article published yes Davide Desio author ea10a57b-23f6-11ef-9085-80d8596d52ef0000-0001-9840-3809 Robert Seiringer author 4AFD0470-F248-11E8-B48F-1D18A9856A870000-0002-6781-0521 RoSe department GradSch department We present an abstract Dyson expansion for perturbations that are merely relatively form-bounded, and apply it to the polaron problem. For a large class of polaron-type models, including the Fröhlich and Nelson models, we prove that the vacuum expectation value of the heat semi-group is a completely monotone function of the square of the total momentum. Consequently, the ground-state energy is a concave function of the square of the momentum, a result recently proved for the Fröhlich model in [14] using a probabilistic approach via Wiener integrals. https://research-explorer.ista.ac.at/download/22639/22641/2026_LettersMathPhysics_Desio.pdf application/pdfno Springer Nature2026 eng 1573-0530 2512.1344310.1007/s11005-026-02107-2 1164 D. Desio, R. Seiringer, Letters in Mathematical Physics 116 (2026). Desio D, Seiringer R. Dyson expansion for form-bounded perturbations and applications to the polaron problem. <i>Letters in Mathematical Physics</i>. 2026;116(4). doi:<a href="https://doi.org/10.1007/s11005-026-02107-2">10.1007/s11005-026-02107-2</a> Desio D, Seiringer R. 2026. Dyson expansion for form-bounded perturbations and applications to the polaron problem. Letters in Mathematical Physics. 116(4), 87. Desio, Davide, and Robert Seiringer. “Dyson Expansion for Form-Bounded Perturbations and Applications to the Polaron Problem.” <i>Letters in Mathematical Physics</i>. Springer Nature, 2026. <a href="https://doi.org/10.1007/s11005-026-02107-2">https://doi.org/10.1007/s11005-026-02107-2</a>. Desio, D., &#38; Seiringer, R. (2026). Dyson expansion for form-bounded perturbations and applications to the polaron problem. <i>Letters in Mathematical Physics</i>. Springer Nature. <a href="https://doi.org/10.1007/s11005-026-02107-2">https://doi.org/10.1007/s11005-026-02107-2</a> D. Desio and R. Seiringer, “Dyson expansion for form-bounded perturbations and applications to the polaron problem,” <i>Letters in Mathematical Physics</i>, vol. 116, no. 4. Springer Nature, 2026. Desio, Davide, and Robert Seiringer. “Dyson Expansion for Form-Bounded Perturbations and Applications to the Polaron Problem.” <i>Letters in Mathematical Physics</i>, vol. 116, no. 4, 87, Springer Nature, 2026, doi:<a href="https://doi.org/10.1007/s11005-026-02107-2">10.1007/s11005-026-02107-2</a>. 226392026-08-03T13:21:14Z2026-08-04T05:57:21Z