A beautiful problem from a Hungarian competition

We need to fill each cell in a 6 × 6 6\times6 grid with a distinct integer from 1 to 36. There are two rules:

  • Every pair of consecutive numbers are in adjacent cells that share an edge.
  • Any two cells containing a multiple of 4 cannot share an edge nor a vertex.

Over all valid configurations, how many of the 36 cells could contain the number 36 ? 36?


Note : Below is a 3 × 3 3\times 3 grid adhering to the rules above.


The answer is 4.

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2 solutions

Áron Bán-Szabó
May 20, 2017

We will prove that 36 36 can only be in a corner. It's easy to see that every even number will be placed on the same colour. We have nine numbers from 1 1 to 36 36 which have the 4 4 divisor: 4 , 8 , 12 , 16 , 20 , 24 , 28 , 32 , 36 4, 8, 12, 16, 20, 24, 28, 32, 36 . We can suppose that even numbers will be placed on the red cells. In the figure we dismembered the table to nine pieces of 2 × 2 2\times 2 square. There cannot be two-divisible by 4 4 -numbers in one 2 × 2 2\times 2 square, so in every 2 × 2 2\times 2 square there will be exactly one number which is divisible by 4 4 . The 6 × 6 6\times 6 grid has a rotational symmetry, so we can suppose that in the middle 2 × 2 2\times 2 square this number (which is divisble by 4 4 ) is in the F cell. Then in the A , B , C , E , G , I , J , K A, B, C, E, G, I, J, K cells there must not be a number, which is a multiply of 4 4 . So in the I J M N IJMN 2 × 2 2\times 2 square the number, which is divisible by 4 4 , can only be in cell N N (because M M is a blue cell). Similarly in the C D G H CDGH 2 × 2 2\times 2 square the only possible place for the number, which is divisble by 4 4 , is cell H H . Now we can see that in the K L O P KLOP 2 × 2 2\times 2 square the number, which is a multiply of 4 4 , can't be in K , L , O K, L, O cells, so it must be on the P P cell. We will show that the only number, which can be on the P P cell, is 36 36 . If it were different (for example 4 a 4*a ) then 1 < 4 a < 37 1<4*a<37 would be true, so 4 a + 2 4a+2 and 4 a 2 4a-2 would have to be placed on the grid. These numbers can't be on the N , H N, H cells (because 4 4 is not a divisor of 4 a 2 4a-2 and 4 a + 2 ) 4a+2) , so in cell K K there would have to be 2 2 numbers, which would not be possible. So 36 36 has to be in a corner, and there are 4 4 corners. A possible numbering:

another solution :

27 26 1 2 3 4

28 25 24 23 6 5

29 30 21 22 7 8

32 31 20 19 10 9

33 34 17 18 11 12

36 35 16 15 14 13

And I am sure there are no more solution if we role out reflection and rotation.

Jamal Senjaya - 4 years ago

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Are you sure?

Áron Bán-Szabó - 4 years ago

How do you know if there are no other solutions?

Pi Han Goh - 4 years ago
Bob Liddington
May 29, 2017

This is a comment on Aron's very elegant solution. I came up with the correct solution using a related semi-empirical approach.

However, I noticed that Aron's solution had the same arrangement of 4-multiples as mine did across the entire board - i.e. 2 regimented columns of 3 on the left, a 3rd column offset at the right edge, separated by 2 columns with no multiples of 4. My actual numbers and orientation were different of course, but the pattern of 4-multiples was identical. I wonder whether this is the only valid arrangement. Anyone?

You might be interested in Jamal's comment in the other solution...

Pi Han Goh - 4 years ago

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