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Ari Aiolfi

Publications and source records attributed to Ari Aiolfi.

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The Dirichlet problem for the minimal surface equation on unbounded helicoidal domains of $\mathbb{R}^{m}$

We consider a helicoidal group $G$ in $\mathbb{R}^{n+1}$ and unbounded $G$-invariant $C^{2,\alpha}$-domains $\Omega\subset\mathbb{R}^{n+1}$ whose helicoidal projections are exterior domains in $\mathbb{R}^{n}$, $n\geq2$. We show that for all $s\in\mathbb{R}$, there exists a $G$-invariant solution $u_{s}\in C^{2,\alpha}\left( \overline{\Omega}\right) $ of the Dirichlet problem for the minimal surface equation with zero boundary data which satisfies $\sup_{\partial\Omega}\left\vert \operatorname{grad}u_{s}\right\vert =\left\vert s\right\vert $. Additionally, we provide further information on the behavior of these solutions at infinity.

math.DG

On the existence of foliations by solutions to the exterior Dirichlet problem for the minimal surface equation

Given an exterior domain $Ω$ with $C^{2,α}$ boundary in $\mathbb{R}^{n}$, $n\geq3$, we obtain a $1$-parameter family $u_γ\in C^{\infty}\left(Ω\right) $, $\left\vert γ\right\vert \leqπ/2$, of solutions of the minimal surface equation such that, if $\left\vert γ\right\vert <π/2$, $u_γ\in C^{\infty}\left( Ω\right) \cap C^{2,α}\left( \overlineΩ\right) $, $u_γ|_{\partialΩ}=0$ with $\max_{\partialΩ}\left\Vert \nabla u_γ\right\Vert =\tanγ$ and, if $\left\vert γ\right\vert =π/2$, the graph of $u_γ$ is contained in a $C^{1,1}$ manifold $M_γ\subset\overlineΩ\times\mathbb{R}$ with $\partial M_γ=\partialΩ$. Each of these functions is bounded and asymptotic to a constant \[ c_γ=\lim_{\left\Vert x\right\Vert \rightarrow\infty}u_γ\left( x\right) . \] The mappings $γ\rightarrow u_γ\left( x\right) $ (for fixed $x\inΩ$) and $γ\rightarrow c_γ$ are strictly increasing and bounded. The graphs of these functions foliate the open subset of $\mathbb{R}^{n+1}$ \[ \left\{ \left( x,z\right) \inΩ\times\mathbb{R}\text{, }-u_{π/2}\left( x\right) <z<u_{π/2}\left( x\right) \right\} . \] Moreover, if $\mathbb{R}^{n}\backslashΩ$ satisfies the interior sphere condition of maximal radius $ρ$ and if $\partialΩ$ is contained in a ball of minimal radius $\varrho$, then \[ \left[ 0,σ_{n}ρ\right] \subset\left[ 0,c_{π/2}\right] \subset\left[ 0,σ_{n}\varrho\right] , \] where \[ σ_{n}=\int_{1}^{\infty}\frac{dt}{\sqrt{t^{2\left( n-1\right) }-1}}. \] One of the above inclusions is an equality if and only if $ρ=\varrho$, $Ω$ is the exterior of a ball of radius $ρ$ and the solutions are radial.

math.DG

A Moser/Bernstein type theorem in a Lie group with a left invariant metric under a gradient decay condition

We say that a PDE in a Riemannian manifold $M$ is geometric if,$\ $whenever $u$ is a solution of the PDE on a domain $Ω$ of $M$, the composition $u_ϕ:=u\circϕ$ is also solution on $ϕ^{-1}\left( Ω\right) $, for any isometry $ϕ$ of $M.$ We prove that if $u\in C^{1}\left( \mathbb{H}^{n}\right) $ is a solution of a geometric PDE satisfying the comparison principle, where $\mathbb{H}^{n}$ is the hyperbolic space of constant sectional curvature $-1,$ $n\geq2,$ and if \[ \limsup_{R\rightarrow\infty}\left( e^{R}\sup_{S_{R}}\left\Vert \nabla u\right\Vert \right) =0, \] where $S_{R}$ is a geodesic sphere of $\mathbb{H}^{n}$ centered at fixed point $o\in\mathbb{H}^{n}$ with radius $R,$ then $u$ is constant. Moreover, given $C>0,$ there is a bounded non-constant harmonic function $v\in C^{\infty }\left( \mathbb{H}^{n}\right) $ such that \[ \lim_{R\rightarrow\infty}\left( e^{R}\sup_{S_{R}}\left\Vert \nabla v\right\Vert \right) =C. \] The first part of the above result is a consequence of a more general theorem proved in the paper which asserts that if $G$ is a non compact Lie group with a left invariant metric, $u\in C^{1}\left( G\right) $ a solution of a left invariant PDE (that is, if $v$ is a solution of the PDE on a domain $Ω$ of $G$, the composition $v_{g}:=v\circ L_{g}$ of $v$ with a left translation $L_{g}:G\rightarrow G,$ $L_{g}\left( h\right) =gh,$ is also solution on $L_{g}^{-1}\left( Ω\right) $ for any $g\in G),$ the PDE satisfies the comparison principle and% \[ \limsup_{R\rightarrow\infty}\left( \sup_{g\in B_{R}}\left\Vert \operatorname*{Ad}\nolimits_{g}\right\Vert \sup_{S_{R}}\left\Vert \nabla u\right\Vert \right) =0, \] where $\operatorname*{Ad}\nolimits_{g}:\mathfrak{g}\rightarrow\mathfrak{g}$ is the adjoint map of $G$ and $\mathfrak{g}$ the Lie algebra of $G,$ then $u$ is constant.

math.DG

On the Size of Minimal Surfaces in $\mathbb{R}^4$

The Gauss map $g$ of a surface $Σ$ in $\mathbb{R}^4$ takes its values in the Grassmannian of oriented 2-planes of $\mathbb{R}^4$: $G^+(2,4)$. We give geometric criteria of stability for minimal surfaces in $\mathbb{R}^4$ in terms of $g$. We show in particular that if the spherical area of the Gauss map $|g(Σ)|$ of a minimal surface is smaller than $2π$ then the surface is stable by deformations which fix the boundary of the surface.This answers a question of Barbosa and Do Carmo in $\mathbb{R}^4$.

math.DG