Abstract. - Topologically stable symmetry defects in ordered media can be classified by some homotopy groups. As an example we establish a complete classification of such defects for crystals.]]>

Abstract : An energy band in a solid contains an infinite number of states which transform linearly as a space group representation induced from a finite dimensional representation of the isotropy group of a point in space. A band representation is elementary if it cannot be decomposed as a direct sum of band representations; it describes a single band. We give a complete classification of the inequivalent elementary band representations.]]>