Test resolution convergence for Jeans instability including drag forces. The dashed line denotes the analytic solution for only gas. The solid line shows the analytic solution for 128 fluids including gas and dust species with a linear distribution from t _s = 10⁻⁴ to 10 and uniform density distribution with a total dust-to-gas ratio ϵ ⁰ = 0.01. The open circles indicate that the numerical solutions for different numbers of cells converge to the solid line. The vertical dotted line indicates ﹩k/{k}_{{\rm{J}}}=1/\sqrt{1+{\epsilon }^{0}}﹩.