Partial summing of infinite range of diagrams for the two-phonon massoperator of polaron described by Fr\"{o}hlich Hamiltonian is performed usingthe Feynman-Pines diagram technique. Renormalized spectral parameters of groundand first excited (phonon repeat) polaron state are accurately calculated for aweak electron-phonon coupling at $T=0$ K. It is shown that the strongerelectron-phonon interaction shifts the energy of both states into low-energyregion of the spectra. The ground state stays stationary and the excited onedecays at a bigger coupling constant.
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