The Cube Life of Hexagon Flowers

Regular hexagons can be put together in a flower-like way:

From the picture above you can see that 7 7 hexagons are necessary to build the hexagon-flower on the left, and 19 19 hexagons are necessary to build the hexagon-flower on the right.

Neither 7 7 nor 19 19 are perfect cubes. As we add more hexagons to make larger hexagon-flowers, will we ever be able to make a hexagon-flower whose total amount of hexagons is a perfect cube ?

Inspiration

No Yes

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1 solution

Pi Han Goh
Jun 27, 2018

The number of regular hexagons used follows the sequence a n = 3 n ( n + 1 ) + 1 = ( n + 1 ) 3 n 3 a_n= 3n(n+1) +1 = (n+1)^3 - n^3 . See the solution in the Inspiration link for the proof.

Suppose a n a_n can be expressed as a (positive) perfect cube, then this would contradict Fermat's last theorem , which is absurd!

You found a nice way to express the proof.

Clearly the intent of the author of the problem was to exclude the degenerate case of a flower with 1 hexagon.

Steven Perkins - 2 years, 11 months ago

Nice solution! Here is a visual aid showing that a n = ( n + 1 ) 3 n 3 a_n = (n + 1)^3 - n^3 :

David Vreken - 2 years, 11 months ago

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