How is this formula for the speed of sound derived?

[Superfluid universe]

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.

 

[Superfluid universe]

Could you please help me with the formula?
Thank you. :)

 

[stevendaryl]

Could you please help me with the formula?
Thank you. :)

It's basically derived here: https://upload.wikimedia.org/wikipe...o_Physical_Chemistry_Lecture_5_Supplement.pdf

They're using a slightly different formulation, but I think that they are equivalent.

 

[Superfluid universe]

Thank you! :)