In 1983 Piers Coleman introduced the slave boson formulation of the Hubbard operators, which enabled valence fluctuations to be treated within a field-theoretic approach. In this approach, the spinless configuration of the ion is represented by a spinless "slave boson" | f 0 ⟩ ⟩ = b † † | 0 ⟩ ⟩ , whereas the magnetic configuration | f 1 : σ σ ⟩ ⟩ = f σ σ † † | 0 ⟩ ⟩ is represented by an Abrikosov slave fermion. From these considerations, it is seen that the Hubbard operators can be written as