Limiting Pointwise Decay for the compressible isentropic Navier-Stokes equations

Abstract: We study the long-time pointwise behavior of small localized perturbations of a constant state for the one-dimensional compressible isentropic Navier-Stokes equations. After subtracting the two Burgers diffusion waves, the convergent sum of all higher-order diffusion waves, and the cross-family viscous corrections, we prove a cone-preserving pointwise estimate for the exact physical remainder and, in particular, [ |R_i(x,t)|\leq C E_N\log(2+t)Ψi(x,t), \qquad \sup{x\in\mathbb R}Ψi(x,t)\leq C(1+t)^{-1}. ] Here (E_N) measures the size of the initial data and (Ψ_i) is the cone-resolved weight; both are defined precisely in the main theorem below. Thus $$|R_i(t)|</em>{L^\infty}\leq C E_N(1+t)<sup>{-1}\log(2+t)$$. The key new idea is to apply a familywise Cole--Hopf transformation to the spatial antiderivative of the remainder, which exactly eliminates the critical same-family first-order feedback. We further construct an approximate Green function adapted to the two characteristic families and combine it with Gaussian-mode extraction and a Kawashima-type energy argument. This yields a cone-preserving estimate at the limiting decay rate, up to a logarithmic loss.