text: The three-dimensional isotropic and non-relativistic uniform Fermi gas case is known as the Fermi sphere.
  A three-dimensional infinite square well, (i.e. a cubical box that has a side length L) has the potential energy
  The states are now labelled by three quantum numbers n, n, and n. The single particle energies are
  where n, n, n are positive integers. In this case, multiple states have the same energy (known as degenerate energy levels), for example .