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In mathematics, a comodule or corepresentation is a concept dual to a module. The definition of a comodule over a coalgebra is formed by dualizing the definition of a module over an associative algebra.

Formal definition[edit]

Let K be a field, and C be a coalgebra over K. A (right) comodule over C is a K-vector space M together with a linear map

\rho: M \to M \otimes C

such that

  1. (id \otimes \Delta) \circ \rho = (\rho \otimes id) \circ \rho
  2. (id \otimes \varepsilon) \circ \rho = id,

where Δ is the comultiplication for C, and ε is the counit.

Note that in the second rule we have identified M \otimes K with M\,.


  • A coalgebra is a comodule over itself.
  • If M is a finite-dimensional module over a finite-dimensional K-algebra A, then the set of linear functions from A to K forms a coalgebra, and the set of linear functions from M to K forms a comodule over that coalgebra.
  • A graded vector space V can be made into a comodule. Let I be the index set for the graded vector space, and let C_I be the vector space with basis e_i for i \in I. We turn C_I into a coalgebra and V into a C_I-comodule, as follows:
  1. Let the comultiplication on C_I be given by \Delta(e_i) = e_i \otimes e_i.
  2. Let the counit on C_I be given by \varepsilon(e_i) = 1\ .
  3. Let the map \rho on V be given by \rho(v) = \sum v_i \otimes e_i, where v_i is the i-th homogeneous piece of v.

Rational comodule[edit]

If M is a (right) comodule over the coalgebra C, then M is a (left) module over the dual algebra C, but the converse is not true in general: a module over C is not necessarily a comodule over C. A rational comodule is a module over C which becomes a comodule over C in the natural way.


Montgomery, Susan (1993). Hopf algebras and their actions on rings. Regional Conference Series in Mathematics 82. Providence, RI: American Mathematical Society. ISBN 0-8218-0738-2. Zbl 0793.16029.