摘要：For a number of diffusive processes involving heat and mass
transfer, a convenient and easy way to solve for penetration time or
depth is to consider an averaged quantity called mean action time.
This approach was originally developed by Alex McNabb, in
collaboration with other researchers. It is possible to solve for
mean action time without actually solving the full diffusion
problem, which may be nonlinear, and may have internal moving
boundaries. Mean action time satisfies a linear Poisson equation,
and only works for finite problems. We review some nice properties
of mean action time, and discuss some recent novel applications.
