might come out in the wee hours of Friday morning for some of you!) And here’s the monthly schedule of Thursday posts:

• The first Thursday of the month will be a post from Justin
• The second Thursday of the month will be a rerun!! Did you know we have over 150 posts on this site?? And we’ve been posting for almost five years??
• The third Thursday of the month will be a post from Anna
• The last Thursday of the month will be a post from Paul

And for those mysterious months with five Thursdays (ooh, when will that be, I wonder?)… There will be a surprise!

And now… for some math!

First up is a little game called SolveMe Mobiles! This game is full of little puzzles in which you have to figure out what each of the different shapes in a mobile weighs. You’re given different clues in different puzzles. So, for instance, in the puzzle to the left, you’re given the weight of the red circle and you have to figure out how much a blue triangle is. But you’re not given the weight of the whole mobile… Hmmm…

And this one, to the right, gives you the weight of one of the shapes and of the whole mobile– but now there are three shapes! Tricky!

Even better, you can build your own mobile puzzle for others to solve! I made this one, shown below– like my use of a mobile within a mobile?

Next up, I found a beautiful Tumblr account that I’d like to share with you full of pictures of found mathematical objects. It’s called… Mathematical Objects! (How clever.) The author of the site writes that the aim of the blog is to “show that mathematics, aside from its practicality, is also culturally significant. In other words, mathematics not only makes the trains run on time but also fundamentally influences the way we view the world.

“Counting to One Hundred with my Four-Color Pen”

Some of the images are mathematical art, like the one above; others are more “practical,” such as plans for buildings or images drawn from science.

Do you ever see an interesting mathematical object in the wild and feel the urge to take a picture of it? If so, go ahead and send it to us! We’d love to see what you find.

I’m very excited to share this last find with you all. It was sent to me by a wonderful math teacher, Mark Dittmer, and his math students. This year, they were inspired by Math Munch to make their own fun online math sites! I think what they made is super awesome– and I want to share it with you. I’ll be featuring some of their work in my next few posts, one thing at a time to give each its own day in the sun. First up is this adorable adventure story about the residents of Number Land. I hope you enjoy it!

Bon appétit!

# Mathy Clocks, Spirolaterals, and Mandalas

Welcome to this week’s Math Munch!

Hermann’s Abacus Clock. What time was I working on this post?

A few months ago, the Math Munch team got an email from retired mathematician Hermann Hoch with a lead to his amazing website full of (among other things)… clocks! One of the things Hermann does with his spare time in retirement is make creative math-y clocks using html. He calls them “html5 experiments”– and they really do take math art to the next level!

There are many fascinating clocks on Hermann’s site. (Be careful, or you might spend too much time watching the seconds go by!) One of my favorites is a clock he calls the Mondriaan Clock. The display is inspired by the art of Dutch painter Piet Mondriaan, who was known for his paintings of overlapping squares and rectangles in primary colors. The clock also comes with the exciting prompt– “wait until time creates golden ratios for us”! At what time will one of the rectangles in the image have dimensions that approximate the Golden Ratio? Hermann says that this question isn’t easy– he hasn’t even found all of the times himself! (And I’m sure he’d love to know– post your ideas in the comments below.)

Next up, I’ve been obsessed with Spirolaterals lately. What’s a Spirolateral, you ask? It’s a shape made by drawing segments of different lengths (say, 2, 3, and 4) one after another in a cycle (say, right, up, left, and down) until the shape closes up (or doesn’t, and you know it never will). If you follow those instructions (drawing on grid paper helps), you make this flower-like shape:

You can make Spirolaterals (or Loop-de-Loops, as they’re also called) with any numbers and using any turning angle. This Spirolateral uses three numbers and a turning angle of 90 degrees. (See the square corners?) But what if you use four numbers? Five numbers? Thirteen numbers? You can try drawing by hand- and then coloring them in, to make a beautiful mathematical creation. The Spirolateral below uses the first 50 digits of pi!

But you also don’t have to draw them by hand. The two Spirolaterals shown here were both drawn using a computer program! My favorite program for drawing Spirolaterals with 90 degree turns is this one, made by Chris Lusto. He gives great instructions and allows you to use as many numbers as you like!

But what if you wanted to make a Spirolateral with a… 109 degree turn? Wouldn’t that be cool! Well, yes, it is cool–

You can make this and other crazy Spirolaterals at this awesome website, brought to us by The Mathenaeum.

Finally, I leave you with this mesmerizing video of Dearing Wang drawing a Mandala. If you thought you’d never use your skills with a straight-edge and compass you worked so hard to develop in Geometry class– think twice. And for you straight-edge and compass nerds, keep an eye out for his pentagon construction! Is it perfect??

Bon appetit!

How long did it take me to make this post? (Hint: This clock tells time in base 6!)

# Fold and Cut, My Favorite Spaces, and Hook

Welcome to this week’s Math Munch!

Before you watch this video, think about this question: Do you think you could fold a piece paper so that you could cut a square out of it using exactly one straight cut? How about a triangle? Hexagon? Christmas tree shape??

Give it a try. Then watch this video:

Pattern for a very angular swan, by Erik Demaine

Surprised? As you may have seen in the video, using the “fold and cut” process you can make any shape with straight sides! Isn’t that crazy? I learned about this a few years ago, and now cutting weird shapes out of paper using just one cut is one of my favorite things to do.

The person who proved this amazing result is one of my favorite mathematicians, Erik Demaine. (You might remember him from our post a few years ago about origami mazes.) I think it’s really interesting that this idea that’s now a mathematical theorem appeared throughout history as a magic trick and a method for cutting out five-pointed stars to make American flags. Check out this website about the fold and cut problem to learn more about the history of the theorem, Demaine’s method for cutting out any straight-edged shape, and other related problems.

Evelyn wearing a Borromean ring cowl. Sweet!

I found out about this video from another favorite mathematician of mine, Evelyn Lamb. Evelyn writes a blog about math for Scientific American called Roots of Unity that’s really fun to read. Check it out if you get the chance!

She has a series of posts called “A Few of My Favorite Spaces” (cue Sound of Music song, “My Favorite Things”). Favorite spaces, you may ask? I’m not familiar with spaces plural. There’s more than just regular old 3D space? Yes, in fact there are! And if you read Evelyn’s blog you’ll learn about how mathematicians like to invent new spaces with bizarre properties– and sometime find out that what they thought was a completely new space actually resembles something very familiar.

Such as… The “house with two rooms.” As I understand it, this a box (“house”) with two floors and two tunnels in it– one punched from the top of the box and another from the bottom. The top tunnel lets you get from the roof of the house to the ground floor; the bottom tunnel lets you get from below the house to the second floor.

If you want to see someone making this crazy house in Minecraft and hear a much better explanation of what the house is like, here’s a video!

Ok, so what’s the point? Well, it turns out you can squish (just squish– no ripping or gluing) this house all the way down to a single point. This means that in topology (the type of math that involves a lot of squishing), the crazy tunnel house space is the same as the really boring space of just one point. I might want to live in a house with all these tunnels– but I definitely don’t want to live in a point. But in topology-world, they’re the same space. Huh.

To learn more about the house with two rooms (aka, point) and other crazy spaces, check out Evelyn’s blog!

Finally, speaking of squishing things down to a point, I want to show you a fun new game I found that involves a lot of squishing– Hook! Here’s a trailer video for the game:

You can find this game online at Kongregate. Enjoy!

Bon appetit!

# HYPERNOM, Euclid the Game, and Math Quilts

Welcome to this week’s Math Munch! And, welcome to a new school year! Back to school means back to Math Munch– and we’re super excited to share some great new things that we found over the summer.  The first of which is…

… THIS.

(GIF hoisted from the amazing Aperiodical)

That’s an image from this crazy new game called HYPERNOM, invented by some of our favorite people– Vi Hart, Henry Segerman, and Andrea Hawksley!

Noming through tasty tasty tetrahedra.

In this game, you wiggle around in a projection of 4-dimensional space, eating (or, better put, NOMING– NOM NOM NOM) 4-dimensional objects. Such as the dodecahedra (polyhedron with faces made from regular pentagons) that come together to form the 4-dimensional shape (called a polytope) you’re moving around in.

This game is MINDBLOWING. Really. You can play it on your computer– but I got to play it wearing a helmet that plunged me into the fourth dimension and left me feeling very dizzy.

The math behind HYPERNOM is kind of complicated but VERY interesting. If you’d like to learn more about the game and the 4-dimensional math it involves, check out this post from Aperiodical. Or, watch the talk that Vi, Henry, and Andrea gave about HYPERNOM at this year’s Bridges Mathematical Art conference!

Next up, the Math Munch team went back to school a few weeks ago, too– literally! And this member of the Math Munch team is taking a math class! My homework assignment last week was to play a new game called Euclid: The Game.

On my way to constructing an equilateral triangle. What should I do next?

The game is pretty much exactly what it sounds like. You get to use just a straight-edge and compass (but a virtual straight-edge and compass, powered by Geogebra, because it’s a computer game!) to make Euclid’s constructions. For instance, the first challenge is to make an equilateral triangle– and all you can do is draw circles and lines! How would you do that?

I love this game for learning geometry because it lets you see how Euclid and his mathematicians peers thought about geometry– but you don’t have to use a real compass! The game saves your constructions so you can use them later– so if you ever want to make an equilateral triangle again, you don’t have to start from scratch. The game also gives you points if you make your construction with the least number of steps or without using any new tools. Give it a try!

Finally, I recently ran across the beautiful mathematical quilts of Elaine Ellison. Elaine is a former high school math teacher from Indiana who now creates and gives talks about making mathematical quilts. Her quilts explore some of the most interesting types of mathematics– from tessellations (like the Escher-inspired fish tessellation quilt to the left), to conic sections, to strange geometric spaces.

 “Poincare Plane” “Parabolas”

Elaine has a website and a YouTube channel devoted to her gorgeous quilts. Check them out! Here’s a taster:

# Sphericon, National Curve Bank, and Cardioid String Art

Welcome to this week’s Math Munch!

Behold the Sphericon!

What is that? Well, it rolls like a sphere, but is made of two cones attached with a twist– hence, the spheri-con! The one in the video is made out of pie (not sure why…), but you can make sphericons out of all kinds of materials.

It was developed by a few people at different times– like many brilliant new objects. But it entered the world of math when mathematician Ian Stewart wrote about it in his column in Scientific American. The wooden sphericon was made by Steve Mathias, an engineer from Sacramento, California, who read Ian’s article and thought sphericons would be fun to make. To learn more about how Steve made those beautiful wooden sphericons, check out his site!

Even if you’re not a woodworker, like Steve, you can still make your own sphericon. You can start with two cones and make one this way, by attaching the cones at their bases, slicing the whole thing in half, rotating one of the halves 90 degrees, and attaching again:

Or you can print out this image, cut it out, fold it up, and glue (click on the image for a larger printable size):

If you do make your own sphericon (which I recommend, because they’re really cool), watch the path it makes as it rolls. See how it wiggles? What shape do you think the path is?

I found out about the sphericon while browsing through an awesome website– the National Curve Bank. It’s just what it sounds like– an online bank full of curves! You can even make a deposit– though, unlike a real bank, you can take out as many curves as you like. The goal of the National Curve Bank is to provide great pictures and animations of curves that you’d never find in a normal math book. Think of how hard it would be to understand how a sphericon works if you couldn’t watch a video of it rolling?

There are lots of great animations of curves and other shapes in the National Curve Bank– like the sphericon! Another of my favorites is the “cycloid family.” A cycloid is the curve traced by a point on a circle as the circle rolls– like if you attached a pen to the wheel of your bike and rode it next to a wall, so that the pen drew on the wall. It’s a pretty cool curve– but there are lots of other related curves that are even cooler. The epicycloid (image on the right) is the curve made by the pen on your bike wheel if you rode the bike around a circle. Nice!

You should explore the National Curve Bank yourself, and find your own favorite curve! Let us know in the comments if you find one you like.

String art cardioid

Finally, to round out this week’s post on circle-y curves (pun intended), check out another of my favorite curves– the cardioid. A cardioid looks like a heart (hence the name). There are lots of ways to make a cardioid (some of which we posted about for Valentine’s Day a few years ago). But my favorite way is to make it out of string!

String art is really fun. If you’ve never done any string art, check out the images made by Julia Dweck’s class that we posted last year. Or, try making your own string art cardioid! This site shows you how to draw circles, ovals, cardioids, and spirals using just straight lines– you could follow the same instructions, replacing the straight lines you’d draw with pieces of string attached to tacks! If you’re not sure how the string part would work, check out this site for basic string art instructions.

Bon appetit!

# Sequence Day, Penguins, and a little folding

Welcome to this week’s Math Munch! And… happy Sequence Day!

If you didn’t know that today was Sequence Day, don’t feel bad– I didn’t know until I ran across this article written by Aziz Inan, an electrical engineering professor at the University of Portland. Why is today Sequence Day? Well, because all of the digits in the sequence 0, 1, 2, 3, 4, 5 appear in today’s date– 3-4-2015!

This particular Sequence Day isn’t super special. There will be another one with the exact same sequence on April 3 (4-3-2015). But, according to Aziz, April 3 will be the last Sequence Day of this year– and the last until 2031! Aziz made this chart of all the Sequence Days that will happen this century. There are 48 all together– and if you look carefully at the chart, you may notice some interesting patterns.

See how the Sequence Days mostly occur at the beginnings of decades, in the first half of the month, and never later in the year than June? Why do you think that might be? Also, the last Sequence Day of the 21st century is in 2065. That means we’ll have to wait almost 40 years for the next Sequence Day after that– until 2103! But, in the scheme of things, this actually isn’t so bad– there were no Sequence Days at all in the last century. (Why might that be?)

We all know about days like Pi Day (coming up soon on 3-14-15 — and it’s a special one because we’ve got those two additional digits this year!), but, as Aziz likes to show, lots of days can be mathematical holidays– if you just look carefully enough. Maybe you’ll find a mathematical holiday of your own! If you do, let us know. We love any excuse to have a party!

Next up, you think penguins are cute, right? Well, take a look at this:

You may have heard the narrator say, “Something more organized is going on.” Well, several mathematicians wondered what that more organized thing was… and it turns out to be very mathematical!

How many penguins do you see?

Francois Blanchette, a mathematician at the University of California, Merced, had the idea to use math to study how penguins keep warm while watching penguin movies like this one. He studies the math of something called fluid dynamics, which, basically, is how things like water and air flow. Francois and several other mathematicians at the University of Erlangen-Nuremberg in Germany noticed that when one penguin in a huddle moves just a little bit, it triggers a chain reaction in which all of the other penguins move in an organized way to keep warm. Their tiny movements cause the huddle to organize into the best shape for all penguins to keep warm during the cold of winter.

Huddle up, little guy!

Scientists and mathematicians are only now realizing all of the amazing ways that math comes into play in the lives of animals, especially in large groups. It seems that penguins are only the beginning! To learn more about the organization of large groups of animals, I suggest you check out this awesome PBS documentary about animal swarms.

Finally, we haven’t heard from Vi Hart in a while. If you’ve been feeling the need for some math art fun in your life, check out this video I dug up from the archives. Origami meets Pythagorean Theorem– what could be better?

Stay warm, and bon appetit!