Hi:
I have come across a PDE of the following form in my research:
C_1 \alpha(t,r) + C_2 \partial_t \alpha(t,r) + C_3 \partial_r \alpha(t,r) + C_4 \beta(t,r) + C_2 \partial_t \beta(t,r) + C_3 \partial_r \beta(t,r) = 0
where the coefficients C_i are all functions of t and r: C_i =...