  Rules are given for determining special directions in the Brillouin zone
which optimize the descrip-tion of various physical quantities with Gamma1 type
symmetry. We consider the cubic, hexagonal, tetragonal and trigonal (e.g. Bi)
lattice. These rules allow us to construct, in all of momentum space,
quantities which have the full symmetry of the Brillouin zone such as the Fermi
surface, momentum density and others, from the knowledge of these quantities
for a limited number of spe-cial directions. These results can also be used for
determining the projections which should be measured either in Compton
scattering or in positron annihilation experiments in order to recon-struct
properly the electron momentum density. Such a treatment is approximately
equivalent to applying Gaussian quadrature in calculations of the radial
functions that occur in the expansion of data into lattice harmonics.
