Tova Brown explores the second part of Hilbert’s Hotel Paradox. What’s infinity plus infinity?
Running an infinite hotel has its perks. Even when the rooms are full you can always find space for new guests, so you proudly welcome everyone who appears at your door.
When two guests arrive at once, you make room. When ten guests arrive, you accommodate them easily. When a crowd of hundreds appears, you welcome them all.
Is there no limit to your hospitality?
Tova Brown’s introduction to Hilbert’s Hotel Paradox, a riddle about the nature of infinity…
Once upon a time, there was a hotel with an infinite number of rooms. You might be thinking this is impossible, and if so you’re right. A hotel like this could never exist in the real world.
But fortunately we’re not talking about the real world, we’re talking about math. And when we do math we can make up whatever rules we want, just to see what happens.
[Feature photo above from the public domain, and title background (below) by frankieleon (CC BY 2.0) via Flickr.]
Have you made a New Year’s resolution to spend more time with your family this year, and to get more exercise? Problem-solvers of all ages can pump up their (mental) muscles with the Annual Mathematics Year Game Extravaganza. Please join us!
For many years mathematicians, scientists, engineers and others interested in math have played “year games” via e-mail. We don’t always know whether it’s possible to write all the numbers from 1 to 100 using only the digits in the current year, but it’s fun to see how many you can find.
Use the digits in the year 2016 to write mathematical expressions for the counting numbers 1 through 100. The goal is adjustable: Young children can start with looking for 1-10, middle grades with 1-25.
You know you’re a math teacher when you see a penny in the parking lot, and your first thought is, “Cool! A free math manipulative.”
My homeschool co-op math students love doing math with pennies. They’re rather heavy to carry to class, but worth it for the student buy-in.
This month, I’m finishing up the nearly 150 new illustrations for the upcoming paperback edition of my Let’s Play Math book. I’m no artist, and it’s been a long slog. But a couple of the graphics involved pennies—so when I saw that penny on the ground, it made me think of my book.
And thinking of my book made me think it would be fun to share a sneak peek at coming attractions…
Real mathematics is intriguing and full of wonder, an exploration of patterns and mysterious connections. It rewards us with the joy of the “Aha!” feeling. Workbook math, on the other hand, is several pages of long division by hand followed by a rousing chorus of the fraction song: “Ours is not to reason why, just invert and multiply.”
Real math is the surprising fact that the odd numbers add up to perfect squares (1, 1 + 3, 1 + 3 + 5, etc.) and the satisfaction of seeing why it must be so.
Did your algebra teacher ever explain to you that a square number is literally a number that can be arranged to make a square? Try it for yourself:
Each new set of pennies must add an extra row and column to the current square, plus a corner penny where the new row and column meet. The row and column match exactly, making an even number, and then the extra penny at the corner makes it odd.
Can you see that the “next odd number” pattern will continue as long as there are pennies to add, and that it could keep going forever in your imagination?
The point of the penny square is not to memorize the square numbers or to get any particular “right answer,” but to see numbers in a new way—to understand that numbers are related to each other and that we can show such relationships with diagrams or physical models. The more relationships like this our children explore, the more they see numbers as familiar friends.
A large jar of assorted coins makes a wonderful math toy. Children love to play with, count, and sort coins.
Add a dollar bill to the jar, so you can play the Dollar Game: Take turns throwing a pair of dice, gathering that many pennies and trading up to bigger coins. Five pennies trade for a nickel, two nickels for a dime, etc. Whoever is the first to claim the dollar wins the game.
Or take the Penny Birthday Challenge to learn about exponential growth: Print out a calendar for your child’s birthday month. Put one penny on the first day of the month, two pennies on the second day, four pennies on the third day, etc. If you continued doubling the pennies each day until you reach your child’s birthday, how much money would you need?
Warning: Beware the Penny Birthday Challenge! Those pennies will add up to dollars much faster than most people expect. Do not promise to give the money to your child unless the birthday comes near the beginning of the month.
The first time I did pennies on a calendar with my homeschool co-op class was during December, so we called it the Penny Christmas Challenge:
I told the kids that if their grandparents asked what gift they wanted for Christmas, they could say, “Not much. Just a few pennies…”
The Penny Square, Dollar Game, and Penny Birthday Challenge are just three of the myriad math tips and activity ideas in the paperback edition of Let’s Play Math: How Families Can Learn Math Together and Enjoy It. Coming in early 2016 to your favorite online bookstore…
Here’s a new video from Jo Boaler at YouCubed.org.
There is a huge elephant standing in most math classrooms, it is the idea that only some students can do well in math. Students believe it, parents believe and teachers believe it. The myth that math is a gift that some students have and some do not, is one of the most damaging ideas that pervades education in the US and that stands in the way of students’ math achievement.
—Jo Boaler
Unlocking Children’s Math Potential
The YouCubed site is full of encouragement and help for families learning math.
— and plenty more!
High school math teacher Chris Rime has done it again. Check out his November 2015 printable math calendars for Algebra 1 (in English or Spanish), Algebra 2, and Geometry students. Enjoy!
At home:
Post the calendar on your refrigerator. Use each math puzzle as a daily review “mini-quiz” for your children (or yourself).
In the classroom:
Post today’s calculation on the board as a warm-up puzzle. Encourage your students to make up “Today is…” puzzles of their own.
As a puzzle:
Cut the calendar squares apart and trim off the dates. Then challenge your students to arrange them in ascending (or descending) order.
Make up problems to fill a new calendar for next month.
And if you do, please share!
Gordon Hamilton of Math Pickle posted Rock – Low unique number game for grades K–2. If you have a set of active kids and a few minutes to spare, give it a try!
“Each game takes about 45 seconds,” Hamilton says. “This is part of the key to its success. Children who have not learned the art of losing are quickly thrown into another game before they have a chance to get sad.”
The experience of mathematics should be profound and beautiful. Too much of the regular K-12 mathematics experience is trite and true. Children deserve tough, beautiful puzzles.
What are the best numbers to pick? Patrick Vennebush hosted on online version of the game at his Math Jokes 4 Mathy Folks blog a few years back, though we didn’t have to bend over into rocks—which is a good thing for some of us older folks.
Vennebush also posted a finger-game version suitable for small groups of all ages, called Low-Sham-Bo:
You may want to count “Ready, set, go!” for throwing out fingers, so the numbers in the count don’t influence the play.
The official name for this sort of game is Lowest Unique Bid Auction.
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This blog is reader-supported.
If you’d like to help fund the blog on an on-going basis, then please join me on Patreon for mathy inspiration, tips, and an ever-growing archive of printable activities.
If you liked this post, and want to show your one-time appreciation, the place to do that is PayPal: paypal.me/DeniseGaskinsMath. If you go that route, please include your email address in the notes section, so I can say thank you.
Which I am going to say right now. Thank you!
“Active Math Game: Rock” copyright © 2015 by Denise Gaskins. .
Interrupt your regular math programming to try this fantastic math doodling investigation!
Anna Weltman wrote a math/art book. It’s great fun for all ages, full of fantastic mathematical explorations — including spirolateral math doodles.
To make a spirolateral, you first pick a short series of numbers (1, 2, 3 is a traditional first set) and an angle (90° for beginners). On graph paper, draw a straight line the length of your first number. Turn through your chosen angle, and draw the next line. Repeat turning and drawing lines, and when you get to the end of your number series, start again at the first number.
Some spirolaterals come back around to the beginning, making a closed loop. Others never close, spiraling out into infinity—or at least, to the edge of your graph paper.
Martin Gardner, “Worm Paths” in Knotted Doughnuts and Other Mathematical Entertainments.Articles by Robert J. Krawczyk:
Anna Weltman appeared on Let’s Play Math blog once before, with the game Snugglenumber. And she’s a regular contributor to the wonderful Math Munch blog.
High school math teacher Chris Rime posted three wonderful review calendars for middle and high school students on his blog.
The links at Chris’s blog will let you download editable Word docx files. If you’re cautious about internet links and prefer PDF, here you go:
Chris writes:
There are no explicit instructions about process being more important than the answer on these, so you’ll need to stress that in class.
I remind students that everyone already knows the answer to each of the questions, and that one of the things we’re practicing is explaining our reasoning…
Enjoy!
And if anyone else has a math review calendar to share, for any grade level, please add your link in the comment section below.
Counting all the fractional variations, my massive blog post 30+ Things to Do with a Hundred Chart now offers nearly forty ideas for playing around with numbers from preschool to prealgebra.
Here is the newest entry, a variation on #10, the “Race to 100” game:
(11.5) Play “Odd-Even-Prime Race.″ Roll two dice. If your token is starting on an odd number, move that many spaces forward. From an even number (except 2), move backward — but never lower than the first square. If you are starting on a prime number (including 2), you may choose to either add or multiply the dice and move that many spaces forward. The first person to reach or pass 100 wins the game.
[Hat tip: Ali Adams in a comment on another post.]
And here’s a question for your students:
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This blog is reader-supported.
If you’d like to help fund the blog on an on-going basis, then please join me on Patreon for mathy inspiration, tips, and an ever-growing archive of printable activities.
If you liked this post, and want to show your one-time appreciation, the place to do that is PayPal: paypal.me/DeniseGaskinsMath. If you go that route, please include your email address in the notes section, so I can say thank you.
Which I am going to say right now. Thank you!
“New Hundred Chart Game: Odd-Even-Prime Race” copyright © 2015 by Denise Gaskins. Image at the top of the post copyright © geishaboy500 (CC BY 2.0).
Denise Gaskins' Let's Play Math 
