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  • There we go.

  • See, most people forgot about the Rubik's Cube back in the eighties, so it had its'

  • heyday and then disappeared the mathematicians remain obsessed with it.

  • They wanted to find out the number of moves it would take to solve the most

  • difficult combination.

  • People probably are aware that there is some maths involved in a Rubik's Cube

  • and the group is the number of different combinations that there are on the

  • Rubik's Cube. It's all the different ways that you can scramble up a cube

  • Other things do have permutations and symmetry there's just something nice and

  • tactile and you can gather and you can see everything

  • as it happens.

  • And this number would be called God's number. If you have perfect knowledge of

  • how to do the Rubik's Cube you would be able to solve the most difficult

  • position in its perfect

  • most efficient algorithm.

  • And they managed to work out that it was less than 30.

  • And over the years, they started to whittle this down closer and closer.

  • No matter how much you scramble up a Rubik's Cube, you can solve it in 20 or fewer twists.

  • It's called God's number because you have to be some kind of omniscient being

  • to work out those 20 moves. It's just so difficult to calculate what those moves are.

  • I couldn't look at a cube and say I need to do these 12 moves or whatever

  • but there are people who can so they do Rubik's Cube solving competitions and

  • they have obviously speed challenges and see who can solve it the fastest but

  • one of the other things that they do is a kind of efficient solve and the

  • competition there is look at a Rubik's Cube and then just write down a list of

  • moves that will solve it

  • Which I think is probably more impressive than people who can solve it really quickly.

  • It was called the super flip. Now the super flip is essentially completely solved

  • except each edge position was flipped over.

  • If you take one of those pieces off and turn it around, you get a whole new universe

  • and you can't get back to the solved Rubik's Cube if you do that.

  • Four point three times ten to the nineteen it's a massive number, that is a massive number of combinations

  • It's 43 quintillion 252 quadrillion 3 trillion 274 billion 489 million 856 thousand

  • It's just a mind-numbing number of possible arrangements

  • So this was a big craze in the 1980s. Big craze.

  • In the 1880s, exactly 100 years earlier, there was a similar puzzle that was also a big craze.

  • They had their code that checked the reduced number of cases.

  • They ran it on the Google servers and they did. They exhaustively checked and we know now for

  • a fact that God's number is 20.

  • Every Rubik's puzzle can be solved in 20 or fewer moves.

  • You might think, well how do we know how? How do we know that?

  • Did we just check all 43.25 billion billion different ways of solving it and we effectively did that.

  • And there are 12 factorial ways to arrange the edges.

There we go.

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ルービックキューブのスピード解法をスローモーションで解く - Numberphile (Speed Solve of a Rubik's Cube in Slow Motion - Numberphile)

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    林宜悉 に公開 2021 年 01 月 14 日
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