Problem 189

Tri-colouring a triangular grid

Consider the following configuration of 64 triangles:

We wish to colour the interior of each triangle with one of three
colours: red, green or blue, so that no two neighbouring triangles have
the same colour. Such a colouring shall be called valid. Here, two
triangles are said to be neighbouring if they share an edge.

Note: if they only share a vertex, then they are not neighbours.

For example, here is a valid colouring of the above grid:

A colouring C' which is obtained from a colouring C by rotation or reflection is considered *distinct* from C unless the two are identical.

How many distinct valid colourings are there for the above configuration?

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These problems are part of
Project Euler
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CC BY-NC-SA 2.0 UK
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