The aim is to find
u1,
u2,
u3, and
u4 which are located at the interior points
P12,
P22,
P11, and
P21, respectively.
We need to construct
4 equations since we have
4 unknows. Now if we apply the central difference equation to the point
P11, we get the first equation:
u21+u12+u01+u10−4u11=0
u21=u4,
u12=u1, and
u11=u3 are some of the points that you want to find while
u10=P10 is one of the boundary points that you have already found. (
P10=1)
You don't understand how boundary conditions are set to find the boundary points? Is that what you mean by your question? If yes, let us find again the boundary point
P10. I think that this will be a good example to understand the idea.
P10=P(h,0)=P(31,0)= ?
Now look at the boundary conditions. Does it tell us anything when
x=31? No. Does it tell us anything when
y=0? Yes. Then,
P10=1.