For completeness, the tensor F μ μ ν ν = ∂ ∂ μ μ A ν ν − − ∂ ∂ ν ν A μ μ is the Maxwell tensor, also known as the electromagnetic field strength, U ( 1 ) field strength or more geometrically the curvature of the U ( 1 ) connection A μ μ . The four-vector gauge field A μ μ is also known as the four-potential.