self-adjoint operator
densely defined operator on a Hilbert space whose domain coincides with that of its adjoint and which equals its adjoint; symmetric operator whose adjoint's domain equals its own domain
positive element
self-adjoint (or Hermitian) element A of a C*-algebra A is called positive if its spectrum σ (A) consists of non-negative real numbers
almost Mathieu operator
operator that arises in the study of the quantum Hall effect