Antisymmetry reduces these 16 equations to just six independent equations. Because it is usual to define F by F μ μ ν ν = η η μ μ α α F α α β β η η β β ν ν , the constitutive equations may, in vacuum, be combined with the Gauss–Ampère law to get: ∂ ∂ β β F α α β β = μ μ 0 J α α .