On the notions of symmetry and aperiodicity for Delone sets
  Non-periodic systems have become more important in recent years, both theoretically and practically. Their description via Delone sets requires the extension of many standard concepts of crystallography. Here, we summarise some useful notions of symmetry and aperiodicity, with special focus on the concept of the hull of a Delone set. Our aim is to contribute to a more systematic and consistent use of the different notions. 
Michael Baake (Bielefeld) and Uwe Grimm (Milton Keynes)
math.MG cond-mat.mtrl-sci