Linear representations in Char 0

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    Linear Representations
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SUMMARY

In characteristic zero, all linear representations of reductive groups and finite groups are semisimple, as established by Maschke's Theorem. The discussion explores whether any linear representation of a non-finite group can also be semisimple, specifically examining the integers under addition as a candidate. This group is infinite and not defined by a finite number of equations, making it a suitable example for further analysis. The conversation suggests investigating the implications of irreducibility in two-dimensional representations, particularly concerning eigenvectors.

PREREQUISITES
  • Understanding of linear representations in group theory
  • Familiarity with semisimplicity and Maschke's Theorem
  • Knowledge of eigenvectors and their significance in linear algebra
  • Basic concepts of reductive groups and their properties
NEXT STEPS
  • Research the properties of linear representations of infinite groups
  • Study the implications of irreducibility in two-dimensional representations
  • Explore the concept of eigenvectors in the context of linear transformations
  • Investigate further examples of groups that challenge the semisimplicity assumption
USEFUL FOR

Mathematicians, particularly those specializing in group theory, linear algebra, and representation theory, will benefit from this discussion.

dg0666
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In characteristic zero any linear representation of a reductive group is semisimple. Also in characteristic zero any linear representation of a finite group is semisimple (Maschke's Thm). However is any linear representation of any group semisimple in characteristic zero?
 
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Evidently we must try to think of a group that is not finite, nor an algebraic group (let's avoid all of them). What is there to think of? First we hit the integers under addition - it is not finite, and not defined by a finite number of equations as a subset of R/C/Q/ any field, so we've got a hope. It is generated by a single element so that is good - a rep is just assigning it to a matrix and we're done. From there you should try to think about what it means for a 2-d rep, say (hint!) to be irreducible (again, hint, do all matrices have two eigenvectors?).
 

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