M.C. Escher 7
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In 1937 a life-changing event occurred - he went home to see his parents! By chance, his brother Beer had chosen the same time to visit. Mauk showed his brother his sketches and Beer was struck by the similarity to work on crystalline structures in papers he had read. He later sent Mauk details and he managed to find them in libraries in Brussels where he was then living under German occupation. One was by Professor George Pólya in which he laid out the 17 possible wallpaper designs. There was even a sketch of them. He stated he thought an artist could make use of this knowledge! Escher was astounded and immediately cut 'development 1' and sent it to him.

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Pólya 17 symmetries
The "17 Wallpaper Groups"