I am unable to find any definition of what rough eigenvalues are. My intuition tells me that this definition only makes sense when we specify some space, say $H$, and suppose we have an operator $O$, and $e_j$ is an operator of $O$.
Then, $e_j$ is a rough eigenvalue of $0$ if the $H$-norm of $e_j$ is not finite?
Am I correct?
Also, it would be very nice if I could have an example to help me understand, if possible.