I am reading the paper *Nodal sets of eigenfunctions of the Laplacian on Surfaces* by Donnelly and Fefferman available [here](https://www.ams.org/journals/jams/1990-03-02/S0894-0347-1990-1035413-2/S0894-0347-1990-1035413-2.pdf). I have a problem understanding Lemma 5.10. To my understanding, what follows is the content of the Lemma 5.10


Let $\mathbb{D}$ be the unit disc in $\mathbb{R}^2$ centered at the origin. Fix $m \in \mathbb{N}$. Then there is a constant $C\_m >0$ such that for any smooth $g :\mathbb{D} \rightarrow \mathbb{R}$ satisfying the following condition
\begin{align\*}
\max\_{|\alpha| \leq m} \left| \frac{\partial^{\alpha}g}{\partial z^{\alpha}}(0) \right| \geq 1
\end{align\*}
there are a pair of perpendicular directions $v\_1$ and $v\_2$ and some $k \leq m$ (here $v\_1, v\_2$ and $k$ can possibly depend on the function $g$) such that the following holds
\begin{align\*}
|\partial\_{v\_1}^{k}g(0)| \geq C\_m~\mbox{and}~|\partial\_{v\_2}^{k}g(0)| \geq C\_m
\end{align\*}


Here are my questions regarding this Lemma.


1. Firstly, is my understanding of the statement of Lemma 5.10 correct? More specifically, is it claimed that there is a universal constant $C\_m$ (independent of the function $g$)?
2. If the answer to my first question is Yes, I cannot really see how the proof provided in the paper establishes existence of the universal constant $C\_m$.


Thanks in advance!



Lemma from Donnelly-Fefferman's paper
$\newcommand{\al}{\alpha}
\newcommand{\G}{\mathcal{G}}
\newcommand{\PP}{\mathcal{P}}$
You are missing condition (ii) in formula (5.9) in that paper. That formula is 
\begin{equation}
\begin{aligned}
&\text{(i)}\quad \sup\_{|\al|\le c\_4} \Big| \frac{\partial^{\al}g}{\partial z^{\al}}(0) \Big|\ge b\_6, \\ 
&\text{(ii)}\quad \sup\_{|z|<1/2}\sup\_{|\al|\le c\_4+1} \Big| \frac{\partial^{\al}g}{\partial z^{\al}}(z)\Big|\le b\_7. 
\end{aligned}
\tag{5.9} 
\end{equation}
It appears that, in the paper, the $a\_i$'s, $b\_i$'s, and $c\_i$'s denote positive real constants, which may depend only on such constants previously mentioned in the paper. 


Then Lemma 5.10 there seems to say that for some nonnegative integer $k\le c\_4$ 
and some perpendicular unit vectors $v\_1$ and $v\_2$ we have
\begin{align\*}
|\partial\_{v\_1}^{k}g(0)|\ge b\_{11}~\mbox{and}~|\partial\_{v\_2}^{k}g(0)|\ge b\_{11}. 
\end{align\*}


This is indeed true, with $b\_{11}>0$ depending only on $c\_4,b\_6,b\_7$. 
Indeed, let $\G$ be the set of all smooth functions $g$ satisfying (5.9), and let $\PP$ be the set of the Taylor polynomials (at $0$) of order $\lfloor c\_4\rfloor$ of the functions $g\in\G$. The set $\PP$ is obviously compact in the natural topology on $\PP$. 
So, there exists 
\begin{align}
 \mu&:=\min\_{g\in\G}\max\_{k,v\_1,v\_2}(|\partial\_{v\_1}^{k}g(0)|\wedge|\partial\_{v\_2}^{k}g(0)|) \\
& =\min\_{P\in\PP}\max\_{k,v\_1,v\_2}(|\partial\_{v\_1}^{k}P(0)|\wedge|\partial\_{v\_2}^{k}P(0)|)\in[0,\infty),
\end{align}
where $\max\_{k,v\_1,v\_2}$ is over all nonnegative integers $k\le c\_4$, 
and all perpendicular unit vectors $v\_1$ and $v\_2$. 


Let $P$ be a minimizer for $\min\_{P\in\PP}$. By condition (5.9)(i), $P$ is a nonzero polynomial of degree $\le c\_4$ and hence has a a nonvanishing term of order $k\le c\_4$. So, $\partial\_v^{k}P(0)=0$ only for finitely many unit vectors $v$. So, we can find perpendicular unit vectors $v\_1$ and $v\_2$ such that $|\partial\_{v\_1}^{k}P(0)|\wedge|\partial\_{v\_2}^{k}P(0)|>0$. 
Thus, $\mu>0$. It remains to let $b\_{11}:=\mu>0$, which depends only on $c\_4,b\_6,b\_7$. 


