The crystallographic restriction theorem in its basic form was based on the observation that the rotational symmetries of a crystal are usually limited to 2-fold, 3-fold, 4-fold, and 6-fold. However, quasicrystals can occur with other diffraction pattern symmetries, such as 5-fold; these were not discovered until 1982 by Dan Shechtman.
The crystallographic restriction theorem is easy to understand
The crystallographic restriction theorem is something I don't understand, therefore it is at the very least not easy for one person to understand
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