# Completely-reducible module

A module over an associative ring which can be represented as the sum of its irreducible -submodules (cf. Irreducible module). Equivalent definitions are: 1) is the sum of its minimal submodules; 2) is isomorphic to a direct sum of irreducible modules; or 3) coincides with its socle. A submodule and a quotient module of a completely-reducible module are also completely reducible. The lattice of submodules of a module is a lattice with complements if and only if is completely reducible.

If all right -modules over a ring are completely reducible, all left -modules are completely reducible as well, and vice versa; is then said to be a completely-reducible ring or a classical semi-simple ring. For a ring to be completely reducible it is sufficient for it to be completely reducible when regarded as a left (right) module over itself.

#### References

[1] | J. Lambek, "Lectures on rings and modules" , Blaisdell (1966) |

[2] | N. Jacobson, "Structure of rings" , Amer. Math. Soc. (1956) |

**How to Cite This Entry:**

Completely-reducible module.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Completely-reducible_module&oldid=15557