Colouring a CUBE !!!
I got this problem from a friend and couldn't solve it ...
Imagine a cube which needs to be painted with different colors on all it's faces.
In how many different ways can I do this if I have 6 different colors ?
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Santanu Banerjee
7 years, 9 months ago
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14 votes
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Presumably you would regard two cube colourings the same if one can be obtained from the other by rotation.
When counting colourings, start by rotating all the colourings so that the red face is on the bottom. There are clearly 5 choices for the colour of the top face, opposite the red one.
There are at first sight 4!=24 colourings of the four side faces. However, since we can still spin the cube about its vertical axis (and maintain the red face at the bottom), these 24 colourings come in 6 groups of four, where the colourings in each group of four are essentially identical.
Thus we have 5 choices for the colour of the face opposite red, and then 6 ways to colour the other four faces. There are 30 different colourings.
How does this relate to 6!=720? There are exactly 24 rotations of the cube (the "do-nothing" rotation, 6 rotations through 90∘ about axes through the midpoints of opposite faces, 3 rotations through 180∘ about axes through the midpoints of opposite faces, 6 rotations through 180∘ about axes through midpoints of opposite edges and 8 rotations through 120∘ about axes through pairs of opposite vertices.
If you take any colouring, there are a total of 24 positions into which that colouring can be rotated, and so 24 of the 720 are essentially the same colouring. Thus there are 720÷24=30 essentially different colourings.
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Seems legit.
I am just wondering, how come there are 24 rotations, according to my calculation it should be 12. Let's consider we are rotating the cube about the top edge, its also rotating about bottom edge at the same time, whichever direction you rotate. Meaning, we are getting 4 rotated distinct configurations, for rotating 2 sides at the same time. And hence, the total no. of position should be 3∗4=12. And if I am right, the corresponding answer should be 720÷12=60
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I have described all 24 rotations.
What do you mean by "top edge"?
I think you mean rotations about the (axis through the midpoints of the) top and bottom faces. For each of the three possible axes, there are indeed four rotations (through 0∘,90∘,180∘,270∘. But the rotation through 0∘ is the "do-nothing" rotation, which is the same rotation for each of the three choices of axis. Thus you are triple-counting this rotation. Your count has found the identity ("do nothing") rotation, the 6 rotations through ±90∘ about the axes through midpoints of opposite faces, and the 3 rotations through 180∘ about the same axes. Thus we have found just 10 (not 12, because of the triple-counting of the identity). However, you have missed the other two types of rotation, about axes through midpoints of opposite pairs of edges, and about axes through opposite faces.
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Oops, sorry, I got ahead of myself. I should have read your post thoroughly. My bad. Thanks, for clearing it up.
It's 6!/24. There are 6! ways in which a cube held in particular position be colored. But there are exactly 24 positions which resemble the same color configuration if cube rotated accordingly. A cube can be placed on 6 different sides and then can be rotated 4 times horizontally by keeping the top face same and 24(=6∗4) different positions be achieved of the same color configuration. Hence the number of ways 6!/24.
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this is an easier way of counting the 24 cube rotations.
I got it at last...
First I will consider a face to be fixed with a specific colour and find out the relative arrangements of the rest.
When I have fixed a colour , the opposite face can be painted in the rest 5 colours , thus there are 5 choices...
Now the rest four face I can calculate using the circular arrangements concept
Similar to Permutations on a Circular Table...
That is 4 colours can be painted on the remaining 4 faces in ( 4 - 1 ) !
Thus the answer should be 5 * ( 4 - 1 ) ! == 30
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Thanks a lot....this is a much better way of doing it than thinking about the 24 possible ways in which a cube can be rotated to give the same set
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But I thought of an alternative way...which probably is actually the same thing... 1. Fix one side 2. Using 5C4 select 4 out of the remaining 5 colours 3. Arrange them in a circular pemutation in 3! ways 4. The remaining color automatically occupies the side opposite to our original color... ANS: 5C4*3!=30 Its just that though I accepted your way...I needed to be sure of my thinking!!
If you want a tough problem, how many colourings are possible (up to rotation) if there are still 6 colours, but they can be repeated (so you could have a colouring with four red faces, one white and one green, for example)?
You can paint the two Opposite faces in 6*5=30 ways.The order of the rest of the coloring does not matter.Sweet and Simple.
Guys...can u please explain the 24 rotations of the cube that give the same set...I didnt quite understand it...Thanks in Advance!!
Here's the fancy way to do problems like these: Burnside's Lemma AoPS also has an introduction to this technique.
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Excellent idea. And the answer to my supplementary question is... ?
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Your Answer :: It's probably coming out to be 2226 ....
What happens if it's a 12 Faced Polygon
and we have 12 predefined colours. No colour repeated ...
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2226: yes.
The dodecahedron has rotation group of order 60 ((a) six axes through midpoints of opposite faces, four nontrivial rotations each, (b) fifteen axes through midpoints of opposite edges, one nontrivial rotation each, (c) 10 axes through pairs of opposite vertices, two nontrivial rotations each, (d) the identity. 6×4+15×1+10×2+1=60).
If we have twelve colours, and none are repeated, there are 6012!=7983360 different colourings.
And if colours can be repeated ...?
I don't think that was a reply to your problem..?
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Possibly. However, in the original problem, the orbits all have the same size (24), so counting them by Burnside's Lemma is not necessary.
As the Cube is symmetrical,paint the top side with any one of the six colors.Then the adjacent 4 sides can be permuted in 3! ways (circular permutation) and respective colors in {5 \choose 4} ways which makes it 30.And paint the remaining side with the remaining color.Hence my answer is 30.
If all the faces of cubes are identical than there is only one way but if all thhe faces are distinct than there are 6! Ways
if you want that each side has a different color each side calling A, B, C, D, E, F on side A will have 6 color options of which only one is needed for the B side will have 5 colors remaining for C will have 4 for D will have 3 for E will have 2 and for F only stay one color: that according to the principle of multiplication would be: 6! : 6x5x4x3x2x1 720 possible combinations
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Don't forget that if you rotate a cube with a certain colouring, it stays the same cube.
if one of the side is painted with one color than we have 5 color left and than if paint 2 sides we have 4 colors and so on ...... so,it will be 6! i.e. 65432*1=720
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No it wont be..Each of unique coloring will be repeated 24 Times !!!
The word "different" is the key here. First consider that every face has a fixed orientation in space. Then definitely you can color them "differently" in 6! ways. Now if the faces do not have fixed orientation (that is exactly what is meant here) then we do not have that many different colorings. Because if we obtain same configuration of face-colorings in space by appropriately rotating different colorings, then those colorings are considered to be the "same". Hence the central question is how many different "rotational symmetries" a cube possesses. The answer is non-trivial and is 24. Hence number of different colorings for a cube is 6!/24=30.
...6!/24 IS IT RIGHT?
But I think it should be less than 6! since it can be rotated about a central axis which gives 4 same combinations for the lateral faces. We can choose the first 4 colours by 6C4 ways. Now these 4 colours can be arranged in (4-1)! ways since there are 4 same combinations for the lateral faces. The last two colours can be selected only in one way as it can be seen in an upside-down way too. Hence, the number of ways may be 6C4 x 3!, but I still have a doubt whether the cube can be seen in some way as well.