Approximate point spectrum
= Approximate point spectrum
{title2=$\sigma_{\mathrm{ap}}(T)$}
The approximate point spectrum consists of the scalars $\lambda$ for which there are unit vectors $x_k$ satisfying $\lVert(T-\lambda I)x_k\rVert\to0$. Equivalently, $T-\lambda I$ is not bounded below.
This is a part of the <spectrum of a bounded operator>.