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

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 $ν_p^r[v](x)$ defined variationally on balls $B_r(x)$. In this paper, in a domain $\Om\subset\RR^N$, $N\ge 2$, we consider the operator $μ_p^\ve$, acting on continuous functions on $\ol{\Om}$, defined by the formula $μ_p^\ve[v](x)=ν^{r_\ve(x)}_p[v](x)$, where $r_\ve(x)=\min[\ve,\dist(x,\Ga)]$ and $\Ga$ denotes the boundary of $Ω$. We first derive various properties of $μ^\ve_p$ 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: $$ u(x)=μ_p^\ve[u](x) \ \mbox{ for every } \ x\in\Om,\quad u=g \ \mbox{ on } \ \Ga, $$ 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 $\ve\to 0$.