In superconductors, scattered electrons cover the entire surface of the Fermi sphere (circle in the figure, valency = 3). In the MP scheme in the article concerned, the shaded wedge confines coverage, causing errors in results.

The Monkhorst–Pack scheme is applied in a recent article in (2019 Supercond. Sci. Technol. 32 125013) implying exclusion of the umklapp phonons. But in a superconductor scattered electrons can land anywhere on the Fermi surface so that umklapp phonons must be included. The authors should take care to explain if and how the original Monkhorst–Pack scheme was modified to accommodate umklapp phonons for electron-phonon scattering.

The Wheeler equation, for electromagnetic disturbances in a gravitational field, was found by Fiziev to have exact solutions both above and below the event horizon, in the form of waves propagating both inwardly and outwardly. This observation can be interpreted and verified from the optical point of view, entirely on the basis of the Schwarzschild metric for length contraction and time dilation, in order to derive a differential version of Snell's law for the Schwarzschild black hole. It reveals interesting physics, including the correct amount of light deflection by the Sun, internal and external Oppenheimer-Snyder cones of the black hole, properties of its phonon sphere and the conclusion that light-rays are kept below the horizon by length contraction and time dilation rather than deflection.

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With a generic model for the electron-phonon spectral density, two simple expressions are derived to estimate the transition temperature and gap-to-temperature ratio in conventional superconductors. They entail that on average the numerical value of the phonon exchange factor, λ, is limited to 2.67, so that room temperature superconductivity may be attained only with a Debye temperature of about 1800 K or higher, in materials that may or may not involve hydrogen. They also show that a Be-Pb alloy may become a superconductor at ∼44 K.