modified:   newtonsDiffusion.m

seeing convergence for V=1, but blowing up around V=10

plot shows that something is fundamentally wrong, even at V=5.  The plot range for \psi is too small, and doesn't match the boundary conditions.

Getting NaN's ... where could there be a division by zero?
restore proper sign for b.  I'd plotted f, not x, and that's why it seemed to be reversed.

modified:   newtonsDiffusion.m