Secular resonant dressed orbital diffusion I : method and WKB limit for
  tepid discs
  The equation describing the secular diffusion of a self-gravitating
collisionless system induced by an exterior perturbation is derived while
assuming that the timescale corresponding to secular evolution is much larger
than that corresponding to the natural frequencies of the system. Its two
dimensional formulation for a tepid galactic disc is also derived using the
epicyclic approximation. Its WKB limit is found while assuming that only
tightly wound transient spirals are sustained by the disc. It yields a simple
quadrature for the diffusion coefficients which provides a straightforward
understanding of the loci of maximal diffusion within the disc.
