“…For a domain with a cusp, the resulting operator is not necessarily self-adjoint. If the cusp is, roughly, less sharp than quadratic, then the form is bounded from below, and the spectrum is discrete (see [16,22,24] and, e.g., [11] for a recent study of the corresponding eigenvalue sequence itself). But, as it was shown in [22,24], the nature of the problem operator may become completely different, as it may lose its semi-boundedness, if the cusp is sharper than quadratic, see also [4, § 5].…”