Friedel's law, named after Georges Friedel, is a property of Fourier transforms of real functions.
, its Fourier transform has the following properties.
are called Friedel's pairs.
is antisymmetric: Friedel's law is used in X-ray diffraction, crystallography and scattering from real potential within the Born approximation.
Note that a twin operation (a.k.a.
Opération de maclage) is equivalent to an inversion centre and the intensities from the individuals are equivalent under Friedel's law.