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For example, consider the third boundary value problem: \begin{align} &\frac{\partial u}{\partial t}-\sum_{i,j=1}^n a_{ij}(x,t) \frac{\partial^2 u}{\partial x_i \partial x_j} + \sum_{i=1}^n b_i(x,t) \frac{\partial u}{\partial x_i} + c(x,t) u = f(x,t), \quad (x,t) \in Q_T \\ &\frac{\partial u}{\partial \mathbf{n}} + b(x,t) u = g(x,t), \quad x \in \partial \Omega, \ 0<t \leq T, \\ &u(x,0)=u_0(x), \quad x \in \Omega, \end{align} where $Q_T=\Omega \times (0,T]$, $0<T<+\infty$, $\Omega ⊂ \mathbb{R}^n$ is open and bounded, and $\partial \Omega \in C^2$. Suppose $a_{ij} \in C(\overline{Q_T})$, $a_{ij}=a_{ji}$ $(i,j=1,\dots,n)$, and there exist positive constants $\lambda$ and $\Lambda$, s.t. \begin{equation} \lambda |\xi|^2 \leq \sum_{i,j=1}^n a_{ij}(x,t) \xi_i \xi_j \leq \Lambda |\xi|^2, \quad \forall (x,t) \in \overline{Q_T}, \ \xi \in \mathbb{R}^n. \end{equation} Suppose also that $b_i, c \in L^\infty(Q_T)$ $(i=1,\dots,n)$, $f \in L^p(Q_T)$ $(1<p<+\infty)$, $b \in C^{1,1/2}(\partial \Omega \times [0,T])$ and is nonnegative, $g \in W^{2,1}_p(Q_T)$, $u_0 \in W^2_p(\Omega)$, and the compatibility condition \begin{equation} \frac{\partial u_0(x)}{\partial \mathbf{n}}+b(x,0) u_0(x)=g(x,0), \quad x \in \partial \Omega \end{equation} is satisfied. It is asserted in the lecture note that the problem admits a unique solution $u \in W^{2,1}_p(Q_T)$, and the following $L^p$ estimate holds: \begin{equation} \|u\|_{W^{2,1}_p(Q_T)} \leq C (\|f\|_{L^p(Q_T)} + \|g\|_{W^{2,1}_p(Q_T)} + \|u_0\|_{W^2_p(\Omega)}), \end{equation} where $C$ is a positive constant which only depends on $n,\lambda,\Lambda,\Omega,T,\|b_i\|_{L^\infty(Q_T)},\|c\|_{L^\infty(Q_T)},\|b\|_{C^{1,1/2}(\partial \Omega \times [0,T])}$ and the continuous modulus of $a_{ij}$.

The question is, is $C$ monotonically increasing with respect to $n,\lambda,\Lambda,T,\|b_i\|_{L^\infty(Q_T)}$, etc.? The lecture note I referred omits the proof and I haven't found the proof elsewhere, therefore I have no idea how the constant $C$ depends on those parameters exactly. Can anyone give me some hints or tell me where I can find the proof? Thank you very much.

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    $\begingroup$ Usually it is proved that $C$ stays bounded when $\lambda^{-1}, \Lambda, T, \|b_i\| \dots \le M$. If you define $C(M)$ as the best constant for which the estimate hold with the constraint $M$, this is increasing in $M$. $\endgroup$ Commented Apr 4, 2021 at 9:51
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    $\begingroup$ This has been posted on (and recieved an Answer) on MSE math.stackexchange.com/questions/4088765/… $\endgroup$ Commented Apr 4, 2021 at 10:29
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    $\begingroup$ @GiorgioMetafune Thank you very much! I posted this question because the validity of an outcome in my thesis depends on the monotonicity of $C$, but inspired by your comment, I find it is enough to assure the validity provided $C$ is locally bounded. I will still try to work through the proof and explicitly keep track of the constants involved as it is a great exercise, as is mentioned in the answer math.stackexchange.com/a/4088860/709713. $\endgroup$
    – Wentao Hu
    Commented Apr 4, 2021 at 16:06

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