I How is this formula for the speed of sound derived?

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c² = (n/m) ∂²U/∂n²

where

U = vacuum energy density as a function of the quasiparticle density
n = quasiparticle number density
m = bare mass of quasiparticle

Is there a book, article where this formula is explained?
Thank you.
 
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Could you please help me with the formula?
Thank you. :)
 
Thank you! :)
 

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