When there is no heat generation in the medium, the finite difference equation
for an interior node further simplifies to Tnode (Tleft Ttop Tright
Tbottom)/4, which has the interesting interpretation that the temperature of each
interior node is the arithmetic average of the temperatures of the four neighboring
nodes. This statement is also true for the three-dimensional problems
except that the interior nodes in that case will have six neighboring nodes instead
of four.