books.google.com - Symmetry is one of the most important organizing principles in the natural sciences. It has been usual in mathematics to identify the study of symmetry with the study of groups. It is a basic premise of this book, that there are aspects of symmetry which are more faithfully represented by a generalization...http://books.google.com/books/about/Inverse_Semigroups_The_Theory_of_Partial.html?id=2805q4tFiCkC&utm_source=gb-gplus-shareInverse Semigroups: The Theory of Partial Symmetries
