Consider the sequence $\{\lambda_n(\Omega)\}$ of eigenvalues of the Dirichelet $p$Laplacian in a bounded domain $\Omega \subset \mathbb{R}^N$ obtained via the Lusternik–Schnirelmann minmax approach. Let $\varphi_n$ be an eigenfunction associated to $\lambda_n(\Omega)$. We are interested in the estimates for the number of nodal domains of $\varphi_n$ which we denote as $\mu(\varphi_n)$.
In the linear case $p=2$, the wellknown Courant nodal domain theorem says that $\mu(\varphi_n) \leq n$ for all $n \geq 1$. Its generalization to the nonlinear case $p \neq 2$ obtained in [^{1}] asserts that
which implies
On the other hand, in the linear case $p=2$, there is a result of Pleijel [^{2}] on the following asymptotic refinement of the Courant nodal domain theorem:
\begin{equation}\label{eq:P} \limsup_{n \to \infty} \frac{\mu(\varphi_n)}{n} \leq \frac{4}{j_{0,1}^2} = 0.69166\ldots, \end{equation}
see, e.g., this post for a discussion.
The aim of the present post is to generalize the result of Pleijel to the $p$Laplacian settings. Pleijel’s approach is purely variational and consists of two main ingredients: the FaberKrahn inequality and the Weyl law.
 The FaberKrahn inequality is easily available for the $p$Laplacian, and it can be formulated as
where $B_1$ is a unit ball in $\mathbb{R}^N$; see, e.g., the discussion here. Therefore, noting that $\lambda_n(\Omega) = \lambda_1(\Omega_i)$ for any $i=1..\mu(\varphi_n)$ where $\Omega_i$ is a nodal domain of $\varphi_n$, we get
Equivalently,
\begin{equation}\label{eq:FKP} \mu(\varphi_n) \leq \frac{\Omega \lambda_n(\Omega)^\frac{N}{p}}{B_1 \lambda_1(B_1)^\frac{N}{p}}. \end{equation}
 The Weyl law is used to estimate $\lambda_n(\Omega)$ in \eqref{eq:FKP} in terms of $n$. Unfortunately, this law is not available for the $p$Laplacian in the required form; see the discussion in [^{3}] and [^{4}]. Instead, we will obtain the simplest explicit Weyltype upper bound for $\lambda_n(\Omega)$, and this will be enough to get a Pleijel’s type result. Let $Q_h$ stands for the $N$dimensional cube with the side length $h$. First, if $h \to 0$, then the number $m$ of cubes $Q_h$ disjointly inscribed in $\Omega$ is given by
Second, by the variational characterization of $\lambda_n(\Omega)$, we can estimate
where $h_n$ is such that there are $n$ disjoint cubes $Q_{h_n}$ inscribed in $\Omega$. We can assume that $h_n$ is maximal.
Third, we know that
Combining the previous three facts, we get
\begin{equation}\label{eq:WP} \lambda_n(\Omega) \leq \lambda_1(Q_{h_n}) = \lambda_1(Q_1) h_n^{p} \approx \lambda_1(Q_1) \left(\frac{n}{\Omega}\right)^\frac{p}{N} \quad \text{as } n \to \infty. \end{equation}
Finally, mixing \eqref{eq:FKP} and \eqref{eq:WP}, we deduce that
\begin{equation}\label{eq:Plp} \boxed{\limsup_{n \to \infty} \frac{\mu(\varphi_n)}{n} \leq \frac{1}{B_1} \left(\frac{\lambda_1(Q_1)}{\lambda_1(B_1)} \right)^\frac{N}{p}.} \end{equation}
Notice that this upper bound does not depend on $\Omega$. Below, we will discuss a possible way how to improve this bound.
All we need now is to get a ‘‘good’’ upper bound for $\lambda_1(Q_1)$ and a ‘‘good’’ lower bound for $\lambda_1(B_1)$.
Let us start with an upper bound for $\lambda_1(Q_1)$. From Proposition 2.7 of [^{5}] we know that
and
where
As lower estimates for $\lambda_1(B_1)$, we use the estimate
see [^{6}] or [^{7}]; and
see [^{8}] and, in general, this post for a discussion of lower bounds.
Thus, substituting all these things into \eqref{eq:Plp}, we get
\begin{equation}\label{eq:Pp<2} \limsup_{n \to \infty} \frac{\mu(\varphi_n)}{n} \leq \frac{\Gamma\left(\frac{N}{2}+1\right)\pi^\frac{N}{2} 2^N (p1)^N}{p^\frac{(2p1)N}{p} \sin(\pi /p)^N} \quad \text{for} \quad p<2 \end{equation}
and
\begin{equation}\label{eq:Pp>2} \limsup_{n \to \infty} \frac{\mu(\varphi_n)}{n} \leq \frac{\Gamma\left(\frac{N}{2}+1\right)\pi^\frac{N}{2} 2^N N^\frac{(p2)N}{2p} (p1)^\frac{N}{p}}{p^\frac{(p+1)N}{p} \sin(\pi /p)^N} \quad \text{for} \quad p>2. \end{equation}
The corresponding plot is depicted below by the increasing line. We see that these upper bounds does not give us a Pleijel constant smaller than $1$ even in the dimension $N=2$, which is quite sad. Note that if $p \to 1$, then the bound \eqref{eq:Pp<2} approaches $\frac{4}{\pi}=1.2732\dots$, while if $p \to \infty$, then the bound \eqref{eq:Pp>2} approaches $\frac{8}{\pi}=2.5464\dots$, see the blue line on figure below.
Let us now discuss a possible improvement of \eqref{eq:Plp} which concerns an improvement of the Weyltype upper bound. For simplicity, let us fix $N=2$. First, we can inscribe in $\Omega$ not a square tiling, but a hexagonal tiling. If $H_r$ stands for a hexagon with the inradius $r$, and if $r \to 0$, then the number $m$ of $H_r$’s disjointly inscribed in $\Omega$ is given by
Therefore, analogously to \eqref{eq:WP} we get
and hence, from \eqref{eq:FKP},
\begin{equation}\label{eq:Plp1} \boxed{\limsup_{n \to \infty} \frac{\mu(\varphi_n)}{n} \leq \frac{2\sqrt{3}}{B_1} \left(\frac{\lambda_1(H_1)}{\lambda_1(B_1)} \right)^\frac{2}{p}.} \end{equation}
Noting that $B_1 \subset H_1$, we get $\lambda_1(H_1) \leq \lambda_1(B_1)$, which yields
Moreover, if $p \to \infty$, then, by the known result from [^{9}], $\lambda_1(H_1)^\frac{1}{p} \to 1$ and $\lambda_1(B_1)^\frac{1}{p} \to 1$, i.e., this upper estimate of the upper estimate \eqref{eq:Plp1} is sharp for $p \to \infty$. See the green line on the figure above.
Thus, unfortunately, even if $n \to \infty$, we cannot show that $\mu(\varphi_n) \leq n$ for all $p>1$ without getting a substantial improvement of the Weyltype upper bound for $\lambda_n(\Omega)$. Such an improvement is clearly a prominent problem which needs to be studied much closer.
The PDFversion of this post can be found here.
Last modified: 18Feb20
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