Discrete form of 2-D forward modeling operator

The basic formula for the vertical component of the 2-D gravity field, (6.13), was derived at the beginning of Chapter 6  

As in the 3-D case, we now divide the domain F into the Nm small rectangular cells, Ffc, r = Uj jF , and assume that the density is constant within each cell, p r) = Pk r er,  

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