Comparison results for the $p$-torsional rigidity on convex domains
Cristian Enache, Mihai Mihailescu, Denisa Stancu-Dumitru
Abstract
For each open, bounded and convex domain $Ω\subset \mathbb{R}^{D},$ $D\geq 2$, and each real number $p>1,$ we denote by $u_{p}$ the $p$\emph{-torsion function} on $Ω$, i.e. the solution of the \emph{torsional creep problem} $Δ_{p}u=-1$ in $Ω$, $u=0$ on $\partial Ω$, where $Δ_{p}u:=\operatorname{div}( \left\vert \nabla u\right\vert ^{p-2}\nabla u) $ is the $p$-Laplacian. Let $T_p(Ω)$ be the $p$\emph{-torsional rigidity} on $Ω$, defined as $T_{p}\left( Ω\right) :=\int_{Ω}u_{p}dx$. Define $T\left( p;Ω\right) :=\left\vert Ω\right\vert ^{p-1}T_{p}\left( Ω\right) ^{1-p}$, where $|Ω|$ stands for the Lebesgue measure of $Ω$. The main purpose of this paper is to compare the values of $T(p;Ω)$ for bounded convex domains having different inradii. We prove that for any $0<a<b$ there exists a constant $γ_{D,p}\in[1/D,1)$, depending only on the dimension $D$ and the parameter $p$, such that $T(p;Ω_b)\leq T(p;Ω_a)$, for all $ Ω_a\in\PP^D(a)$, and $Ω_b\in\PP^D(b)$, if and only if $γ_{D,p}b\geq a$, where $\PP^D(r)$ denotes the family of convex bounded domains in $\mathbb{R}^D$ of inradius $r$. In addition, we discuss the asymptotic equality case, the limiting regimes $p\rightarrow 1^+$ and $p\rightarrow\infty$, and the sharpness of our bounds on model families such as rectangles, orthotopes, ellipses, and triangles}. We also derive a Saint-Venant type comparison result under additional geometric constraints, as a direct consequence of our main theorem.