We study the consistency between the formulas of the sound speed of the degenerate free electron gas at zero and finite temperature. Using the Sommerfeld expansion, we show that the expression of the sound speed of the degenerate free electron gas at finite temperature gives the expression of the sound speed of the degenerate free electron gas at zero temperature when the temperature tends to zero at fixed Fermi energy. The present and non-trivial work completes the previous results about the sound speed of the degenerate free electron gas at zero and finite temperature. It is also valid for the pure neutron or proton gas by considering the rest-mass of interest.
G. Faussurier (Thu,) studied this question.
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