In the land of Fantasia, where people communicate by crystal ball, Wizard Mathys has been placed in charge of keeping the crystal connections clean and clear. He decides to figure out how many different ways people might talk to each other, assuming there’s no such thing as a crystal conference call.

Mathys sketches a diagram of four Fantasian friends and their crystal balls. At the top, you can see all the possible connections, but no one is talking to anyone else because it’s naptime. Fantasians take their siesta very seriously. That’s one possible state of the 4-crystal system.

On the second line of the diagram, Joe (in the middle) wakes up from siesta and calls each of his friends in turn. Then the friends take turns calling each other, bringing the total number of possible connection-states up to seven.

Finally, Wizard Mathys imagines what would happen if one friend calls Joe at the same time as the other two are talking to each other. That’s the last line of the diagram: three more possible states. Therefore, the total number of conceivable communication configurations for a 4-crystal system is 10.

For some reason Mathys can’t figure out, mathematicians call the numbers that describe the connection pattern states in his crystal ball communication system Telephone numbers.

Can you help Wizard Mathys figure out the Telephone numbers for different numbers of people?
T(0) = ?
T(1) = ?
T(2) = ?
T(3) = ?
T(4) = 10 connection patterns (as above)
T(5) = ?
T(6) = ?
and so on.

Hint: Don’t forget to count the state of the system when no one is on the phone crystal ball.

Feature photo at top of post by Christian Schnettelker (web designer) and wizard photo by Sean McGrath via Flickr. (CC BY 2.0) This puzzle was originally featured in the Math Teachers At Play (MTaP) math education blog carnival: MTaP #76.

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Math Concepts: sorting by attribute (card suits), counting up, counting down, standard rank of playing cards (aces low). Players: two or more, best with four to six. Equipment: one complete deck of cards (including face cards), or a double deck for more than six players. Provide a card holder for young children.

How to Play

Deal out all the cards, even if some players get more than others. The player to the dealer’s left begins by playing a seven of any suit. If that player does not have a seven, then the play passes left to the first player who does.

After that, on your turn you may lay down another seven or play on the cards that are already down. If you cannot play, say, “Pass.”

Once a seven is played in any suit, the six and the eight of that suit may be played on either side of it, forming the fan. Then the five through ace can go on the six in counting-down order, and the nine through king can go on the eight, counting up. You can arrange these cards to overlap each other so the cards below are visible, or you can square up the stacks so only the top card is seen.

Players do not need to wait for both the six and eight of a suit to be played before they begin building the fan up or down.
The first player to run out of cards wins the game.

If you want to keep score, count the cards remaining in your hand after one player goes out. After everyone has had a turn as dealer, whoever has the lowest total score is the champion.

Variations

In some traditions, play always begins with the seven of diamonds, so whoever has that card goes first.

Domino Tan

The player to the dealer’s left may lead any card, and then all the suits must start with that number (instead of with seven) and build up and down from there.

Fan Tan Trumps

When the dealer gets to the end of the deck and there aren’t enough cards to give every player one more, the last few cards are turned face up and may be played by anyone as needed. The suit of the last card becomes the trump suit, and cards of that suit may be played on any of the fans, with the card they replace going on the trumps fan. In this case, the cards must be laid out in overlapping rows, not stacked up, so everyone can see where the trumps have gone.

For instance, if spades are trump, then a nine of spades could be played on the eight of hearts, which would leave the nine of hearts without a home—so it has to go on the spades fan.

Exceptions: The seven of the trump suit starts its own fan, like any other seven, and the last card dealt (the one that named the trump suit) must also be played to the trumps fan when its turn comes.

Crazy Tan

Deal only seven cards to each player, and set the rest of the deck out as a draw pile. The first player who cannot play must draw one, which he may play if possible. If not, and the next player also cannot play, she must draw two. If neither of those cards will play, and the next player has nothing to play, he must draw three, and so on, each player drawing one more card than the last person. When one of the players is finally able to lay down a card, this resets the draw count back to zero.

In Crazy Tan, players are allowed to lay down a run (playing several cards in a row of the same suit on a single turn). Or they may play parallel cards (cards of the same rank in different suits, all played in the same turn). Or a player may even lay down parallel runs, if the cards happen to work out that way.

History

Fan Tan may also be called Crazy Sevens. Like any folk game, it is played by a variety of rules around the world. If you search for it on the Internet, you may run into an unrelated Chinese gambling game called Fan Tan, which is similar to Roulette.

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Do you enjoy math? I hope so! If not, browsing the articles linked in this post just may change your mind.

Welcome to the 85th edition of the Math Teachers At Play math education blog carnival—a smorgasbord of links to bloggers all around the internet who have great ideas for learning, teaching, and playing around with math from preschool to pre-college.

By tradition, we start the carnival with a short puzzle or activity. But if you would like to jump straight to our featured blog posts, click here to see the Table of Contents.

Math Concepts: addition to thirty-one, thinking ahead. Players: best for two. Equipment: one deck of math cards.

How to Play

Lay out the ace to six of each suit in a row, face up and not overlapping, one suit above another. You will have one column of four aces, a column of four twos, and so on—six columns in all.

The first player flips a card upside down and says its number value. Players alternate, each time turning down one card, mentally adding its value to the running total, and saying the new sum out loud. The player who exactly reaches thirty-one, or who forces the next player to go over that sum, wins the game.

Variation

For a shorter game, use only the ace to four of each suit. Play to a target sum of twenty-two.

History (and a Puzzle)

Thirty-One comes from British mathematician Henry Dudeney’s classic book, The Canterbury Puzzles. According to Dudeney, “This is a game that used to be (and may be to this day, for aught I know) a favourite means of swindling employed by card-sharpers at racecourses and in railway carriages.”

Dudeney challenges his readers to find a rule by which a player can always win: “Now, the question is, in order to win, should you turn down the first card, or courteously request your opponent to do so? And how should you conduct your play?”

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Six years ago, my homeschool co-op classes had fun creating this April calendar to hand out at our end-of-semester party. Looking at my regular calendar today, I noticed that April this year starts on Wednesday, just like it did back then. I wonder when’s the next time that will happen?

A math calendar is not as easy to read as a traditional calendar — it is more like a puzzle. The expression in each square simplifies to that day’s date, so your family can treat each day like a mini-review quiz: “Do you remember how to calculate this?”

The calendar my students made is appropriate for middle school and beyond, but you can make a math calendar with puzzles for any age or skill level. Better yet, encourage the kids to make puzzles of their own.

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Did you know that playing games is one of the Top 10 Ways To Improve Your Brain Fitness? So slip into your workout clothes and pump up those mental muscles with the Annual Mathematics Year Game Extravaganza!

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 2015 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 must use all four digits. You may not use any other numbers.

Solutions that keep the year digits in 2-0-1-5 order are preferred, but not required.

You may use a decimal point to create numbers such as .2, .02, etc., but you cannot write 0.02 because we only have one zero in this year’s number.

You may create multi-digit numbers such as 10 or 201 or .01, but we prefer solutions that avoid them.

My Special Variations on the Rules

You MAY use the overhead-bar (vinculum), dots, or brackets to mark a repeating decimal. But students and teachers beware: you can’t submit answers with repeating decimals to Math Forum.

You MAY NOT use a double factorial, n!! = the product of all integers from 1 to n that have the same parity (odd or even) as n. Math Forum allows these, but I’ve decided I prefer my arithmetic straight.

12 Days of Christmas is a card game designed by Dr. Gord Hamilton of Math Pickle. It’s designed for 2-8 players, ages 8+, to be played in 20-30 minutes. Simple enough for the whole family to play, yet strategic enough for the game geeks in the family to enjoy along with everyone else. To order, check out the Kickstarter:

But don’t delay! The Kickstarter project ends Christmas Day.

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