 Harmonic coordinate

A harmonic coordinate satisfies ${\Delta }_g{x}^{\lambda }=0$ for a Riemannian metric, or ${\square }_g{x}^{\lambda }=0$ for a Lorentzian metric. Equivalently $g^{\mu \nu }{\Gamma }_{\mu \nu }^{\lambda }=0$ or ${\partial }_{\mu }(\sqrt{|\det g|}{g}^{\lambda \mu })=0$. This common coordinate condition applies in both Riemannian geometry and general relativity.