Note that we need two indices ( j and k) to determine the direction of a single particle. In
this respect, a couple (i, j) is taken to label one particle. Note also that, while the coordinates
x and z depend on both j and k indices, the y-coordinate depends only on the index j.
In the reference where the origin corresponds to the quantity (xH +V0 cos(α)τ, 0, 0), we
may write