This is a glossary of representation theory in mathematics.
The term "module" is often used synonymously for a representation; for the module-theoretic terminology, see also glossary of module theory.
See also Glossary of Lie groups and Lie algebras, list of representation theory topics and Category:Representation theory.
Notations: We write . Thus, for example, a one-representation (i.e., a character) of a group G is of the form .
Representation theory is simple to define: it is the study of the ways in which a given group may act on vector spaces. It is almost certainly unique, however, among such clearly delineated subjects, in the breadth of its interest to mathematicians. This is not surprising: group actions are ubiquitous in 20th century mathematics, and where the object on which a group acts is not a vector space, we have learned to replace it by one that is (e.g., a cohomology group, tangent space, etc.). As a consequence, many mathematicians other than specialists in the field (or even those who think they might want to be) come in contact with the subject in various ways.
Fulton, William; Harris, Joe, Representation Theory: A First Course