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Anonymous Poster

Dice problem

02/23/2009 3:31 PM

A cube of wood has edges of 9cm. What is the minimum number of saw cuts required to produce 729 1cm cubes, and why ?

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#1

Re: Dice problem

02/23/2009 4:39 PM

If your saw is big enough, 24 will suffice.

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#5
In reply to #1

Re: Dice problem

02/24/2009 12:01 AM

Nope

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#2

Re: Dice problem

02/23/2009 4:59 PM

8 cuts per pair of sides will give 9 portions.

9 x 9 x 9 = 729

3 x 8 = 24 cuts.

Where can one buy this zero thickness saw?

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#4
In reply to #2

Re: Dice problem

02/23/2009 9:15 PM

"Where can one buy this zero thickness saw?"

KrisDel of course.

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#6
In reply to #2

Re: Dice problem

02/24/2009 12:11 AM

You're allowed to put the dust back with glue if you really want.

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#7
In reply to #6

Re: Dice problem

02/24/2009 12:24 AM

Would it be OK to stick positive dots onto the dice.

In this case the one will be loaded because of the more weight at the six.

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#8
In reply to #7

Re: Dice problem

02/24/2009 12:36 AM

Nope ; "729 1cm cubes". You'd need the special KrisDel "No-Vol Glue" anyway, unless you could live with some spare dust. "Dice" is "dice" in the 'diced carrots' sense.

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#17
In reply to #8

Re: Dice problem

02/24/2009 12:18 PM

Actually, KrisDel buys my Consolidated Gookumpucky SuperDuper Negative Volume Glue in bulk, add fillers and repackage it as their "No-Vol Glue." I know they are competing with my own product, but I have an agreement with all the label suppliers that they must sell labels without adhesive backing to KrisDel. KrisDel then is forced to use my SuperDuper Sticker Stickum.

Consolidated Gookumpucky - You'll be glad you stuck with our products!

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#18
In reply to #17

Re: Dice problem

02/24/2009 2:41 PM

Hey, that's a labelous statement and you're hereby served with a conjunction !

C'mon, Doug, you're still in with a chance for a GA here ; why is 6 the minimum number of cuts to dice up a 3x3x3 cube ? That's a simplified take on the question as posted, but answering it for this case should be enough.

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#19
In reply to #18

Re: Dice problem

02/24/2009 3:00 PM

Actually, it could be done with 3 cuts. Now, you tell me how!

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#20
In reply to #19

Re: Dice problem

02/24/2009 3:13 PM

Oh go on, tell me......I don't know

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#21
In reply to #20

Re: Dice problem

02/24/2009 10:39 PM

It's simple, and your pic alludes to that. Use an eight-bladed saw. Draw centerlines on the top of the cube and on two adjacent vertical sides. Line up the center point on the saw with the centerlines to make your cuts.

Hey, if the blades have to be zero-width, you might as well have 8 of them! Or use 8 lasers with a beamwidth of one photon.

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#22
In reply to #17

Re: Dice problem

02/25/2009 12:20 AM

you can make it without a single cut.

Hint : you need to contact acme and KrisDel products Inc.

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#3

Re: Dice problem

02/23/2009 6:19 PM

I dropped the infinitely thin saw blade on the floor and I'm having trouble finding it amongst all the infinitely fine sawdust....

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#9

Re: Dice problem

02/24/2009 3:31 AM

Yup, I'm with hendrik on this one.
8 Horizontal cuts gives a stack of 9 slabs.
8 Vertical cts (north south) gives 81 square section beams.
Final 8 vertial cuts (east west) gives 729 cubes.

Del

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#10
In reply to #9

Re: Dice problem

02/24/2009 5:27 AM

Then you can go sit on the silly-step too

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#11

Re: Dice problem

02/24/2009 6:15 AM

I think Kris has sussed it but he's a scaredey squirrel....
He's convinced me that there's a fewer cuts solution but I've taken a vow of silence.
He's lurking in the bushes at the mo'
Del

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#12
In reply to #11

Re: Dice problem

02/24/2009 6:40 AM

Teee heeee...Well I bet you're buying sugar lumps right now just to see !oops...wasn't me. Was never here, didn't do it......

OK, it's 12, but proving it is the cleverful bit. Squirrel beats two cats, and that ain't a bad start to the day

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#13
In reply to #12

Re: Dice problem

02/24/2009 8:11 AM

The 12 cuts is right.

Start with 2 cuts in each direction = 6 cuts

You now have 27 3x3x3blocks

now put the blocks in a straight stripe and give 2 more cuts on the 2 long faces

now stack the 1x1x3 blocks side by side and give the final 2 cuts.

==> 12 cuts

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#14
In reply to #13

Re: Dice problem

02/24/2009 10:31 AM

Oh go on, confess, the Delster told you ?

I'll give you a GA Hendrik, nicely explained. You can have another if you explain why it's the mimimum number of cuts.....

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#15
In reply to #14

Re: Dice problem

02/24/2009 10:41 AM

Wood eye!

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#16
In reply to #15

Re: Dice problem

02/24/2009 11:01 AM

ROFL - thought that'd jangle your bell. Don't be mean - give Hendrick a GA, or I'll sharpen another pencil !

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#23

Re: Dice problem

02/27/2009 12:06 PM

Kris is right it's 12.

To do a Rubik's cube you've got to do at least 6, think about the very central cube and it's obvious that you can't do it in less.

Now for the nine by nine. First you've got to get out the central Rubik, any other plan will leave you wanting. That's six cuts.

Now you've got to get the central cuby out of each of the Rubiks. That's another six cuts (easy logically but practically pretty tricky).

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#24
In reply to #23

Re: Dice problem

02/27/2009 1:22 PM

Hurrah !

Thank you, Randall. The inner-most cube of a 3x3x3 has to be produced by 6 separate cuts. In my mind, it's fair enough to extrapolate that into first using 6 cuts to section a 9x9x9 block, then another 6 cuts to section all those simultaneously.

Such a proof of minimum cuts will undoubtedly give Fyz a touch of the vapours, which seals the deal on me giving you a GA.

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#25

Re: Dice problem

02/28/2009 12:01 PM

To my mind, the main difficulty in proving the result was to show that we cannot share the cuts in such a way that we can manage with only (say) two cuts more than would be required for a cube that is 8x8x8. My solution (there may be better ones) is to recognise that after each cut we need only consider the 'largest' of the two blocks - so at each stage of cutting we only have one block to consider.

Formalising this*:

We may start by partially generalising the problem (trust me**, it works out simpler in the end):
. Consider the problem of cutting a rectangular block*** with sides of length 2p+1, 2q+1, 2r+1, (p, q, r integers) into unit cubes, using a saw that allows arbitrarily long planar cuts.

If we cut this into two pieces across the side of length p (arbitrarily selected), the two pieces will be of different size (except when p=0). The smaller of these pieces need never take more cuts to reach the unit cube stage than the larger one, so we can confine our consideration to the larger piece. Following the same reasoning, there is never a disadvantage to making the larger piece as small as practical - so (cutting along the direction of 'p') we cannot do better than to create a piece whose sides are 2p-1+1, 2q+1, and 2r+1.
Note in passing that, if p=0 (length=2), a single cut will result in two equal size blocks with the relevant side of unit length.
For the next cut, we can choose to cut across whichever side we please; but whatever we do, the new block will only be smaller in a single dimension.

Clearly, if we follow this (optimal****) scheme, it will take p+1 cuts to cut the side of length 2p+1 to unity, q+1 for the side of length 2q+1, and r+1 for the side of length 2r+1 - and the order of making the cuts is irrelevant.

So the total number of cuts needed will be p+q+r+3.
Back to the specific case: for this problem we started with a cube whose sides are of equal length 23+1, so it will take at least twelve cuts before the smallest piece is a unit cube.

*Please let me know where this appears lacking, and I will try to improve it.
**... I'm not a proper doctor
***formally, a rectangular parallelepiped
****that is not to say that no other scheme is as good - only that there won't be any that are better.

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#26
In reply to #25

Re: Dice problem

03/01/2009 2:58 AM

That sounds jolly good and, on the basis that my screensaver cut in whilst I was cogitating, I shall GA it. My mind is racing around other cube sizes, but unfortunately I'm limited to one GA ().

Many thanks for expressing this in a more formal manner. I suspect the completely formal description that might be found in a mathematical journal would be incomprehensible to me ! A nicely worded solution, Fyz.

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#27
In reply to #25

Re: Dice problem

03/02/2009 4:34 AM

Brilliant and beautiful as always.

Just to complete the generalisation: it's clear that to cut any block with sides n,m and l where

2(p-1) <= n < 2p

2(q-1) <= m < 2q

2(r-1) <= l < 2r

You need p+q+r cuts.

Having written this I can see that it was implicit in your solution , but, some of us aren't so fast .

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#28
In reply to #27

Re: Dice problem

03/02/2009 5:28 AM

Thanks! And I think the explicit completion was worthwhile.

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