I was working on a problem in another discussion and I noticed something strange about pdesolve: it appears to assume a boundary condition.
The figure below shows the same equation for a fixed bed reactor solved with two sets of boundary conditions. Because the equation is second order in x, there should be two b.c. on x. The solution on the right for c2 only has one, yet the results are nearly identical to the c solution on the left.
If I change the outlet derivative b.c. for the c system, the results do change. This implies that the second boundary condition is active if supplied.
Also, I have changed the systems so that the first b.c. for both is merely the concentration at x=0. Both systems get solutions that are nearly identical again.
Can anyone confirm my deduction that pdesolve is assuming a missing b.c.?