Coulomb scattering in the massless Nelson model III: ground state wave functions and non-commutative recurrence relations

Let HP,σbe the single-electron fiber Hamiltonians of the massless Nelson model at total momentum P and infrared cut-off σ> 0. We establish detailed regularity properties of the corresponding n-particle ground state wave functions fP,σn as functions of P and σ. In particular, we show that (Formula presented.) where c is a numerical constant, λ00 is a positive function of the maximal admissible coupling constant which satisfies limλ0→0δλ0=0 and χ[σ,κ)is the (approximate) characteristic function of the energy region between the infrared cut-off σ and the ultraviolet cut-off κ. While the analysis of the first derivative is relatively straightforward, the second derivative requires a new strategy. By solving a non-commutative recurrence relation, we derive a novel formula for fP,σn with improved infrared properties. In this representation ∂Pj′∂PjfP,σn is amenable to sharp estimates obtained by iterative analytic perturbation theory in part II of this series of papers. The bounds stated above are instrumental for scattering theory of two electrons in the Nelson model, as explained in part I of this series.