Given an electric field E and a magnetic field B defined on a common region of spacetime, the Riemann–Silberstein vector is F = E + i c B , where c is the speed of light, with some authors preferring to multiply the right hand side by an overall constant ε ε 0 / 2 , where ε 0 is the permittivity of free space. It is analogous to the electromagnetic tensor F, a 2-vector used in the covariant formulation of classical electromagnetism.