Rubik's Cube Algorithm Cut Again, Down to 23 Moves

Bryan writes "The number of moves necessary to solve an arbitrary Rubik's cube configuration has been cut down to 23 moves, according to an update on Tomas Rokicki's homepage (and here). As reported in March, Rokicki developed a very efficient strategy for studying cube solvability, which he used it to show that 25 moves are sufficient to solve any (solvable) Rubik's cube. Since then, he's upgraded from 8GB of memory and a Q6600 CPU, to the supercomputers at Sony Pictures Imageworks (his latest result was produced during idle-time between productions). Combined with some of Rokicki's earlier work, this new result implies that for any arbitrary cube configuration, a solution exists in either 21, 22, or 23 moves. This is in agreement with informal group-theoretic arguments (see Hofstadter 1996, ch. 14) suggesting that the necessary and sufficient number of moves should be in the low 20s. From the producers of Spiderman 3 and Surf's Up, we bring you: 2 steps closer to God's Algorithm!"

I still can't do it.

[ASMworkz]

Call me when it's down to 10 moves! :)

Re:I still can't do it.

[Hankapobe]

Call me when it's down to 10 moves! :)

I can do it in one .... I outsource it.

Re:I still can't do it.

[Kjella]

I can do it in zero, I just declare that it's fine just the way it is and accuse anyone that tries to argue otherwise for being segregationists trying to keep all the different colors apart.

Re:I still can't do it.

[khallow]

This method has firm group theoretic underpinnings too. An alternate way to view this excellent solution is to realize that moves don't matter. Thus, you can say all states of the Cube linked by moves are equivalent in their state of fineness and hence the entire orbit is a single equivalence class of acceptability. This reduces the overly complex and uninteresting problem to a trivial one. Perfectly mathematical thing to do.

Re:I still can't do it.

[Ethanol-fueled]

I took a sniff, "Ugh, it smells like shit!" That's because I shoved it up my ass! bwahahahah!"

So you managed to fit a whole rubik's cube up your ass? CowboyNeal mustnt've been too impressed.

Re:I still can't do it.

[fizzup]

Call me when it's down to 10 moves! :)

I can do it in one .... I outsource it.

I can solve it faster .... I defenestrate [merriam-webster.com] it.

Re:I still can't do it.

[Daimanta]

Yes, but does the window run Linux?

Please, disregard the previous sentence.

Please, disregard the previous sentence.

Re:I still can't do it.

[SQLGuru]

This post intentionally left blank

Re:I still can't do it.

[Herkum01]

When I was a kid I had a rubix cube which one day resulted in this,

Angry Kid + Teacher(target) + Rubix Cube = defenestrate + one half rubix cube + dozens little pieces of a cube.

Afterwards I felt pretty bad about the whole thing...

Re:I still can't do it.

[this great guy]

What did the verb "defenestrate" apply to ?

• 1. the cube
• 2. the kid

Re:I still can't do it.

[ladybugfi]

No, the correct answer is

• 3. The teacher

Re:I still can't do it.

[Richy_T]

Pshaw, everyone knows that teachers should be disposed of on the top of bonfires.

Rich

Re:I still can't do it.

[Fishead]

I like to buy a new one, solved, and in the package, then using some CA (krazy glue) glue it together so nobody can "un-solve" it.

Re:I still can't do it.

[hurfy]

One is easy

http://www.puzzle-shop.de/aggie-cube.html [puzzle-shop.de]

lol, have that one on my shelf still sealed like that too. Totally amazed i remembered the name to google....

Are the ones that need to be a specific direction a lot more moves? The Pacman cube i have is a killer cause all the little ghosts and stuff started out upright 20? years ago and have been laying down sideways ever since :(

Easy

[camperdave]

Call me when it's down to 10 moves!

Step 1: Drop cube in can of paint. Done.

Re:Easy

[felipekk]

Actually, that's more than 10 moves:

1 Drive to Home Depot
2 Buy bucket and paint
3 Drive back home
4 Open the paint enclosure
5 Drop paint into bucket
6 Clean the floor
7 Clean yourself
8 Drop the cube
9 Pickup the cube
10 Realize that white paint won't make it look solved
11 Drive back to Home Depot
12 Buy a new bucket and dark paint
13 Drive back home
14 Open
15 Drop
16 Clean yourself (you actually learned not to mess the floor)
17 Drive to Toys-R-Us
18 Buy another Rubik's cube
19 Drive back
20 Drop the cube
21 Pick it up

Solved!

Re:Easy

[Jay L]

Drop cube in can of paint.

Paint is so 1987. Sears now sells powder coaters for $150.

There is nothing that can't be improved with a little powder coating. Pencils, silverware, desks...

Re:Easy

[Richy_T]

New toy alert!!!

my best time -1 min

[ami.one]

As a kid my best time was 1 min ! Used to just take off all the stickers on all faces and put them back in correct order. Friends were confused though as to why i want to solve it alone in a room and not in front of them.

"Solving the rubik's cube"

[PinkyDead]

I've never heard it called that before.

Be careful, you may go blind.

That's quick

[ricebowl]

And here I used to think my method was faster; but since there's more than 23 stickers on the cube I guess it ain't any more...

Re:That's quick

[Hankapobe]

And here I used to think my method was faster; but since there's more than 23 stickers on the cube I guess it ain't any more...

Try spray paint.

Re:That's quick

[Tumbleweed]

And here I used to think my method was faster; but since there's more than 23 stickers on the cube I guess it ain't any more...

This would definitely faster than my method of taking it apart and reassembling it in the correct order.

Re:That's quick

[gartogg]

Fact: You are wrong.

Re:That's quick

[Chris Burke]

I just buy a new one.

Do the math, quick!

[HiggsBison]

And here I used to think my method was faster; but since there's more than 23 stickers on the cube I guess it ain't any more...

So that would be, um, each face is three by three, um, nine stickers on each face. Then multiply that times the number of sides, so six times nine would be, uh, ...

Forty two.

Re:Do the math, quick!

[microTodd]

Bravo! Good Sir! Nicely done. Subtle, and only a true fan will catch that awesome reference.

Re:Do the math, quick!

[Splab]

And you should probably hand in your geek card and go back to literature and get acquainted with The Hitchhikers Guide To The Galaxy.

Re:Do the math, quick!

[something_wicked_thi]

No.

It's 42.

Trust me.

Re:Do the math, quick!

[bkr1_2k]

Seriously? You didn't get that? Credentials officially revoked.

Now back to our regularly scheduled non-RTFA programming.

Re:That's quick

[nizo]

If you had the original rubix cube, the white center square would pop off, allowing you to unscrew it and easily take apart/reassemble the cube. Way better than the new cheap ones that you have to pop apart and hope you don't break in the process.

I can always do it....

[Brad1138]

in 48 moves or less. Luckily the center sticker is always in the right place so I don't need to move that one.

Re:I can always do it....

[tangent3]

With 6 colours and 9 squares per face, there will always be 2 squares of the same colour per face, so it can always be done in 42 moves or less.

Re:I can always do it....

[syousef]

in 48 moves or less. Luckily the center sticker is always in the right place so I don't need to move that one.

You don't need to remove all 48 stickers. At least one of the outer 8 stickers will match the center on at least one side. So you don't have to unstick them all. So I can do it in 47 moves or less. Nyer!

Re:I can always do it....

[harry666t]

MAN, that's Forty Six & Two!

Re:I can always do it....

[DrEasy]

Now what would be the upper bound on solving the cube by moving the stickers around? What's the worst initial position?

Or...

[Anonymous Coward]

"Combined with with some of Rokicki's earlier work, this new result implies that for any arbitrary cube configuration, a solution exists in either 21, 22, or and 23 moves"

Or 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14 or 15 or 16 or 17 or 18 or 19 or and 20 moves.

Re:Or...

[this great guy]

You could share the script you used to output that sentence...

#!/bin/sh echo "Or 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14 or 15 or 16 or 17 or 18 or 19 or and 20 moves."

Re:Or...

[jibjibjib]

echo -n "Or ";for i in `seq 19`; do echo -n "$i or "; done; echo "and 20 moves."

Re:Or...

[kalirion]

Scripts are for wusses. I'm doing this as a web service.

Re:Or...

[CrazedWalrus]

Of course it can be. Take a solved cube and rotate it one single move. That's an arbitrary configuration, and it's solvable in one move. (Or are we using a definition of "arbitrary" I'm not familiar with?)

Re:Or...

[FrangoAssado]

Well, he did say any arbitrary configuration.

It is currently known that there is at least one configuration that is not solvable in 20 moves or less.

The point being: it is possible to solve a cube from any arbitrary configuration in N moves, where N is 21, 22 or 23 (it's not yet known which).

Re:Or...

[harryjohnston]

He said "I doubt that it is 1 for any arbitrary configuration." That translates to "there is no configuration which can be solved in 1 step" which obviously isn't true. What he presumably meant to say was "I doubt that it is 1 for *all* configurations." (The word "arbitrary" is redundant.)

Maybe I'm just slow...

[Keeper Of Keys]

The point being: it is possible to solve a cube from any arbitrary configuration in N moves, where N is 21, 22 or 23 (it's not yet known which).

On third reading, I get it. I thought, as presumably did the AC above, that Rokicki had shown that from an arbitrary start position it can always be solved in either 21, 22 or 23 moves.

But I think you mean "it is possible to solve a cube from any arbitrary configuration in no more than N moves, where N is 21, 22 or 23". If it starts out only 1 move away from completion, I don't really just have twiddle it round an extra 20 moves, do I?

Re:Maybe I'm just slow...

[FrangoAssado]

But I think you mean "it is possible to solve a cube from any arbitrary configuration in no more than N moves, where N is 21, 22 or 23". If it starts out only 1 move away from completion, I don't really just have twiddle it round an extra 20 moves, do I?

Yes, you're right, the "no more than" makes it clearer (that's what it was intended to mean). Of course there are configurations that don't need more than N moves (for N < 21), you can make them by starting with a solved cube and make N rotations (as with your 1 move example).

Re:Or...

[vux984]

No, it says 21, 22 or 23 moves. This statement is more narrow than the one you are suggesting, and thus more interesting.

So if you have a solved cube and make 1 move... the distance back to solved is 1 move.

To do it in 21 22 or 23 moves ... seems a touch less interesting. back-forth-back-forth...x21

18 moves is the limit

[BadAnalogyGuy]

Mathematically, the limit is a hard 18 (by faces): 6^2 / 2. alternatively by squares per face: ((9 * 6) / 3) ^ 2 / (2^2)

The math isn't hard. It's finding those correct 18 moves that is.

Re:18 moves is the limit

[BadAnalogyGuy]

Sorry, that second one is from another algo.

It should be 2(3^3)/3

Re:18 moves is the limit

[IWannaBeAnAC]

No, that is just a lower bound: by counting the number of possible configurations it can be shown that there exists at least one configuration that takes 18 or more steps to solve. It says nothing about an upper bound, which could (and is!) somewhat larger.

Re:18 moves is the limit

[kestasjk]

And who said abstract math isn't useful?

Re:18 moves is the limit

[Sunthalazar]

There are 2 statements.

1) "there exists" a configuration for which the minimum number of steps is "18".

2) "for all" configurations, "there exists" a solution that takes less than XX steps to solve.

We are trying to find the answer to #2. We know that #1 exists, so we know that the lower bound of a perfect solver (#2) is 18.

The article seems to be saying that the upper bound of #2 is 21-23.

Re:18 moves is the limit

[SQLGuru]

C programmers don't know how to count, anyway.

(I know, I know.....it's an offset from the pointer, so it's really offset 0, offset 1, etc.)

Layne

Re:18 moves is the limit

[mikael]

But there is more than one solution - the centre cube on each face can have any one of four orientations. If you were to paint arrows onto each cube, scrambled the cube, and then solved it, the arrows would not necessarily be aligned with the rest of the cubes on that side.

So there might be actually 4^6 solutions (4096).

Re:18 moves is the limit

[Sparr0]

To actually *prove* your premise, I would probably start by considering the corners. Since each corner has 3 colors, they cannot change position relative to each other [between solved configuration"s"] except by symmetric operations. Nor can the edge cubes, by similar logic. I am not sure how to state it formally. I am also not sure how to extend the concept to prove that the central square cannot be rotated, but given that you "know" that to be true, I would hope a proof is possible (without resorting to mechanical analysis of the cube).

Re:18 moves is the limit

[locofungus]

If you are saying there is no way to rotate a centre face without moving any of the other pieces then you are wrong.

Proof:

F1U1 (front 90, Upper 90) repeated 105 times returns the cube to its original state.

But 105 is 1 mod 4. Therefore the Front and Upper centres have been rotated by 90 degrees at the end of this move.

R1F1 has similar properties, as does R1U1.

Therefore F1U1^105 R1F1^105 R1U1^315 will rotate F centre through 180 and R and U centers through 360 - i.e. one centre will be rotated by 180 degrees.

Funnily enough, only last week I got a cube that had polarization of the centres. I very quickly found a move to rotate one centre 90 clockwise and an adjacent 90 anticlockwise but the move to rotate a single centre through 180 degrees took me a very long time to find. (The above 525 step move is not original to me and was only pointed out to me once I'd found a 12 step move to do the same thing)
Also the cube I had actually had one centre that could be in any orientation when viewed on its own. Only when comparing with the other centres was it possible to deduce that the centre was rotated from it's factory state.

Tim.

Solvable?

[CastrTroy]

The summary says for every solvable cube. What does that mean. Every configuration is a solvable one. If you remove a corner and rotate it, and place it back in the cube, the cube is no longer solvable, but I would argue that it's no longer a rubik's cube either.

Re:Solvable?

[pwnies]

It may not be a rubiks cube, but it would be quite humorous if strategically placed in an "Obsessive Compulsive Puzzle Solvers Anonymous" meeting.

Re:Solvable?

[ampathee]

Not really. Anyone who could solve a cube would find the rotated corner in a minute or two. My group of friends were into rubiks cubes a few years ago, and that trick got old fast.

Re:Solvable?

[Fred Ferrigno]

My friends decided to flip two pieces without telling me, thinking that would really annoy me. They were quite disappointed when I solved the cube, as the second flip perfectly counteracted the effect of the first.

Re:Solvable?

[Kjella]

Not for very long. While nowhere near the minimum number of steps, there are fairly simple techniques to solve a Rubik's cube so they'd quite quickly conclude it's been tampered with.

Re:Solvable?

[Your Pal Dave]

Yeah, when they first come out with books containing solutions I decided to fix a cube so that it was unsolvable and leave it out for someone who had memorized the solution to try and solve it. They always figured out the tampering within a minute or so.

Sadly, it was not nearly as amusing as I had hoped.

At least I still had my RPN calculator to lend to the smartass premeds in chem lab. (Furious punching of keys followed by "Where's the Equals?")

Re:Solvable?

[compro01]

At least I still had my RPN calculator to lend to the smartass premeds in chem lab. (Furious punching of keys followed by "Where's the Equals?")

I had a guy try that on me. He was rather put out when he found I knew how to use it.

Re:Solvable?

[Chris Burke]

When I forgot my trusty TI-81 in a calculus class, a friend tried to do that to me by handing me an HP calculator. I was very confused at first, and my friend had a laugh, but he didn't know how much assembly programming I had done and manipulating stacks wasn't exactly foreign to me.

That's my "i'm smart" story since I have to admit I was never very good at Rubik's Cube. :(

Re:Solvable?

[Your Pal Dave]

That's my "i'm smart" story since I have to admit I was never very good at Rubik's Cube. :(

Oh, I never actually solved the thing, unless you count prying it apart and re-assembling it in the correct order!

Re:Solvable?

[Deanalator]

I once spent and hour or so working on a cube before I realized that one of the two color pieces had two colors from opposite sides of the cube :-(

It is pretty annoying when people do the sticker trick to only solve one side of a cube.

Re:Solvable?

[greg1104]

But if you're given an arbitrary cube, how do you know if it's been tampered with such that it's no longer solvable? It may be the case that the simplest way to determine that, that works in every case, is to try and solve the cube and discover you can't. I don't believe it's a trivial problem to stare at a cube and figure out if a simple change like a rotated corner has been made to it.

Re:Solvable?

[Anonymous Coward]

Nah. It's pretty easy to tell if a scrambled cube is solvable. You can see which individual edge pieces are "flipped" and which corner pieces are correct or rotated (either clockwise or counter-clockwise). In a solvable cube, you must have either 0 flips or an even number of flipped "edge" pieces. If you assign a value of 1 to clockwise turned corner pieces and a value of 2 to counterclockwise pieces, adding up the values must be divisible by 3. Assuming these criteria are met, a scrambled cube can be deemed as "solvable". This is determined in a manner of seconds for us blindfold solvers but can be learned by just about anyone in less than an hour.

Re:Solvable?

[UnknownSoldier]

Someone have a link to the 3x3 parity function?

Re:Solvable?

[Kjella]

Probably because it's more work to find what all the permutations starting from a solved rubik's cube are, instead you start with a general cube and quickly eliminate the unsolvable ones. Techincally you're solving a slightly more general problem with the rubik's cube as a special case, so "solvable cube" is probably correct in the paper but equals rubik's cube in practical life.

Definitions

[Ghoser777]

It really depends if solvability is implied in the definition of a rubik's cube. The game of Solitaire is not always winnable from initial given cards - does that mean that the dealt cards aren't a legal Solitaire game, or just not winnable?

Re:Definitions

[MillionthMonkey]

A Rubik's cube is one of 12 possible (similar looking) cubes you can make if you start with all the pieces taken apart. The set of all possible cube configurations is separated into 12 orbits and a Rubik's cube can only reach configurations within the orbit that it's already in when it's solved. I wouldn't call the other 11 cubes Rubik's cubes. You start by scrambling a solved cube like a normal person, not assembling an unsolved cube from parts to see if you're going to "win" or "lose" with the cube you made. It's not like Solitaire at all.

Re:Solvable?

[mdmkolbe]

Every configuration is a solvable one.

If you take a cube apart and put it back together in random order, it may not be solvable. IIRC there are twelve sets of configurations where it is possible to get between configurations in the same set, but not between configurations not in the same set. (This obviously ignores configurations where you just took the stickers off and repasted them.)

LET THERE BE THREE moves...

[davidsyes]

1. Pour paint on Cube
2. Let Dry
3. PROPHET

I can do it in 2...

[El_Oscuro]

1. Take apart
2. Put back together (wait... why are there parts left over????)

Re:LET THERE BE THREE moves...

[GoodNicksAreTaken]

3. PROPHET

That must be part of the God's algorithm the summary mentions.

Re:LET THERE BE THREE moves... HEHEHE

[Tango42]

This is Slashdot - you're allowed to quote Kirk.

That's great.

[raehl]

Maybe he should next try and find the minimum number of edits to fix the grammar in a Slashdot article submission.

After that, solve for the max number of edits a Slashdot editor will actually do before just posting the article anyway.

Mastermind

[LoudMusic]

I know it's not a Rubik's cube, but I can solve any Mastermind puzzle in seven moves.

http://en.wikipedia.org/wiki/Mastermind_(board_game) [wikipedia.org]

but rubik's hypercube remains unsolved

[circletimessquare]

that 4th dimensional rotational axis means you have to reach forward in time in order to solve one side in the present, without affecting any other sides you solved in the past, meaning you can't twist it to the present, without affecting what you've already solved in the future

rubik's hypercube has me stumped

Re:but rubik's hypercube remains unsolved

[icegreentea]

A 4 spatial dimension hypercube as been solved. There are giant algorithms that allow humans to solve them given enough time (I think its around 1000 moves). A 5d version is also around (and also solved. by computers only I think).

Only one move required...

[FoolsGold]

Blend the fucker - http://www.youtube.com/watch?v=NrqHHBibRvs [youtube.com]

There, saved you from another 22 pointless moves.

Slightly offtopic

[KokorHekkus]

I actually found one of the solutions (obviously not uniquely) for the Rubiks Cube myself. It ended up to be the "corners first"-type of solution which I think is quite a natural way to reach a solution (it's basically a divide and conquer algorithm). If you can put the corners in their right place you only need to use a 8 move permutation to solve the rest which I call "the cross"-pieces.

So I'm curious if anyone else has experienced this as being the obvious but not perfect solution?

Re:Slightly offtopic

[KokorHekkus]

Ehum, of course I meant the same 8 move permutation applied many times, not just once. Sorry.

Re:Slightly offtopic

[locofungus]

Yes, corners first. I don't know about the same 8 move combination but IMO there are only 8 pieces that aren't obvious how to position.

First you do four corners on a face - anybody can do this without any "method". (at least I could)

Then you position the second four corners and rotate them as necessary - this is more tricky.

Then you position the eight edges on opposite faces (leaving the middle slice unsolved). (How to do this was completely obvious to me, YMMV)

Then you need to position and rotate the remaining four edges - again more tricky.

Finally, you might need to reposition the centres (By default I ignore the four centres on the middle slice and minimize the moves for the edges, sometimes leaving me with a four spot pattern to "solve" at the end)

Tim.

Re:Slightly offtopic

[KokorHekkus]

The eight move permutation I use on a Rubiks cube with the corners done is:

Turn "right side" a quarter turn towards you
Turn "bottom layer" a quarter turn to the left
Turn "right side" a quarter turn away from you
Turn "center layer" a quarter turn to the right
Turn "right side" a quarter turn towards you
Turn "bottom layer" a quarter turn to the right
Turn "right side" a quarter turn away from you
Turn "middle layer" a quarter to the left


Those 8 moves will shift places of 3 edge pieces, the two in the middle layer closest to you and the bottom one furthest away from you. And carefully applied that's all I need, with the minor variation that the center layer turn is either left, right or a half full turn. Of course I actually use other move-patterns as well but they're basically just for speed/convenience.

Re:Slightly offtopic

[SuperMonkeyCube]

Corners first, when initially stumbled upon by accidentally solving the corners, does seem rather obvious. Many of the people that were fast in the early days of cubing were using corners first solutions. Any of us that were trying to make funny patterns on the cube were familiar with moving groups of edges around that didn't disturb the corners. Also, the easiest move to flip edges left everything else undisturbed, and flipped one edge on the top layer and three in the middle layer. (That would be RERERERE.) These things combined to make corners first commonplace.

As someone who still carries a cube around, I would have to say that you can still be well under a minute with a corners first solution, and not have to memorize as many routines (15-20 tops) as someone using the more efficient Fridrich F2L solution (76 much longer routines minimum) not to mention the additional difficulty of having to locate two cubes at a time for corner-edge pairs. This is, as a task, rather like human computing. Do you stick with a short program that's less error prone, or do you write a more complicated one that hogs more memory in the hope of getting it done a few cycles sooner? I'm surprised at how many so-called 'nerds' on this thread disregard the cube as some sort of frustrating toy that can't be done except by paint or sticker removal (especially when disassembly is more reliable :P) instead of wanting to understand the behavior of groups and physically grasping algorithm efficiency. Cudos to KokorHekks for solving it himself!

Brute force

[Tango42]

This is just using a powerful computer to solve it by brute force... it's not really an algorithm, it's just a proof of a new upper bound.

(I know, technically speaking, brute force probably does count as an algorithm, but it's not a very impressive one.)

Re:Brute force

[rokicki]

This result required the use of *many* computers to solve *many* positions (approximately four million billion positions), and each found solution was 20 moves or less.

Yes, in some terms it was brute force, but consider how big a number four million billion is, and how long it takes to solve just a single position in 20 moves or less.

Re:Brute force

[mdmkolbe]

The art in using brute force is in pruning down the search space.

For example, calculating the optimal tic-tac-toe game by hand is a lot easier once you notice and exploit the symmetries.

Some "brute force" algorithms move from exponential to cubic or better once you trim them down properly. So any new tricks for improving brute force are of interest.

Re:Brute force

[phliar]

Beauty is in the eye of the beholder....

This is not about using brute-force solvers. Given some problem space, there are many interesting things to think about: optimal algorithms -- in time, and in memory -- of course; but also exploring the structure of the space itself. Interesting symmetries, upper and lower bounds on path and cycle lengths, ... . The possibilities are boundless.

And no one would consider using a brute-force solver, but writing one would be interesting. Give it a try.

Hofstadter

[pez]

Perhaps slightly off-topic, but the Hofstadter cited (via Metamagical Themas) is the same Douglas Scott Hofstader that wrote Goedel, Escher, Bach -- one of the greatest books ever written.

Re:Hofstadter

[DerekLyons]

Anyone who has only read GEB is really missing out - both Metamagical Themas and The Minds I are also real mind openers. They take the themes raised in GEB and explore them further.
 
Fluid Concepts and Le Ton Beau De Marot are highly skippable.

Re:Hofstadter

[Cajun Hell]

A real Hofstadter fan/pedant would have complained about the "Hofstadter 1996" knowing it was off by a decade.

Re:Hofstadter

[DMUTPeregrine]

Douglas Richard Hofstadter is his name. Not Scott. No idea where pez got that.

Recapping what it means

[Anonymous Coward]

When the limit was proved to be no worse than 25, there were lots of comments on Slashdot that misunderstood various aspects of what this means.

Here are clarifications for some common points of confusion:

1. What Tom has shown, that "an arbitrary cube can be solved in 23 moves", it means the nastiest legal cube needs no more than 23 face turns to solve. Obviously many starting configurations can be done in less.

2. This type of research doesn't tell you WHICH 23 moves. Only that it's 100% certain that there exists a 23-moves-or-shorter solution, for any legal cube.

3. It's easy to figure out the total number of permutations of the cube. Given that, it can be determined that 17 face-turns doesn't produce enough different permutations, but 18 does, so there is a definite lower bound of 18 moves, that is, there exists at least some configurations that MUST be 18 moves or more away from solved.

4. Specific configurations have been found that provably need 20 face turns to solve. So the worst-case will never get better than that.

5. It may be possible to narrow the limit further, showing that all cubes can be solved in 22 face turns or less. Maybe 21. Maybe 20. It will never get lower than that.

Put succinctly, as of today, the worst-case number of face-turns to solve a cube is no worse than 23. It's been known for a while that the worst case is no better than 20.

One more question

[gr8_phk]

Can a face turn be 180 degrees, or is that 2 turns? My guess from a normal person perspective is that a 180 degree turn is one move. The programmer and armchair mathematician in me says that a 180 degree turn is two moves, as it passes through one configuration on the way to another. So what is the generally agreed upon definition of a move?

On a related note, when I was a kid some people thought sliding a middle slice was a move, when it was clear to me that it was really 2 "face turns". From the terminology you use, we seem to agree on that. I just thought I'd mention it.

Cube expert

[extrasolar]

You know, if you want to make faster progress I suggest you consult the experts of all things cube [timecube.com].

Good Old Days at A&M

[FurtiveGlancer]

I remember "Computer Tom" sitting near the back of the class with headphones on, programming in BASIC on a SHARP. When the prof dared ask Tom a question, Tom invariably got it right and often pointed out a mistake earlier in the problem. I wonder if he recalls the "Nun-inverting" amplifier?

If you read this Tom, hope we didn't take up too much of your valuable programming time and thanks for the VAX account elevation. :)

So Much Brainpower Wasted

[Nom du Keyboard]

So much brainpower spent on trying to figure out a toy.

How Many Moves Is My Solution?

[Nom du Keyboard]

I just take them apart and reassemble them as solved.