I am dealing with diffusion-reaction equation with three species. I have $L^2$ bound of concentrations. Now I want $L^2$ bound of gradient of concentrations. Somehow if I get $L^4$ or $L^\infty$ bound Of $u_i$, then I can solve my problem. Please help me.
The model is the following system of PDEs for the vector variable ($u_1, u_2, u_3$) $$ \begin{cases} \partial_t u_1- D_1 \Delta u_1=-k_fu_1u_2+k_b u_3+\theta_1 \\ \partial_t u_2- D_2 \Delta u_2=-k_fu_1u_2+k_b u_3+\theta_2 \\ \partial_t u_3- D_3 \Delta u_3=k_fu_1u_2-k_b u_3+\theta_3 \end{cases} $$ where
- $D_i(i=1,2,3), k_b, k_f$ are positive constants and
- $\theta_i(x,t)$ is a function in $L^2$.
Note.- I don't have positive solution of diffusion-reaction system.