Dear all

How to avoid in plot "matrix in singular" in my pde system

> a1 := -2;
                                     -2
> b1 := 3;
                                      3
> c1 := -1;
                                     -1
> L := 15;
                                     15
> b := .25;
                                    0.25
> th := 1.5;
                                     1.5
> pde := diff(u(x, t), x, x, x, x, t...