Can you paint a 10x10 grid with 3 colours such that it doesn't contain any rectangles whose corners are all the same colour? Rectangles must be 2x2 or greater and parallel to the grid's sides. Computers are allowed :)

A similar question about 2 colours is here: Painting a 4x6 grid with 2 colours

Good luck!

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    $\begingroup$ Is there a clever way to accomplish this task? "Computers are allowed" makes it seem like the question is intended to be solved by brute-force search, rather than any creativity. $\endgroup$ – Deusovi Sep 20 '19 at 2:57
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    $\begingroup$ There is likely a clever method given the combinatorial feel of the problem. If such a method exists, computers ought not to be allowed. $\endgroup$ – greenturtle3141 Sep 20 '19 at 4:21
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    $\begingroup$ @Deusovi good question. Even a brute force search program will struggle to find this due to the large search pace, but there are clever algorithms that can solve this. However, there could still be a way that a human can solve this and it will be interesting to see if anyone can do it. By the way, the 4x6 problem I linked to can be done by hand. $\endgroup$ – Dmitry Kamenetsky Sep 20 '19 at 4:27

Solution (not mine):

10x10 solution
Source: Rectangle Free Coloring of Grids, https://arxiv.org/abs/1005.3750 page 28
The authors of that paper found the solution by placing one color manually and letting a computer program figure out the other two colors.

  • $\begingroup$ Indeed this is the paper that inspired me to look into this problem. Originally they posted a challenge for the first person to find a solution for 4 colors on a 17x17. It turns out that even 18x18 is possible! $\endgroup$ – Dmitry Kamenetsky Sep 25 '19 at 6:15


By the pigeonhole principle, every row and every column contains at least four squares of the same colour.
Let us look at this particular colour for each column : this gives a list of $10$ colours. By the pigeonhole principle, at least four of these colours are the same.
In an attempt to prove that it's impossible: in a $10\times 4$ grid, can you place four yellow dots per column in such a way that no four dots are the corners of a rectangle?
It turns out you can place four yellow dots per column in up to five columns while satisfying this condition:
enter image description here
If a solution exists, it will have to include a $4\times 10$ subconfiguration of the above, or similar.

  • $\begingroup$ rot13(Vs lbh pna rahzrengr nyy gur cbffvoyr 10k4 fbyhgvbaf, gura sbe nal bar bs gurz gb or va gur yrsgzbfg 4 pbyhzaf, vg'f evtugzbfg 3 pbyhzaf zhfg zngpu gur yrsgzbfg 3 pbyhzaf bs nabgure 10k4 fbyhgvba. Fvzvyneyl, vg'f evtugzbfg 2 pbyhzaf zhfg zngpu gur yrsgzbfg 2 pbyhzaf bs nabgure 10k4 fbyhgvba. Naq vgf evtugzbfg pbyhza zhfg zngpu gur yrsgzbfg pbyhza bs nabgure 10k4 fbyhgvba. Guvf vf orpnhfr jungrire pbyhzaf 5-7 ner, nal 4-jvqgu fhofrpgvba zhfg or bar bs gur 10k4 fbyhgvbaf.) $\endgroup$ – hdsdv Sep 20 '19 at 8:08
  • $\begingroup$ @hdsdv Not sure I understand: ROT13(gur cvtrbaubyr cevapvcyr qbrf abg nyybj lbh gb fnl fbzrguvat nobhg rnpu naq rirel 4k 10 fhopbasvthengvba). $\endgroup$ – Arnaud Mortier Sep 20 '19 at 8:44
  • $\begingroup$ Lrf, ohg vs jr pna rahzrengr nyy 10k4 fbyhgvbaf, gura nal 4 pbyhzaf bs gur pbzcyrgrq tevq zhfg or bar bs gubfr 10k4 fbyhgvbaf. $\endgroup$ – hdsdv Sep 20 '19 at 8:49
  • $\begingroup$ @hdsdv well, sure, but this is clearly not doable by hand, which was my intended purpose. $\endgroup$ – Arnaud Mortier Sep 20 '19 at 9:19
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    $\begingroup$ @ArnaudMortier I just almost made 10x10 by hand, without one in corner. $\endgroup$ – Jan Ivan Sep 20 '19 at 9:38

Partial - I don't think one can solve it by hand. Closest one I could find, but missing 3x3 (in 12x12, so "maximum" is 12x9 or 9x12):


So, is there a solution? I tried to switch rows and columns, but that leads nowhere.

  • $\begingroup$ Yes a solution exists :) $\endgroup$ – Dmitry Kamenetsky Sep 20 '19 at 10:44

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