Preface: Special issue dedicated to Professor Chaojiang Xu's 65th Birthday (Part I)

On Some Properties of the Curl Operator and Their Consequences for the Navier-Stokes System
Nicolas Lerner & François Vigneron

Dirichlet Eigenvalue Problem of Degenerate Elliptic Operators with Non-Smooth Coefficients
Hua Chen, Hong-Ge Chen & Jin-Ning Li

Subelliptic Harmonic Maps with Values in Metric Spaces of Nonpositive Curvature
Yaoting Gui, Jürgen Jost & Xianqing Li-Jost

First Order Hardy Inequalities Revisited
Xia Huang & Dong Ye

Global-in-Time $L^p$−$L^q$ Estimates for Solutions of the Kramers-Fokker-Planck Equation
Xue-Ping Wang & Lu Zhu

Zero Viscosity-Diffusivity Limit for the Incompressible Boussinesq Equations in Gevrey Class
Feng Cheng

Gevrey Well-Posedness of the Hyperbolic Prandtl Equations
Wei-Xi Li & Rui Xu