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Oct 9, 2018 at 16:13 comment added user126920 Let me make clear that I haven't read the paper very carefully. There could be a much better explanation for why you don't need to worry about the $C^3$ norm of the boundary.
Oct 9, 2018 at 16:11 comment added user126920 This sort of approximation is commonly left implicit, which one could argue is a bit sketchy, but on the other hand, it's tedious to write down and no new ideas are involved. Having said that, I agree the requirement that each $\Omega_k$ be convex makes things somewhat tricky.
Oct 9, 2018 at 12:55 comment added xiaocha123 However, in my opinion, the author did not mention definitely what are the smooth exhausting domains used by himself (maybe I made the mistake here). Hence, in my opinion, it seems that the approximating process in the paper is different from that is presented in Gilbarg-Trudinger's book (see Chapter 8, probably, Theroem 8.34). Precisely, the approximating Dirichlet problems (55) in the paper are still defined on $\Omega$, rather than on certain smooth and strictly convex exhausting domains of $\Omega$ itself. That is the part which confuses me. Thanks!
Oct 9, 2018 at 12:45 comment added xiaocha123 But there is still a problem confusing me. Firstly, I shall present my own understanding: the author verified that the approximating functions $f_{\epsilon,\rho}$ satisfy Condition (C) near the boundary $\partial \Omega$ such that one can obtain the approximating Dirichlet problems (55) by letting $\rho \rightarrow 0^+$, that is, $$ \det (D^2 u_\epsilon)=f_\epsilon \mbox{ in } \Omega, \mbox{ } u_\epsilon =0 \mbox{ on } \partial\Omega.$$
Oct 9, 2018 at 12:07 comment added xiaocha123 @Stanley Snelson Thanks very much for your helpful comment! I believe that the approximation procedure you informed me is a good approach.
Oct 8, 2018 at 15:20 comment added user126920 The actual steps of the approximation procedure would be very similar to the one for linear elliptic equations, which you can find in Chapter 6 of Gilbarg-Trudinger. (I don't have the book in front of me, so I can't give a more precise reference.)
Oct 8, 2018 at 15:07 comment added user126920 The key point is that the estimates don't depend quantitatively on the $C^3$ regularity of the boundary, so you can approximate $\Omega$ with smoother $\Omega_k$ whose boundaries converge in the $C^{2,1}$ norm, and take the limit. You can probably do both approximations at once (letting $\varepsilon = 1/k$) or to keep it simpler, you could do the entire proof as written for $C^3$ domains, get a priori estimates, and finally do the domain approximation.
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