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We have a finite dimensional representation of a finite quiver over, say, the rationals. Is there an algorithm known to decompose this representation into its irreducible components?

A related question: we have two (finite-dimensional) representations of a finite quiver. Is there an algorithm to check if one is a summand of the other?

Thanks a lot for hints and pointers!

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Well, unless you make some restriction on the shape of the quiver, the term "quiver representation" is just as general as "representation of an associative algebra". If you work over a finite field, your quiver is finite (resp. your algebra is finitely generated) and your modules are of finite dimension, then MAGMA can do a lot of those computations (e.g. indecomposable direct summands, composition series, isomorphism checking...). –  Florian Eisele Feb 10 '12 at 17:28
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The first question is ambiguous. Do you want to find to find the composition facors of a representation or the decomposition into indecomposables. In any case, if you are working with finite dimensional representations over a field then the answer to all these questions is: Yes, an algorithm exists. –  Bruce Westbury Feb 10 '12 at 19:34
    
Thanks for the comments. I was thinking of the case where everything is finite and we are in characteristic 0. –  Christian Stump Feb 11 '12 at 11:20
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1 Answer

If you want to decompose a finite-dimensional representation over, say, $\mathbb{Q}$ into its indecomposables over, say, $\overline{\mathbb{Q}}$, there is not only an algorithm but an efficient one (at least theoretically efficient: polynomial-time). See

Chistov, A., Ivanyos, G. and Karpkinski, M. Polynomial Time Algorithms for Modules over Finite Dimensional Algebras, ISSAC 1997.

It works more generally over finite fields, and the real or algebraic closures of number fields.

Not only that, but they also show how to test efficiently if two such representations are equivalent. These two pieces together answer your second question (when is one rep, say $R_1$, contained as a summand in another $R_2$): decompose both into indecomposables, then see if every indecomposable in $R_1$ appears in $R_2$ with greater or equal multiplicity (test each indecomposable of $R_1$ for equivalence to each indecomposable in $R_2$).

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