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I've just been working with my 12-year-old daughter on Cantor's diagonal argument, and countable and uncountable sets.

Why? Because the maths department at her school is outrageously good, and set her the task of researching a mathematician, and understanding some of the maths they did - the real thing.

So what else could we have done - thinking that we know our multiplication tables and fractions, but aren't yet completely confident with equations which have letters for unknown numbers?

I did think of proving that there are infinitely many primes - we can follow an argument - other suggestions welcome.

And incidentally, tell your local high school to do this ...

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How about how Archimedes was too smart to invent Calculus. I believe, at 12, you'r daughter know volumes and their formulae right? –  edit Jun 1 '13 at 18:33
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I think Euler and the 7 bridges of Königsberg problem could be a good choice. –  user1242967 Jun 1 '13 at 18:47
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The Euler bridges problem I will be using with 10/11-year-olds next month as a physical exercise, laid out on the playing field - thanks for reminding me. –  Mark Bennet Jun 1 '13 at 18:51
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@ShuhaoCao You've just reminded me how good some of the problems are in the Professor Layton series for Nintendo DS. –  Mark Bennet Jun 1 '13 at 18:56
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@MarkBennet Hmmm, interesting. I have never played Prof Layton, but based on some google results I see some interesting puzzles. I would say if she is interested in this, some games about combinatorial games may suit her, like the game of Go (usgo.org/what-go). :) –  Shuhao Cao Jun 1 '13 at 19:54
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20 Answers 20

Six people at a dinner party is sufficient to ensure that there are either three mutual strangers or three mutual acquaintances. In fact, six is the smallest number that ensures this phenomenon. This is the diagonal Ramsey number $R(3,3)$, and the proof can be demonstrated with a couple pictures and just a dash of the pigeonhole principle. There are lots of directions she could go after understanding $R(3,3)$ (though most of it is not due to Ramsey).

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I would suggest Euler and his characteristic - for example, use it to show that there are only five regular polyhedra. One advantage of this subject is that one has to only work with pictures and integer numbers.

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That's a great one too. There are all sorts of practical applications too - soccer balls make an interesting study. And why the Eden Project (which has domes in it) has pentagons in its domes as well as hexagons ... –  Mark Bennet Jun 1 '13 at 18:59
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One can even prove the five-colour theorem if the concentration span of the young person is sufficient ... –  Mark Bennet Jun 1 '13 at 19:00
    
I just discovered that a very nice application of Euler's characteristic is the "3 houses" problem, where you have to connect 3 houses to 3 factories without crossing lines, which is impossible (in other words, show that the graph $K_{3,3}$ is not planar). –  Denis Jun 1 '13 at 21:49
    
One can also use the Euler characteristic to study the numbers of hexagons and pentagons on a soccer ball! –  Adam Saltz Jun 3 '13 at 2:40
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If we are talking about cardinals, there are two similar, but important things one can try and understand (which I believe are within the grasp of a 12 year old):

  1. There is a bijection between the integers and the rationals.
  2. There is no bijection between the rationals and the real numbers.

Equally great, Cantor's theorem.


Outside set theory, the existence of irrational numbers, including the irrationality of $\sqrt2$.

Then one can give the most exquisite proof that there are two irrational numbers that one is raised to the power of the second give us a rational number:

$$\sqrt2^\sqrt2\quad\text{ or }\quad\left(\sqrt2^\sqrt2\right)^\sqrt2.$$

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We didn't do bijections, but the conversation will be had ... –  Mark Bennet Jun 1 '13 at 19:24
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Call it 'full pairing' or 'one-to-one correspondence'. –  Berci Jun 1 '13 at 19:25
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I don't think "one-to-one" is a good choice of words to describe a bijection (more just an injection), but "full pairing" does. Just my two pennies worth. –  Andrew D Jun 1 '13 at 19:34
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@Andrew: I don’t agree. The term one-to-one is symmetric: when we say in everyday speech that we can match these objects with those one to one or (more idiomatically) one for one, we’re talking about a bijection, not an injection. Moreover, it’s a well-established term that one must eventually learn anyway. –  Brian M. Scott Jun 2 '13 at 6:20
    
@BrianM.Scott That makes a bit more sense now you explain it - I can't say I'd ever probably use it still, especially seeing as a "one-to-one function" is an injection while a "one-to-one correspondence" is a bijection; they seem like statements which take too long to write out for little gain. –  Andrew D Jun 2 '13 at 8:34
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One topic that might be accessible and interesting to explore are straightedge and compass constructions. Actually 'proving' certain things -- like the impossibility of trisecting an angle -- would probably be a little to advanced there are certainly many interesting things that can be done like bisecting an angle, constructing the square root of an integer, or the existence of irrational numbers. Additionally, this is something that would be quite hands on. After laying out the basic rules and a few basic techniques you could just let your daughter explore and mess around and see what she can come up with.

While this is not necessarily due to one mathematician you could tie this to how mathematicians, especially the ancient Greeks actually viewed numbers.

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Great suggestion! –  Mark Bennet Jun 1 '13 at 19:31
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Even though it implies a paradox, I like the Hilbert hotel. It can be explained to everyone, no matter the age. It deals with the concept of infinity, cardinality can be explained in an easy way, if all rooms are occupied and all costumers are in a room, then the "cardinality" is equal. And so on.

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We've discussed the Hilbert Hotel on car journeys - I have an older daughter too, and that hit her inquisitive years. Thanks for the suggestion - I'd forgotten. –  Mark Bennet Jun 1 '13 at 19:57
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When trying to teach someone about "equations which have letters for unknown numbers" I like to ditch the symbols and instead focus on the concepts they represent. Like this: http://worrydream.com/LadderOfAbstraction/

The have a look at some of the other the amazing work being done at: http://worrydream.com/KillMath/

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A number of combinatorial games may be quite suitable.

  1. Move mountains. Start with any permutation of $[n]$. A legal move consists in moving any number that is a local maximum to the front of the permutation. E.g., the legal moves from $214365$ are to $421365$ and to $621435$. The first player to have no legal move loses. (It’s not too hard to see that this happens precisely when the position $n(n-1)\dots21$ is reached.) This is a good game for getting an understanding of $\mathscr{N}$ and $\mathscr{P}$ positions: the key insight is that if you can move $n$ to the front, either you win outright, or your opponent must make a move that lets you move $n$ to the front again on your next move.

  2. One-heap takeaway. Start with a heap of $n$ counters and a finite set $T$ of positive integers. At each move the player whose turn it is must take $t$ counters from the heap for some $t\in T$. The first player who cannot move loses. If $T=\{1,\dots,m\}$ for some $m$, the game is quite straightforward to analyze fully, and for any $T$ it leads naturally to the equivalence classes of modular arithmetic.

  3. Nim. Start with two-heap Nim; she can probably discover the winning strategy on her own with little or no help. She might well be able to follow the argument for the general strategy for any number of heaps, which is a nice application of binary representation.

A couple of questions to think about:

  1. If you have a two-pan balance and are required to put the object to be weighed in one pan and the balancing weights in the other, what is the most efficient set of standard weights that will allow you to weigh any whole number of ounces from $1$ through some $n$? (Most efficient requires some explanation and probably some examples, and it’s best to use $n$ of the form $2^k-1$.) This leads naturally to binary notation.

  2. If you have a two-pan balance and may put the standard weights on either pan, what is the most efficient set of standard weights that will let you weigh any whole number of ounces from $1$ through some $n$? (Here the most informative values of $n$ are of the form $\frac12(3^k-1)$. This leads naturally to balanced ternary notation, which is kind of fun to play with. E.g., subtraction is just inversion followed by addition, so it doesn’t require learning a separate subtraction table.

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Quick Sort can be a funny exercise if you like Information Technology. Show her how to order ten cards using a recursive quick sort strategy. It is quite simple to show it is the fastest algorithm, just because its operations are a comparasion with some arithmetic inside.

It is a pragmatic stuff because you play cards often, and it is a golder ring connecting deep math to Information Technology in a very elegant way, indeed.

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Unless you're using a very different definition of 'quicksort' than I am, I'm not sure how to easily show that it's the fastest algorithm; even the notion of $O(n\log n)$ is probably beyond the average 12-year-old. Radix sorts/merge sorts are, at least for me, much more practical to do by hand, and the natural sort for cards is arguably a form of radix sort... –  Steven Stadnicki Jun 3 '13 at 0:59
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I immediately thought of the formula for the sum of arithmetic progression. It is easy to understand, really practical and I think I've read somewhere it was proven by Gauss when he was in the 3rd grade.

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I wish that when I was 12 years old or younger, that someone had told me about the MAJOR THEMES of mathematics such as EQUIVALENCE (1+1 is quantitatively equivalent to 2; it is not the SAME as 2), the distinction between OBJECTS and their PROPERTIES (e.g., numbers vs. primeness), ORIGINS OF THE OBJECTS (such as the invention of counting), SHORTHAND (notation, i.e., use of symbols for numbers that already have names and for equivalence and for variables and for functions etc.), TERMINOLOGY (i.e., choices of names for the objects and their properties), REPRESENTATION (such as how to represent fractions), QUESTIONS (such as HOW we COULD/MIGHT represent fractions; and how we might reduce the proliferation of names and symbols for quantities), INGENUITY (such as a base system for naming and symbolizing larger quantities; and using division to represent fractions), and so on. It took me till old age to realize that mathematics is perfectly INTENTIONAL and SENSIBLE and is NOT an exercise in remembering and cleverness.

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If she's into chess, explore various upper bounds for the number of games that can be played, constrained by a rule that if the board reaches the same configuration three times, that the game ends in a draw. (In real chess, if this happens a player has to choose to end it in a draw, but forget about that.)

An extremely naive upper bound is something like $2^{2\cdot65^{32}}(2\cdot65^{32})!$, taking the power set of possible board configurations and assigning an order. But then start considering things like disallowing two pieces on the same square, and the actual constraints on where pieces can be given the game rules, and in what order moves may be made...

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See this link for a discussion chess.com/blog/kurtgodden/the-longest-possible-chess-game. This takes the 50-move-rule into account. There is some counting and use of parity involved. It would be an interesting task to set a bright group. –  Mark Bennet Jun 2 '13 at 19:33
    
Before things get better (via the constraints you mention), they get worse because you need to take into account the possibility of promoted pieces. Also, the configuration of pieces may not be enough to determine whether castling is legal (did the king or rook move previously and then return to its starting position?) and whether an en passant capture is legal (did the pawn to be captured move two squares on the immediately previous move?); so, in addition to the locations of pieces, you sometimes need a little bit of the game's history. –  Andreas Blass Jun 2 '13 at 22:02
    
@Andreas All these things make it an interesting problem to work on because there is no nice and tidy solution. There is always going to be some complication that you can factor in that will allow you to reduce the bound, if you work hard enough. You have alerted me to a miswording in my answer though. –  alex.jordan Jun 2 '13 at 23:07
    
@Andreas Regarding promoted pawns, they are still the same pieces they always were, just with a new face. So possibly such a pawn can be in some specific spot four times, rather than two. And we can replace the $2\cdot65^{32}$ with $4\cdot65^{32}$, if I am counting right. –  alex.jordan Jun 2 '13 at 23:13
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Have you heard of Erik Demaine? He's a mathematician at MIT who researches mathematical origami. Some of the stuff he does might be more interesting to a 12 year old.

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I'm going to add the Russell Paradox to the list - simply because the consequences of considering it lead to some interesting places.

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The story of the grains of wheat on a chessboard (sometimes told as rice) offers several possible candidates for a mathematician, or at least a mathematically aware poet!

This story leads several interesting places, including the Law Of Big Numbers, which is more about economics than mathematics.

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A group of $500$ students eagerly awaited their professor inside a university lecture hall. As the professor took the stage he posed a question to the students.

How many students here today share a birthday with another student in this lecture hall?

A) Less than 10%

B) Less than 25%

C) More than 25%

The professor counted a show of hands. A clear majority of students chose B. “No!”, proclaimed the professor; “C is the correct answer”. The professor then stated he could prove it without knowing a single student’s birthday. Perplexed, the students looked at each other and wondered if a few screws needed tightening.

Simply apply the Pigeonhole Principle, the professor explained. In the extreme, there will be $366$ students, each with a different birthday. The remaining $134$ students will share a birthday with another student. Therefore at least $135=134+1$ students share a birthday with another student. The students erupted in applause!

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Dear professor: what if 364 of the students each have distinct birthdays and the other 136 all share the 'last' day of the year as their birthday? –  Steven Stadnicki Jun 3 '13 at 1:01
    
@StevenStadnicki: Then each of those 136 students is counted 135 times - well over 50%. I think maybe the question might be rephrased as "How many birthdays are shared by two students in this lecture hall?" That would cover cases like this. –  Darrel Hoffman Jun 3 '13 at 3:40
    
@StevenStadnicki. Very good point, I'll revise the answer to more than 25%. All the screws seem tight now :) –  cyclochaotic Jun 3 '13 at 4:04
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Show her Colin Adam's knot book! Seriously interesting and accessible in the first part.

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I'm adding another - the 17 Wallpaper Groups, which the link here says

"A proof that there were only 17 possible patterns was first carried out by Evgraf Fedorov in 1891 and then derived independently by George Pólya in 1924."

Because there is a really nice exposition in "The Symmetries of Things" by John H. Conway, Heidi Burgiel & Chaim Goodman-Strauss. The book has some great illustrations.

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I think the book is one of the best mathematical books of the last ten years - I've give four copies away. –  Mark Bennet Dec 14 '13 at 21:27
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Another fun one is the tessellation of the sphere by regular polygons. You characterize all tessellations of the plane first by way of interior angles, and then show that the same can be done on the sphere. This is a great way of showing that Euclidean geometry isn't the only way to do geometry!

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  1. Anything from N.Vilenkin's "In Search of Infinity" will do: http://www.amazon.com/In-Search-Infinity-N-Ya-Vilenkin/dp/0817638199/ref=sr_1_2?ie=UTF8&qid=1393976309&sr=8-2&keywords=Vilenkin (problems related to infinity, like a space hotel with infinite number of rooms).
  2. "Word substitution" problem: there are 3 words A,B,C, we can substitute any subword B in the word A with the word C. When this process (B->C) can be repeated infinitely for given A,B,C? I'm not sure there is a known solution in general, but even finding any A,B,C which provide an infinite example is interesting.
  3. Graph theory problems (Königsberg Bridge Problem,...).
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The Monty Hall Problem has stumped smart people: it's fun and you can run experiments.

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