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# Nine Eigen

##### Age 16 to 18 Challenge Level:

This problem involves the action of matrices on vectors in three dimensions. The first few questions look at fixed vector directions; the latter questions look at fixed vectors. As you consider each point, make use of geometric or algebraic arguments as appropriate. Draw diagrams and construct particular examples of matrices and vectors if needed. If there is no definitive answer to a given part, try to give examples of when the question posed is or is not true.

In the questions below: $R, S$ are rotation matrices; $P, Q$ are reflection matrices; $M$ is neither a rotation nor a reflection.
1. Which of the different types of matrices can leave no vector directions fixed?
2. Which of the different types of matrices can leave exactly one vector direction fixed?
3. Which of the different types of matrices can leave more than one vector direction fixed?
4. Is it ever the case that $RS$ can leave a vector invariant?
5. Is it ever the case that $PQ$ can leave a vector invariant?
6. Is it ever the case that $M$ will leave the direction of a vector invariant?
7. Can a matrix with determinant zero leave a vector fixed?
8. Can a matrix with determinant greater than $1$ leave a vector fixed?
9. Can a matrix leave exactly two vectors fixed?