# MATH with my KIDS

## An alien is thinking of a number…

…and you have to guess what it is.

You were captured by a malevolent alien, who would really like to eat you for dinner. He told you his name, but since it is unpronounceable to humans, you like to call him Evil Eddie. Evil Eddie has a morbid sense of fairness and so, before roasting you with an apple stuck in your mouth and serving you with a side of glowing, purple, French-cut string beans, has locked you in a chamber carved out of the bedrock of a strange world. Before the polycarbide door slides shut with a hiss, Eddie tells you that you he is thinking of a number. If you can guess it, he will reluctantly release you. If not, well…let’s just hope that you can guess the number.

As you look around the cavern you see, in the dim and flickering light (does it come from torches? You don’t see any.), an enormous stone bowl, filled to the brim with glass marbles. How many are there? Thousands at least. Tens of thousands, perhaps. On the other side of the chamber is an identical bowl, but this one is empty. In the middle of the room, hanging from the ceiling is a rope. You hear Eddie’s voice thundering into the room “when you think that you know the number, put that many marbles in the empty bowl. Then pull the rope.”

Fortunately you have a friend. In an adjacent chamber is a benevolent alien. You don’t know his name, so you decide to call him Manvel. By some contrived plot device that the author has not taken the time to think of, you know that Manvel:

1) can read Evil Eddie’s mind (hence, knows the number),

2) is going to try to transmit this number to you, and

3) is going to do so by tapping out the digits (through the wall) in some base (could be base 10 (that’s what we normally use), could be base 2 (called binary. We think of computers as using base 2), could be any whole number base.

You listen and hear …tap… long pause …tap tap tap tap tap… long pause …tap tap tap… then silence.

You have the digits! They are 153. As stated above, you now know the number is:

$(1\times b^2)+(5\times b)+3$

You just don’t know what $b$ is. (I’ve glossed over the issue of big endian versus little endian order, but we are assuming, by aforementioned plot device, that big endian order has been used by Manvel).

So…how many marbles do you put in the bowl?

to be continued…

## Sequences and Creative Math for Kindergartners

I visited my kids’ classes for career day in May. I have a son in Kindergarten and a daughter in pre-K. I also visited two other kindergarten classes. I told the kids that I am a research mathematician: mathematician means I do math. Research means I do math that nobody has ever seen before, math that I make up.

I mentioned that math is lots of things: It’s thinking about shapes and numbers and patterns, and other stuff. This day we were going to do numbers and patterns.

I showed them a board covered with three lines of (blank) sticky notes. Under each note was a number.The point is to try to guess which number is coming next.

We started with the blue line of numbers.

Once I had uncovered the 1 and 2 almost everyone expected the next number to be 3.

When I showed them that it was a 1, they caught on pretty quickly.

They had fun shouting out the 1-2-1-2-1-2 pattern as I frantically tried to keep up with them on the board.

Next, the pink ones.

I started by showing the 1-2-1 with a warning that I was being tricky. A 2 should come next, right? Wrong! But I didn’t tell them what it was yet. I put the sticky notes back on and explained that I was going to pull them off in a different order to see if it gave them a hint. (At this, some kids suggested I pull them off starting at the right instead of the left.)

I pulled off every other sticky: 1-skip a sticky-1-skip a sticky-1-skip… Okay, done with the ones. Next we have:

2-skip a sticky-2-skip a sticky-2-skip… (pulling off every other sticky that is left after the first pass). Okay, done with the twos. So the next one should be (the one that everyone thought should be a 2):

It’s a 3! And most of the students guessed that it was a 3!

And so it goes: 3-skip a sticky-3-skip a sticky-3 skip…

Next we have the fours: 4-skip a sticky (at this point we hit the end of the board, but if the board were longer…we would continue 4-skip a sticky-4-skip a sticky-4-skip…

Only one pink number left…

Everyone knows it’s a 5 by now! If we had a long enough board we would have 5-skip a sticky-5-skip a sticky-5-skip…

The pink numbers are:

1-2-1-3-1-2-1-4-1-2-1-3-1-2-1-5-1

Some interesting things (I think) about these pink numbers: Once you’ve done the ones and twos the row looks exactly like it would if you had pulled off the same patterns on the blue row! Nevertheless, most kids pick up on the fact that 3 should come next (well…I did give a big clue “done with the ones! . . . Done with the twos!”)

These pink numbers are called the ruler sequence, which comes up in the towers of Hanoi problem.

Now for the greens (the stickies look yellow in these pictures, but they are really green (yellowish-green)).

This row is really sneaky (but usually the kids understand it before their teachers! A testament to the freedom that young minds who haven’t had mathphobia drilled into them are capable of!), so I give a big hint. What do we have so far (on the green row)? One. How many ones? One one.

So that is what comes next. You start with a 1 then you say “look, it’s 1 one,” and that’s what you put next 1-1.

Now what do you have? 1-1-1. You have 3 ones. So next comes:

3-1.

Now you have 1-1-1-3-1. Four ones and one three. So next is:

4-1-1-3.

Now you have:

1-1-1-3-1-4-1-1-3. That is, six ones, two threes, and one four. So next will be:

6-1-2-3-1-4.

And now you have:

1-1-1-3-1-4-1-1-3-6-1-2-3-1-4. That is eight ones, one two, three threes, two fours and one six.

There is only room on the board to put 8-1. But if we had more room we would keep going. If we had paper going all the way around the school… I asked the kids if they thought we would ever get a 5. We haven’t so far! (At this someone always pointed out that we got a 5 on the pink line. A good observation! But we are just looking at the green line now.) What about 10? If we kept going would we get a 10? Would we ever get a 20? Or 100? Would we ever get a thousand or a million or a billion?

I’m pretty sure that you do get a 5 and a 10. I don’t know about 20, 100, 1,000, 1,000,000 or 1,000,000,000. I’m guessing that you do get them… Why? well, my reasoning is this: in some sense you have infinitely many chances to hit those numbers. Maybe at one point you will have 20 ones. If not, maybe you will get 20 twos. Of course maybe you will skip right over 20 twos. So maybe you will get 20 threes or 20 fours or 20 fives or…, you get the idea.

I told the kids that I didn’t know if you ever get 20 or 100 or 1,000. All I know is that you keep getting bigger and bigger numbers (can you see why?). I pointed out to the kids that this is a math question that I don’t know the answer to, and (as far as I know) neither does anybody else. This is math research, and it’s math research that they can do! They can figure out if we ever get a 5 or a 10. A million…probably they won’t figure that out until they learn some computer programming…

The kids were perfectly willing to accept that this is a math question that has an answer, but nobody knows that answer yet.

As for me, I’m conjecturing (conjecture=educated guess) that if you continue with the blue numbers, you will eventually hit any positive, whole number that you might care about. I already told you my reasons for making this conjecture. Can you prove it? Can you prove me wrong? If you’re a teacher, give the problem to your students. Maybe they’ll have an idea about it. Then let me know.

Finally, I wanted to let the kids make up their own number sequences. I handed out sheets of paper with five slots for numbers and five sticky notes. I told them that they could write any numbers they want in the blanks. They could be tricky, or they could be easy (like 1-1-1-1-1). Then, cover them up, and see if someone can guess the numbers.

Do they have to be able to articulate some reasoning for the patterns behind their numbers? No. But if they can, that’s great. There doesn’t have to be any reasoning at all. The point of this exercise was for the kids to do some free-form creative math. Just release your inhibitions (actually the kindergartners don’t have many inhibitions. The grown-ups on the other hand…) and write down some silly or crazy or boring or weird or hard-to-guess or happy, or sad pattern. The kids loved this part. They especially loved seeing if I could guess their numbers.

Let me know what you think about these sequences and this activity!

## Synchronicity

I first heard the word synchronicity when I was sitting in an airport, waiting for a plane, working on this math problem (for fun) and over-hearing a man’s cell phone conversation. I thought that it was a good word for this phenomenon. Here it is:

Have you noticed that 3 times 7 is 21 and if you expand 7 base 3 it is 21. That is:

$21=3\times7$

$21_3=7$

This seems like an interesting coincidence. Could it be that this happens for any other numbers? Maybe it happens infinitely often, maybe never. Asking such questions is where the adventure begins. Of course we are implicitly working base 10 here. That is, in the first line the string of numerals “21″ is interpreted as a base 10 number. In the second one it is (explicitly) interpreted base 3. You could allow 10 to be replace by other bases also.

If we restrict our attention to base 10 (in the first line), I claim that, by exhaustive computer search, there are only five other instances of this phenomenon. They are:

$1,\!066,\!338,\!805,\!156,\!287,\!287,\!067$ $=9\times118,\!482,\!089,\!461,\!809,\!698,\!563$ and $1,\!066,\!338,\!805,\!156,\!287,\!287,\!067_9$ $=118,\!482,\!089,\!461,\!809,\!698,\!563$

$1,\!124,\!161,\!329,\!714,\!632,\!881,\!704$ $=9\times124,\!906,\!814,\!412,\!736,\!986,\!856$ and $1,\!124,\!161,\!329,\!714,\!632,\!881,\!704_9$ $=124,\!906,\!814,\!412,\!736,\!986,\!856$

$2,\!305,\!867,\!155,\!177,\!711,\!644,\!802$ $=9\times256,\!207,\!461,\!686,\!412,\!404,\!978$ and $2,\!305,\!867,\!155,\!177,\!711,\!644,\!802_9$ $=256,\!207,\!461,\!686,\!412,\!404,\!978$

$2,\!306,\!166,\!776,\!784,\!312,\!535,\!170$ $=9\times256,\!240,\!752,\!976,\!034,\!726,\!130$ and $2,\!306,\!166,\!776,\!784,\!312,\!535,\!170_9$ $=256,\!240,\!752,\!976,\!034,\!726,\!130$

$5,\!744,\!341,\!611,\!556,\!736,\!174,\!883$ $=9\times638,\!260,\!179,\!061,\!859,\!574,\!987$ and $5,\!744,\!341,\!611,\!556,\!736,\!174,\!883_9$ $=638,\!260,\!179,\!061,\!859,\!574,\!987$