Both the SBEs and SLEs contain an identical homogeneous part driven either by a classical field (SBEs) or by a spontaneous-emission source (SLEs). This homogeneous part yields an eigenvalue problem that can be expressed through the generalized Wannier equation that can be solved analytically in special cases. In particular, the low-density Wannier equation is analogous to bound solutions of the hydrogen problem of quantum mechanics. These are often referred to as exciton solutions and they formally describe Coulombic binding by oppositely charged electrons and holes. The actual physical meaning of excitonic states is discussed further in connection with the SBEs and SLEs. The exciton eigenfunctions are denoted by ϕ ϕ λ λ ( k ) where λ λ labels the exciton state with eigenenergy E λ λ and ℏ ℏ k is the crystal momentum of charge carriers in the solid.