Two-dimensional representations of the electron density distribution

Generally, in order to apply the concepts we have just described, simplified representations are used for the triple Fourier series expressing the electron density at a given point of the crystal. 

Fourier sections are cross-sections of the distribution p(x,y,z), at a given altitude z. They are expressed as follows  

Although these Fourier maps are two-dimensional representations, calculating them requires for all of the structure factors to be known. 

Bragg [BRA 66] imagined a second two-dimensional representation of a crystal s stmcture. This time, it consists of a projection, parallel to the vector c, onto the basis plane defined by the vectors a and b, of the electron density in the cell defined by the triplet (a, b, c). 

If A is the area of side ab and, as we defined previously, if the cell s volume is Vc = Nabc, then  

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