Let $\Delta$ be an open triangle in $\mathbb{R}^2$ and $u\in C^0(\overline{\Delta})\cap C^\infty(\Delta)$ be the convex function satisfying
$$
\det D^2u=1,\quad u|\_{\partial\Delta}=0.
$$
Classical results on Monge-Ampère equations imply that there exists a unique such $u$. I need informations on the boundary regularity of $u$, but could not find any literature on regularity of such Monge-Ampère equations near a boundary line segment.



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> > **Question 1.** What can be said about the boundary regularity of $u$? In particular, is $\|\nabla u(x)\|$ bounded as $x\in\Delta$ approaches a point on $\partial \Delta$ which is not a vertex?
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**Remarks.**


1. The second question is equivalent to asking whether $u$ has infinite slope at a non-vertex point $p\in\partial\Delta$, or more precisely, whether the limit
$$
\lim\_{t\rightarrow 0^+}\frac{u((1-t)p+tq)}{t}\in [-\infty,0)
$$
equals $-\infty$ for some $q\in\Delta$ (which implies the same limit for every $q\in \Delta$).
2. It can be shown $u$ has finite slope at a vertex (namely, the above limit is finite if $p\in\partial\Delta$ is a vertex): assuming $p=0$ w.l.o.g., by Comparison Principle, $u$ is minorized by a convex function $v$ of the form 
$$
v(x)=c|x|^\alpha-L(x),
$$
where the constant $\alpha\in (1,2)$ is arbitrary, $c>0$ is a constant to make $\det D^2v=c^2\alpha^2(\alpha-1)|x|^{2\alpha-4}$ bigger than $1$ on $\Delta$, and $L$ is a linear function on $\mathbb{R}^2$ to make $v|\_{\partial\Delta}\leq 0$; whereas $v$ has value $0$ and finite slope at $x=0$.
3. Attempting to apply Comparison Principle near non-vertex boundary points, one is lead to the following problem:



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> > **Question 2.** Let $D=\{x\in\mathbb{R}^2\mid |x|<1,\, x\_1>0\}$ be the half-disk. Is there a *nonnegative* convex function $u\in C^0(\overline{D})$ which vanishes on the boundary line segment $\{x\_1=0,\, |x\_2|\leq 1\}$, such that $\det D^2u\geq \lambda>0$?
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I tried to construct such a $u$ of the form $u(x\_1,x\_2)=f(x\_1)g(x\_2)$ (with $f$ and $g$ nonnegative, convex and $C^2$) but was lead to the conclusion that such $u$ can never satisfy the hypotheses.



Boundary regularity for the Monge-Ampère equation $\det D^2u=1$
The answer to both questions is no.


For Question $2$: After subtracting a function of the form $cx\_1$ we may assume that $u \geq 0$ and $u(t,0) = o(t)$. (Take $c$ to be the slope of the tangent line to $u(t,0)$ at $t = 0$; then $u - cx\_1 \geq 0$ on the $x\_1$ axis and vanishes on the $x\_2$ axis, so by convexity is nonnegative in $D$). Then $\{u < \epsilon\}$ contains the triangle $T\_{\epsilon}$ with vertices $\pm e\_2$ and $4l\_{\epsilon}e\_1$ where $\epsilon^{-1}l\_{\epsilon} \rightarrow \infty$ as $\epsilon \rightarrow 0$. It is straightforward to check that the paraboloid $P\_{\epsilon} = 2\epsilon(l\_{\epsilon}^{-2}(x\_1 - l\_{\epsilon})^2 + 4x\_2^2)$ satisfies $P\_{\epsilon} > \epsilon \geq u$ on $\partial T\_{\epsilon}$, vanishes at $(l\_{\epsilon},0)$, and $\det D^2P\_{\epsilon} \rightarrow 0$ as $\epsilon \rightarrow 0$. For $\epsilon$ small this violates the comparison principle.


Question $1$ can be reduced to Question $2$ as follows: if there exists a sequence of points approaching the flat boundary where the graph of $u$ has supporting planes of uniformly bounded slope, then in the limit we get a supporting plane to the graph of $u$ with bounded slope that vanishes on one of the edges. Then subtract the plane, translate, rotate and rescale.


However, near the flat boundary I suspect there is some expansion of $u$ in powers of "singular building blocks," maybe of the form $x\_1(-\log x\_1)^{1/2}$ and $x\_2$. (This is motivated by the observation that $w := x\_1(-\log x\_1)^{1/2}(x\_2^2-1)$ satisfies $0 < \lambda < \det D^2w < \Lambda$ in the domain $\{|x\_2| < c(-\log (x\_1))^{-1/2}\}$ near the origin, which has boundary that separates from the $x\_2$ axis slower than any polynomial).


