Density susceptibility
= Density susceptibility
{title2=$\chi_\rho(q)=\delta\rho_q/\delta\mu_q$}
The static <density susceptibility> is the linear density response to a chemical-potential perturbation. In the quadratic uniform contact <Bose gas>, $\chi_\rho(q)=[g+q^2/(4m\rho_0)]^{-1}$. Its inverse is a density stiffness. At zero <wavevector> $\partial\rho/\partial\mu=1/g$ at mean field, while the isothermal <compressibility> includes the additional factor $1/\rho^2$.