Variational $p$-harmonious functions: existence and convergence to $p$-harmonic functions

TL;DR

This paper introduces a variational approach to $p$-harmonious functions, establishing existence, uniqueness, and convergence of solutions to the $p$-Laplace equation using a game-theoretic mean value property.

Contribution

It develops a new variational operator $oldsymbol{ ext{μ}}_p^oldsymbol{ ext{ε}}$, proves existence and uniqueness of associated functions, and shows their convergence to the $p$-harmonic solution.

Findings

Existence and uniqueness of variational $p$-harmonious functions with boundary data.

The family of variational solutions converges uniformly to the $p$-harmonic solution.

The operator $oldsymbol{ ext{μ}}_p^ ext{ε}$ has key properties like continuity and monotonicity.

Abstract

In a recent paper, the last three authors showed that a game-theoretic $p$-harmonic function $v$ is characterized by an asymptotic mean value property with respect to a kind of mean value $\nu_p^r[v](x)$ defined variationally on balls $B_r(x)$. In this paper, in a domain $\Om\subset\RR^N$, $N\ge 2$, we consider the operator $\mu_p^\ve$, acting on continuous functions on $\ol{\Om}$, defined by the formula $\mu_p^\ve[v](x)=\nu^{r_\ve(x)}_p[v](x)$, where $r_\ve(x)=\min[\ve,\dist(x,\Ga)]$ and $\Ga$ denotes the boundary of $\Omega$. We first derive various properties of $\mu^\ve_p$ such as continuity and monotonicity. Then, we prove the existence and uniqueness of a function $u^\ve\in C(\ol{\Om})$ satisfying the Dirichlet-type problem: $$ u(x)=\mu_p^\ve[u](x) \ \mbox{ for every } \ x\in\Om,\quad u=g \ \mbox{ on } \ \Ga, $$ for any given function $g\in C(\Ga)$. This result holds, if we assume the existence of a suitable notion of barrier for all points in $\Ga$. That $u^\ve$ is what we call the \textit{variational} $p$-harmonious function with Dirichlet boundary data $g$, and is obtained by means of a Perron-type method based on a comparison principle. \par We then show that the family $\{ u^\ve\}_{\ve>0}$ gives an approximation scheme for the viscosity solution $u\in C(\ol{\Om})$ of $$ \De_p^G u=0 \ \mbox{ in }\Om, \quad u=g \ \mbox{ on } \ \Ga, $$ where $\De_p^G$ is the so-called game-theoretic (or homogeneous) $p$-Laplace operator. In fact, we prove that $u^\ve$ converges to $u$, uniformly on $\ol{\Om}$ as $\ve\to 0$.