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A note on relativistic kinetic Schauder estimates

Weinan Wang

math.AParXiv:2608.03126

Abstract

In this note, motivated by DW26, we construct counterexamples to momentum-only Schauder estimates for linear kinetic equations with relativistic transport. The construction exploits a structural compatibility between the nonlinear momentum-to-velocity map and a single fixed pair of smooth diffusion and drift coefficients: after conjugation, the diffusion becomes the Laplacian and all induced first-order terms cancel. Thus the obstruction is realized for one fixed operator, uniformly elliptic on every bounded momentum set, rather than through a frequency-dependent family of coefficients. We further show that the failure persists with zero external forcing by constructing uniformly positive stationary solutions for which the hidden spatial oscillation is encoded in a bounded zeroth-order coefficient whose momentum Hölder seminorm tends to zero. Consequently, the full Lorentzian Hölder control used in HSTT cannot in general be replaced by momentum-only Hölder control.

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