Name: Pancake Numbers - Numberphile
Uploaded: 2020-03-27T18:06:09.000Z
Duration: 8 min 55 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Relative clauses (Restrictive vs. nonrestrictive)

So today I want to tell you about the pancake number, which is among the most ridiculously named numbers in maths.

But it is technically to do with pancakes, or at least pancakes are one Nice way to explain what this number is.

We have a pan full of pancakes here on dhe.

The idea with this is there no, all the same size, who have five different sizes of pancake.

And we also have a very inconsiderate chef who makes pancakes all different sizes and then served them up in the wrong order.

If you've got a stack of pancakes of all different sizes, the most satisfying way is to have them in decreasing order of science with the smallest one of the top of the biggest one of mom.

So we need to try and get this stack of pancakes back into order.

I use my pancake flipper, stick that on a plate.

The question now is, how do we put these pancakes back into the right order?

There is a rule about how I'm allowed to shuffle these around.

I'm allowed to take the top section of pancakes and turn it over, and that's all I can do.

So, in this case, and what do I want to do?

No, in fact, first I'm gonna flip these top two, and the reason for that will become apparent shortly.

And if I hadn't put those took top two at the start, I would have had not to do this now.

So that's five flips, which is fine, because I know that any stack of pancakes can be fixed in five or fewer flips.

This one, I suspect, could have been in slightly fewer five.

But in this case, I've used exactly five, which is within the bounds.

So any sack of five pancakes can be done in five or fewer flips there.

This is currently in the state where need zero flips, but they're obviously some that could be done in fewer than five on the most you'll need for five pancakes.

Doing it in the most efficient possible way is five.

So the pancake number for five pancakes is there for five.

And this works for any number of pancakes in a stack, so I can show you what happens with three pancakes.

There are six possible ordering CZ for three pancakes so I could have small, medium large three at the bottom again, but instead have to and then want to at the bottom, then three, then one like that, one at the bottom and then two and then 32 at the bottom 13 like that on finally one of the bottom three and then to that gives me all six possible ordering Tze and for each one of these, I can work out how many flips I need to get it back into the sort of standard correct way up.

So in this case, it zero flips, it's already there.

This could be done in one which is just to flip the whole thing.

This one could be done in one flip this one.

If I flip those top two and then flip the whole thing, I'll get back to sorted.

So I'm gonna call that 12 flips this one.

If I flipped the whole thing and then flip those top two around, that'll be two flips on DDE.

In this case, this is the most difficult one, because this can't meet on in fewer than three flips, so I can do it in three.

I would maybe say Flip the whole thing, and then you flip the top two and let the whole thing, but it can't be done in fewer than three.

So for three pancakes, the pancake number is three.

Yes, it does look a little bit partly so.

After that, it starts to go on, so every so often it goes up by two instead of going up by one.

So it's not quite always just the number of pancakes that you've got, but it is actually something that people try and calculate the trying work out.

What is the pancake number four Given number of pancakes on Dhe.

It's actually an ongoing question because for 20 pancakes we don't know what the pancake number is.

We don't actually really know what the answer is to this.

And I guess it's partly because it's just a massive computation.

If you've got 20 pancakes in the stack, the number of possible way she could order 20 pancakes in a stack is massive for any given.

Ordering the number of possible sets of flips you could do is even Maciver, you know, to check all of those and see which one is the most efficient would take a lot of computer time, so no one's managed to crack that yet.

I think something like that s so yeah, we know it's more than 22 on.

In fact, we know it's less than I think 35.

And the reason the person that we have to credit with the fact that we know it's less than 35 is Bill Gates.

So Bill Gates that the guy off of Microsoft when he was a university in 1979 when he was 24 he did a paper.

Unfortunately, paper wasn't called like on the pancake number.

It was called something like algorithms for prefix reversals.

Because that's the technical computer science term for this.

If you're taking the first section and reversing it, that's a prefix reversal.

They proved that there is an upper bound for the pancake number for if you know how many pancakes you've got.

They proved that it's five enforced, five over three, which means that for 20 pancakes it's 105 over three.

Since then, they've actually worked out better upper bound for it.

If they've now shown that anything is less than 18 and over 11 which was found in 2009 which was exactly 40 years later, they managed to improve on that bound.

So there is a way that will always work for any given kind of messed up stack of pancakes.

So the trick is that you take whichever pancake is the biggest pancake.

That's not currently where it's supposed to be on, Put it to the top.

And then I put that one toe where it's meant to be if you keep repeating that so if now safe Pancake three wasn't in the right place.

You do the same thing, you put it to the top, and then each time you you're kind of definitely gonna place at least one pancake.

If you're looking, you will always do it in two and minus three.

So for n pancakes, in the case of five, it would always do it in seven or fewer.

But I know that for five pancakes, pancake number is five, which means there is a way to do all of them in five or fewer.

So that wouldn't be necessarily the most efficient method.

The nice thing about this is they actually originated from, like, a real world problem s.

Oh, there was a mathematician, Jacob Goodman, who was trying to solve a problem in the real world.

And this thing occurred to him is like, Oh, that's an interesting puzzle, you know?

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Pancake Numbers - Numberphile

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