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Ruth Padel

In representation theory, a subrepresentation of a representation $(\pi ,V)$ of a group G is a representation $(\pi |_{W},W)$ such that W is a vector subspace of V and $\pi |_{W}(g)=\pi (g)|_{W}$ .

A nonzero finite-dimensional representation always contains a nonzero subrepresentation that is irreducible, the fact seen by induction on dimension. This fact is generally false for infinite-dimensional representations.

If $(\pi ,V)$ is a representation of G, then there is the trivial subrepresentation:

V^{G}=\{v\in V\mid \pi (g)v=v,\,g\in G\}.

If $f:V\to W$ is an equivariant map between two representations, then its kernel is a subrepresentation of $V$ and its image is a subrepresentation of $W$ .

References

Fulton, William; Harris, Joe (1991). Representation theory. A first course. Graduate Texts in Mathematics, Readings in Mathematics. Vol. 129. New York: Springer-Verlag. doi:10.1007/978-1-4612-0979-9. ISBN 978-0-387-97495-8. MR 1153249. OCLC 246650103.

Credit: Wikipedia https://en.wikipedia.org/wiki/Subrepresentation

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