Fourier’s transform

I br the majority of physicaJ systems, the Lagrangian is invariant to any space translation. In this case, all the equations of the theory cam be simplified by means of Fourier s transformation, i.e. by considering the characteristic values in the momentum space. 1 he rules for drawing graphs of the characteristic values in the momentum space are given. 

In any case, as based on Fourier s transformations that connect the electronic density (as real measure) with the structure factor (as complex measure), the Parseval s theorem is formulated which stipulates that the mean square value on each side of the Fourier s transformations are proportional, i.e., it can be written like (Cantor Schimmel, 1980) ... 

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