Many thanks for your kind advice. Substitution needs concentration indeed! Best regards, ChG
You forgot the underscore v[rho_] =. Also, I would avoid defining rho0 = Subscript[rho, 0], because this is a function of rho, not a constant, as I suppose you want.
v[rho_] =
rho0 = Subscript[rho, 0]
rho
