The Resolvent of the Nelson Hamiltonian Improves Positivity


We give a new proof that the resolvent of the renormalised Nelson Hamiltonian at fixed total momentum P improves positivity in the (momentum) Fock-representation, for every P. The argument is based on an explicit representation of the renormalised operator and its domain using interior boundary conditions, which allows us to avoid the intermediate steps of regularisation and renormalisation used in other proofs of this result.

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Appendix A: Technical Lemmas

Appendix A: Technical Lemmas

Here we reprove the key Lemmas of [12, 20] for the special case of the (massive or massless) Nelson model at fixed momentum.

Lemma A.1

The family of operators Gλ has the following properties:

  1. a)

    For every λ > 0, the operator Gλ is bounded;

  2. b)

    \( {ran} G_{\lambda } \subset D({L_{P}^{s}})\) for any 0 ≤ s < 1/4, and for all λ0 > 0

    $$ \sup_{\lambda\geq \lambda_{0}} \|(L_{P}+\lambda)^{s} G_{\lambda} \|_{\mathscr{B}(\mathscr{H}_{P})} <\infty; $$
  3. c)

    Gλ maps D(dΓ(ω)1/2) to itself and there exists C > 0 so that for all λ > 0 and ψD(dΓ(ω)1/2)

    $$ \|\mathrm{d} {\Gamma}(\omega)^{1/2} G_{\lambda} \psi\|_{\mathscr{H}_{P}} \leq C \lambda^{-1/4} \|\mathrm{d} {\Gamma}(\omega)^{1/2} \psi\|_{\mathscr{H}_{P}}; $$
  4. d)

    There exists λ0 so that for all λ > λ0 the operator 1 − Gλ is boundedly invertible on P and D(dΓ(ω)1/2), and

    $$ \sup_{\lambda>\lambda_{0}}(\|(1-G_{\lambda})^{-1}\|_{\mathscr{B}(\mathscr{H}_{P})} + \|(1-G_{\lambda})^{-1}\|_{\mathscr{B}(D(\mathrm{d} {\Gamma}(\omega)^{1/2})} )<\infty. $$


For a) and b) it is sufficient to prove that

$$ -a(v)(L_{P}+\lambda)^{s-1} $$

defines a bounded operator on P, uniformly in λ. To prove this, we insert a factor of \(\sqrt {\omega (\xi )/\omega (\eta )}\) and its inverse, and then use that ab ≤ (a2 + b2)/2 as well as the symmetry in ξ,η, to obtain for n ≥ 1

$$ \begin{array}{@{}rcl@{}} &&\|a(v)(L_{P}+\lambda)^{s-1}\psi\ensuremath{^{(n)}}\|^{2}_{\mathscr{H}_{P}\ensuremath{^{(n-1)}}} \\ &&=n \int\limits_{\mathbb{R}^{3(n-1)}} \mathrm{d} Q \int\limits_{\mathbb{R}^{3}} \mathrm{d} \xi \int\limits_{\mathbb{R}^{3}} \mathrm{d} \eta \frac{v(\eta)\psi\ensuremath{^{(n)}}(Q,\xi) \omega(\xi)^{1/2}}{(L_{P}(Q,\eta)+\lambda)^{1-s}\omega(\eta)^{1/2}} \frac{v(\xi) \overline{\psi}\ensuremath{^{(n)}}(Q,\eta)\omega(\eta)^{1/2}}{(L_{P}(Q,\xi)+\lambda)^{1-s}\omega(\xi)^{1/2}}\\ &&\leq n \int\limits_{\mathbb{R}^{3(n-1)}} \mathrm{d} Q \int\limits_{\mathbb{R}^{3}} \mathrm{d} \xi \frac{\omega(\xi) |\psi\ensuremath{^{(n)}}(Q,\xi)|^{2}}{L_{P}(Q,\xi)+\lambda} \int\limits_{\mathbb{R}^{3}} \mathrm{d} \eta \frac{|v(\eta)|^{2}}{(L_{P}(Q,\eta)+\lambda)^{1-2s}\omega(\eta)}. \end{array} $$

By the Hardy-Littlewood rearrangement inequality we have for s < 1/4

$$ \begin{array}{@{}rcl@{}} {\int}_{\mathbb{R}^{3}} \frac{|v(\eta)|^{2} \mathrm{d} \eta}{(L_{P}(Q,\eta)+\lambda)^{1-2s} \omega(\eta)} &\leq& g^{2} {\int}_{\mathbb{R}^{3}} \frac{\mathrm{d} \eta }{(\eta^{2} + m^{2})((P-\eta-{\sum}_{j=1}^{n-1}k_{j})^{2}+\lambda)^{1-2s}} \\ & \leq& g^{2} {\int}_{\mathbb{R}^{3}} \frac{\mathrm{d} \tau }{|\tau|^{2} (\tau^{2}+ \lambda)^{1-2s}}, \end{array} $$

which is uniformly bounded for λλ0. Together with (57) and the symmetry of ψ(n) this gives

$$ \begin{array}{@{}rcl@{}} \|a(v)(L_{P}+\lambda)^{s-1}\psi\ensuremath{^{(n)}}\|^{2}_{\mathscr{H}_{P}\ensuremath{^{(n-1)}}} &\leq& n C\int\limits_{\mathbb{R}^{3(n-1)}} \mathrm{d} Q \int\limits_{\mathbb{R}^{3}} \mathrm{d} \xi \frac{\omega(\xi) |\psi\ensuremath{^{(n)}}(Q,\xi)|^{2}}{L_{P}(Q,\xi)+\lambda} \\ & =& C \int\limits_{\mathbb{R}^{3n}} \mathrm{d} K \frac{ \sum\limits_{j=1}^{n} \omega(k_{j}) |\psi\ensuremath{^{(n)}}(K)|^{2}}{L_{P}(K)+\lambda} \\ &\leq& C \| \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}}^{2}. \end{array} $$

To prove A.1, we proceed as in (57) to obtain (denoting \({\Omega }(K)={\sum }_{j=1}^{n} \omega (k_{j})\))

$$ \begin{array}{@{}rcl@{}} &&\|a(v)(L_{P}+\lambda)^{-1}\mathrm{d} {\Gamma}(\omega)^{1/2}\psi\ensuremath{^{(n)}}\|^{2}_{\mathscr{H}_{P}\ensuremath{^{(n-1)}}} \\ & &\leq 2n \int\limits_{\mathbb{R}^{3(n-1)}} \mathrm{d} Q \int\limits_{\mathbb{R}^{3}} \mathrm{d} \xi \omega(\xi) |\psi\ensuremath{^{(n)}}(Q,\xi)|^{2} \int\limits_{\mathbb{R}^{3}} \mathrm{d} \eta \frac{\Omega(Q, \eta)|v(\eta)|^{2} }{(L_{P}(Q,\eta)+\lambda)^{2}\omega(\eta)}. \end{array} $$

By scaling and rearrangement, we have

$$ {\int}_{\mathbb{R}^{3}} \frac{|v(\eta)|^{2} \mathrm{d} \eta}{(L_{P}(Q,\eta)+\lambda)^{2}} \leq \lambda^{-1}g^{2}{\int}_{\mathbb{R}^{3}} \frac{\mathrm{d} \tau}{|\tau| (\tau^{2} +1)^{2}}, $$


$$ {\int}_{\mathbb{R}^{3}} \frac{|v(\eta)|^{2} \mathrm{d} \eta}{(L_{P}(Q,\eta)+\lambda)^{2}\omega(\eta)} \leq ({\Omega}(Q)+\lambda)^{-3/2} g^{2}{\int}_{\mathbb{R}^{3}} \frac{\mathrm{d} \tau}{|\tau|^{2} (\tau^{2} +1)^{2}}. $$

Using symmetry as in (59) this implies

$$ \begin{array}{@{}rcl@{}} \|a(v)(L_{P}+\lambda)^{-1}\mathrm{d} {\Gamma}(\omega)^{1/2}\psi\ensuremath{^{(n)}}\|^{2}_{\mathscr{H}_{P}\ensuremath{^{(n-1)}}} \leq C \lambda^{-1/2} \|\mathrm{d} {\Gamma}(\omega)^{1/2} \psi\ensuremath{^{(n)}}\|^{2}_{\mathscr{H}_{P}\ensuremath{^{(n)}}}, \end{array} $$

and proves c) by taking adjoints.

To prove d) observe that b) and c) imply that

$$ \|G_{\lambda}\|_{\mathscr{B}(\mathscr{H}_{P})} + \|G_{\lambda}\|_{\mathscr{B}(D(\mathrm{d} {\Gamma}(\omega)^{1/2}))} \leq C \lambda^{-s} $$

for s < 1/4. Thus for large enough λ the inverse of 1 − Gλ in both spaces exists and is given by the Neumann series, whose norm is bounded by \((1-C\lambda _{0}^{-s})^{-1}\). □

Lemma A.2

For any ε > 0 there exists C > 0 such that for all λ > 0, n ∈ ℕ0 and K ∈ ℝ3n

$$ |T_{\mathrm{d},\lambda}(K)| \leq C (L_{P}(K) + \lambda)^{\varepsilon}. $$


We treat only the case n > 0, the case n = 0 is similar but simpler. We have (with \({\Omega }(K)={\sum }_{j=1}^{n} \omega (k_{j})\))

$$ \begin{array}{@{}rcl@{}} |T_{\mathrm{d},\lambda}(K)| &= & \left|{\int}_{\mathbb{R}^{3}} |v(\xi)|^{2} \frac{(P-\sum\limits_{j=1}^{n}k_{j} - \xi)^{2} - \xi^{2} + {\Omega}(K) + \lambda}{(\xi^{2} +\omega(\xi))(L_{P}(K, \xi)+ \lambda)} \mathrm{d} \xi\right| \end{array} $$
$$ \begin{array}{@{}rcl@{}} &\leq & {\int}_{\mathbb{R}^{3}} |v(\xi)|^{2} \frac{(P-\sum\limits_{j=1}^{n}k_{j})^{2} + 2 |\xi||P-\sum\limits_{j=1}^{n}k_{j}|}{(\xi^{2} +\omega(\xi))(L_{P}(K, \xi)+ \lambda)}\mathrm{d} \xi \end{array} $$
$$ \begin{array}{@{}rcl@{}} &&+ {\int}_{\mathbb{R}^{3}} |v(\xi)|^{2} \frac{\Omega(K) + \lambda}{(\xi^{2} +\omega(\xi))(L_{P}(K, \xi)+ \lambda)} \mathrm{d} \xi . \end{array} $$

To simplify the notation, we set \(p:=P-{\sum }_{j=1}^{n}k_{j}\). The first term in (65) and the term (66) are bounded by almost identical arguments, so we only give the details for one of them.

Using that (ξ2 + ω(ξ)) ≥ ξ2 − 4εω(ξ)2ε ≥|ξ|2 − 2ε (for 1 ≥ ε > 0) in (66), to avoid a singularity in ξ = 0, and then scaling out \(\sqrt {\Omega (K)+\lambda }\), we obtain by rearrangement

$$ \begin{array}{@{}rcl@{}} (66) &\leq & ({\Omega}(K) + \lambda)^{\varepsilon} g^{2}\int\limits_{\mathbb{R}^{3}} \frac{\mathrm{d} \xi}{\xi^{3-2\varepsilon}\Big(\Big(\frac{p}{\sqrt{\Omega(K)+\lambda}}-\xi\Big)^{2} +1\Big)} \\ & \leq & ({\Omega}(K) + \lambda)^{\varepsilon} g^{2}{\int}_{\mathbb{R}^{3}} \frac{\mathrm{d} \xi}{\xi^{3-2\varepsilon}(\xi^{2} +1)}. \end{array} $$

For the second term in (65) we have

$$ \begin{array}{@{}rcl@{}} {\int}_{\mathbb{R}^{3}} |v(\xi)|^{2} \frac{2 |\xi||p|}{(\xi^{2} +\omega(\xi))((p-\xi)^{2} +{\Omega}(k) +\omega(\xi)+ \lambda)} \mathrm{d} \xi\\ \leq g^{2} {\int}_{\mathbb{R}^{3}} \frac{2 |p|}{\xi^{2}((p-\xi)^{2} +\lambda)} \mathrm{d} \xi. \end{array} $$

Scaling by |p|≠ 0 then yields

$$ {\int}_{\mathbb{R}^{3}} \frac{2 |p|}{\xi^{2}((p-\xi)^{2} + \lambda)} \mathrm{d} \xi \leq {\int}_{\mathbb{R}^{3}}\frac{2}{\xi^{2}(\frac{p}{|p|}-\xi)^{2}}\mathrm{d} \xi=C, $$

where C is independent of p since the last integral is invariant by rotations. Combining these bounds proves the claim. □

Lemma A.3 (cf. [19, Lem.3.8])

There is C > 0 so that the inequality

$$ \| T_{{\mathrm{od}}, \lambda} \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}} \leq C \| \mathrm{d} {\Gamma}(\omega)^{1/2} \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}} $$

holds for all λ > 0 and n ∈ ℕ.


We may write

$$ \begin{array}{@{}rcl@{}} T_{{\mathrm{od}}, \lambda} \psi\ensuremath{^{(n)}}(K)= -\sum\limits_{j=1}^{n} {\int}_{\mathbb{R}^{3}}\frac{v(k_{j})\omega(\xi)^{1/2}\psi\ensuremath{^{(n)}}(\hat K_{j}, \xi)}{(L_{P}(K, \xi) + \lambda)^{1/2} \omega(k_{j})^{1/2}} \frac{v(\xi)\omega(k_{j})^{1/2}\mathrm{d} \xi}{(L_{P}(K, \xi) + \lambda)^{1/2} \omega(\xi)^{1/2}}. \end{array} $$

By the Cauchy-Schwarz inequality we obtain

$$ \begin{array}{@{}rcl@{}} |T_{{\mathrm{od}}, \lambda} \psi\ensuremath{^{(n)}}(K)|^{2} &\leq &\left( \sum\limits_{j=1}^{n} {\int}_{\mathbb{R}^{3}}\frac{|v(k_{j})|^{2} \omega(\xi)|\psi\ensuremath{^{(n)}}(\hat K_{j}, \xi)|^{2} }{(L_{P}(K, \xi) + \lambda) \omega(k_{j})}\mathrm{d} \xi \right) \end{array} $$
$$ \begin{array}{@{}rcl@{}} & &\times \left( \sum\limits_{\ell=1}^{n} {\int}_{\mathbb{R}^{3}}\frac{|v(\eta)|^{2}\omega(k_{\ell})}{(L_{P}(K, \eta) + \lambda) \omega(\eta)}\mathrm{d} \eta\right). \end{array} $$

The factor (72) is bounded by (writing \({\Omega }(K)={\sum }_{j=1}^{n} \omega (k_{j})\) and \(p=P-{\sum }_{j=1}^{n} k_{j}\))

$$ \begin{array}{@{}rcl@{}} && \sum\limits_{\ell=1}^{n} {\int}_{\mathbb{R}^{3}}\frac{|v(\eta)|^{2}\omega(k_{\ell})}{(L_{P}(K, \eta) + \lambda) \omega(\eta)}\mathrm{d} \eta \\ &&\leq g^{2} \sum\limits_{\ell=1}^{n} \omega(k_{\ell}) ({\Omega}(K)+\lambda)^{-1/2} {\int}_{\mathbb{R}^{3}}\frac{1}{\Big(\Big(\frac{p}{\sqrt{\Omega(K)+\lambda}} - \eta\Big)^{2} +1\Big) |\eta|^{2}}\mathrm{d} \eta\\ &&\leq g^{2} \sum\limits_{\ell=1}^{n} \omega(k_{\ell}) {\Omega}(K)^{-1/2} {\int}_{\mathbb{R}^{3}}\frac{1}{(\eta^{2} +1) |\eta|^{2}}\mathrm{d} \eta, \end{array} $$

where in the final step we have used the Hardy-Littlewood inequality on the integral.

We use this to estimate the integral of |Tod,λψ(n)(K)|2 over K and obtain

$$ \begin{array}{@{}rcl@{}} && \| T_{{\mathrm{od}}, \lambda} \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}}^{2} \\ & \leq& C \sum\limits_{j, \ell=1}^{n}{\int}_{\mathbb{R}^{3n}} {\int}_{\mathbb{R}^{3}}\frac{ \omega(k_{\ell}) {\Omega}(K)^{-1/2} |v(k_{j})|^{2} \omega(\xi)|\psi\ensuremath{^{(n)}}(\hat K_{j}, \xi)|^{2} }{(L_{P}(K, \xi) + \lambda) \omega(k_{j})} \mathrm{d} \xi \mathrm{d} K. \end{array} $$

By renaming the variables kj = η, ξ = kj in the j-th integral, and using the symmetry of ψ(n), this becomes

$$ \begin{array}{@{}rcl@{}} \| T_{{\mathrm{od}}, \lambda} \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}}^{2} &\leq& \sum\limits_{j\neq \ell=1}^{n}{\int}_{\mathbb{R}^{3n}} {\int}_{\mathbb{R}^{3}}\frac{\omega(k_{\ell}) {\Omega}(\hat K_{j}, \eta)^{-1/2} |v(\eta)|^{2} \omega(k_{j})|\psi\ensuremath{^{(n)}}(K)|^{2} }{(L_{P}(K, \eta) + \lambda) \omega(\eta)} \mathrm{d} \eta \mathrm{d} K \end{array} $$
$$ \begin{array}{@{}rcl@{}} &&+ \sum\limits_{j=1}^{n}{\int}_{\mathbb{R}^{3n}} {\int}_{\mathbb{R}^{3}}\frac{ {\Omega}(\hat K_{j}, \eta)^{-1/2} |v(\eta)|^{2} \omega(k_{j})|\psi\ensuremath{^{(n)}}(K)|^{2} }{(L_{P}(K, \eta) + \lambda) } \mathrm{d} \eta \mathrm{d} K. \end{array} $$

The first term is bounded by

$$ \begin{array}{@{}rcl@{}} (75) &\leq & \sum\limits_{j=1}^{n} {\int}_{\mathbb{R}^{3n}} {\int}_{\mathbb{R}^{3}}\frac{\Omega(\hat K_{j})^{1/2} |v(\eta)|^{2} \omega(k_{j})|\psi\ensuremath{^{(n)}}(K)|^{2} }{(L_{P}(K, \eta) + \lambda) \omega(\eta)} \mathrm{d} \eta \mathrm{d} K \\ &\leq & C {\int}_{\mathbb{R}^{3n}} \sum\limits_{j=1}^{n} \omega(k_{j}){\Omega}(K)^{-1/2} {\Omega}(\hat K_{j})^{1/2}|\psi\ensuremath{^{(n)}}(K)|^{2} \mathrm{d} K \\ &\leq & C {\int}_{\mathbb{R}^{3n}} {\Omega}(K)|\psi\ensuremath{^{(n)}}(K)|^{2} \mathrm{d} K, \end{array} $$

by the same scaling argument as before.

By the same reasoning, the second term satisfies the bound

$$ \begin{array}{@{}rcl@{}} (76) &\leq &\sum\limits_{j=1}^{n}{\int}_{\mathbb{R}^{3n}} {\int}_{\mathbb{R}^{3}}\frac{ |v(\eta)|^{2} \omega(k_{j})|\psi\ensuremath{^{(n)}}(K)|^{2} }{(L_{P}(K, \eta) + \lambda) \omega(\eta)^{1/2} } \mathrm{d} \eta \mathrm{d} K \\ & \leq & C {\int}_{\mathbb{R}^{3n}} {\Omega}(K) |\psi\ensuremath{^{(n)}}(K)|^{2} \mathrm{d} K \end{array} $$

and this proves the claim. □

Lemma A.4

There is a C > 0 so that the inequality

$$ \| S_{{\mathrm{od}}, \lambda} \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}} \leq C \| \mathrm{d} {\Gamma}(\omega)^{1/2} \psi\ensuremath{^{(n)}} \|_{\mathscr{H}_{P}\ensuremath{^{(n)}}} $$

holds for all λ > 0 and n ∈ ℕ.


As τ+,λ(K) ≥ 0, the proof is identical to that of Lemma A.3. □

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Lampart, J. The Resolvent of the Nelson Hamiltonian Improves Positivity. Math Phys Anal Geom 24, 2 (2021).

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  • Nelson model
  • Energy renormalization
  • Positivity improving operators

Mathematics Subject Classification (2010)

  • 81Txx
  • 46N50