Maxwell's equations
Here  is the 4-current,  is the field strength tensor  (written as a 4 × 4 matrix),  is the Levi-Civita symbol, and  is the 4-gradient (so that  is the d'Alembertian operator).  (The  in the first equation is implicitly summed over, according to Einstein notation.) The first tensor equation says the same thing as the two inhomogeneous Maxwell's equations: Gauss' law and Ampere's law with Maxwell's correction.  The second equation say the same thing as the other two equations, the homogeneous equations: Faraday's law of induction and the absence of magnetic monopoles.