A boundary of red and blue squares is given. Can you fill in the interior such that each 5-square pattern consisting of an interior cell plus its four nearest neighbours always contains an even number of red squares?
Example:
Now try this one:
Answer
My answer :
And the number of red cells for each pattern :
every interior pattern has an even number of red cells.
How to find it :
I hoped there were a lot of symmetry so I had only a few cells to choose ! All the yellow cells are deduced from the blue cells by symmetry.
Then a few random tries gave me the answer
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