Comment on “Fast-slow mode coupling instability for coasting beams in the presence of detuning impedance”

We show untenability of key points of the recent article of N. Biancacci, E. Metral, and M. Migliorati [Phys. Rev. Accel. Beams 23, 124402 (2020)], hereafter the article and the authors. The main Eqs. (23) are shown to be incorrect even as an approximation. The article claims the solution of this pair of equations to be in “excellent agreement” with the pyheadtail simulations for CERN PS, which is purportedly demonstrated by Fig. 6. Were it so, it would be either a signal of a mistake in the code or a consequence of a nonsmoothness of the simulations.