If a polynomial has multiple roots, we may modify its coefficients a little bit so that all its roots become simple. Similarly, for the Schrödinger equation, we may perturb the Hamiltonian operator a little bit and remove the degeneracy of its eigenvalues.

- First excited states of helium
- The reason for the removal of the
*l*degeneracy [Lev2, p.271, l.-16-l.-3]. - The reason for the removal of the exchange degeneracy [Lev2, p.271, l.-2-p.272, l.11].

- The reason for the removal of the