  The probability density function of the random flight with isotropic initial
conditions is obtained by an expansion in the number of collisions and the in
the spatial harmonics of the solution, as in a Fourier series. The method holds
for any dimension and is worked out in detail for the three dimensional case.
In this case the probability density functions conditional to 1 and 2
collisions are also found using a different method, which yields them in terms
of elementary functions and the polylogarithm function Li$_2$. The latter
method is exact in the sense that one does not have to truncate a series, as in
the first method. This provides a reference to decide where to truncate the
series. A link is provided to a web page where the reader may download the
series truncated at 132 collisions; for times larger than 100 times the average
inter-collision time, the Gaussian approximations is used. The case in which
the initial condition is a particle moving along a fixed direction is briefly
considered.
