Monthly Archives: January 2015

Composition Series

Positive integers can be uniquely factored as a product of primes. Here, we would like to prove a counterpart for modules. Now there are two ways to “factor” a module M; a more liberal way takes a submodule N which gives us composition … Continue reading

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Radical of Module

As mentioned in the previous article, we will now describe the “bad elements” in a ring R which stops it from being semisimple. Consider the following ring: Since R is finite-dimensional over the reals, it is both artinian and noetherian. However, R is not … Continue reading

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Noetherian and Artinian Rings and Modules

We saw the case of the semisimple ring R, which is a (direct) sum of its simple left ideals. Such a ring turned out to be nothing more than a finite product of matrix algebras. One asks if there is a … Continue reading

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The Group Algebra (III)

As alluded to at the end of the previous article, we shall consider the case where K is algebraically closed, i.e. every polynomial with coefficients in K factors as a product of linear polynomials. E.g. K = C is a common choice. Having assumed … Continue reading

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The Group Algebra (II)

We continue our discussion of the group algebra. Constructing K[G]-modules Recall that such a module V is also called a representation of G over K, and corresponds to a group homomorphism (i) Given a K[G]-module V, a submodule W of V is precisely a vector subspace W such that g(W) ⊆ W for … Continue reading

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The Group Algebra (I)

[ Note: the contents of this article overlap with a previous series on character theory. ] Let K be a field and G a finite group. The group algebra K[G] is defined to be a vector space over K with basis , where “g” here is … Continue reading

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