Show that eigen values of a matrix remain invariant under rotation.
HINT:Consider $QAQ^T$ where $Q^TQ=I$
I am unable to answer this question.I don't know what is meant by rotation of a matrix.
On using the hint I have shown that if $\lambda $ is an eigen value of $A$ corresponding to $v$ then $\lambda $ is an eigen vector of $QAQ^T$ corresponding to $Qv$.
But I don't understand how that solves the problem of rotation of matrices or what is meant by that. Neither do I understand why should I find eigen values of $Q^TAQ$.