Squares Within Squares

Alice says, "Whoa, look! I can put together these 13 squares to make a bigger square."

What is the largest N N such that we cannot put together N N squares (of any size) to form a square?

If you think the answer is infinite, please put the answer as 99999.


The answer is 5.

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1 solution

Geoff Pilling
Nov 14, 2016

For positive n n , you can construct a solution for the following two cases:

  • 2 n + 2 2n+2
  • n + 3 n+3 (If there is a solution for n n )

The first one is done by drawing a square, then drawing n n squares along each of two adjacent edges and 1 1 at the corner.

The second one can be done by taking any arrangement of n n squares that forms a square and subdividing one square into 4 4 parts which will add 3 3 to the total square count.

This means that you are good to go for all even N > 2 N > 2 :

  • 4 , 6 , 8 , 10 , . . . 4,6,8,10,...

And, you can add three to any of those, giving you:

  • 7 , 9 , 11 , 13 , . . . 7,9,11,13,...

Now that you have a sequence of three (namely 6 , 7 6,7 and 8 8 ) you can make any higher number by adding 3 3 to the lowest of the three, in this case 6 6 , giving you 9 9 , producing a new sequence of three ( 7 , 8 7,8 and 9 9 ). Rinse wash and repeat! :)

The only numbers that can't be constructed in this way, then, are 2 , 3 2, 3 , and 5 5 , the largest of which is 5 \boxed{5}

And after playing around with these three numbers I convinced myself (without a rigorous proof... :-/ ) that there was no way to construct a square with any of these numbers of squares.


For more examples of squares in squares, see http://www.squaring.net .

The approach that I used to show 5 is not possible, is to argue from the largest dissected square S S .

  • If S S is unique, then each side that touches another square must touch at least 2 others. Show that the other squares touch S S on exactly 2 sides, so S S is in the corner. The casework from here is straightforward.
  • If S S is not unique, consider if the 2 squares match up perfectly, or not perfectly, and proceed with casework.

Not as pretty as I would like.

Calvin Lin Staff - 4 years, 7 months ago

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Arguably cleaner, but I agree it's still not too pretty:

  • Each side must be covered by at least two squares.
  • Each square may only (partially) cover two sides.
  • Thus the split is 3-3-2-2, 4-2-2-2, or less.
  • You cannot have the two 3's on opposite sides (use the fact that the whole thing is a square and not a rectangle).
  • In all cases, you have two adjacent sides, covered by two squares each.
  • The remaining portion is either a single square or a L-shape, to be covered by two squares; it's impossible.

Ivan Koswara - 4 years, 6 months ago

Great problem! I hadn't come across this topic before. Apparently this is known as a Mrs. Perkins's Quilt .

Brian Charlesworth - 4 years, 7 months ago

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