If the influence of the physical processes in the simulation region is large
enough to reach boundary, then we have to consider the interaction of
this influence and outer environment. At this time, the boundary will change according to
the result of this interaction. A simple treatment of this case is to look on the
boundary as a linear continuous boundary. Assuming that *f*(*x*_{b}) is the
boundary value,
the value of the inner point adjacent to *x*_{b},
the value of the outer
point adjacent to *x*_{b}. Then for a linear boundary condition we can have

(56) |

Using the newly found we can easily continue our simulation on the boundary.