Regularity of the extremal solutions associated to elliptic systems

Abstract

We examine the elliptic system given by \begin{eqnarray*} \qquad \left\{ \begin{array}{lcl} -Δu =λf(v) \quad \mbox{ in } Ω -Δv =γf(u) \quad \mbox{ in } Ω, u=v =0, \quad \mbox{ on } \pOm \end{array}\right. \end{eqnarray*} where $λ,γ$ are positive parameters, $Ω$ is a smooth bounded domain in $\IR^N$ and $f$ is a $C^{2}$ positive, nondecreasing and convex function in $[0,\infty)$ such that $\frac{f(t)}{t}\rightarrow\infty$ as $t\rightarrow\infty$. Assuming $$0<τ_{-}:=\liminf_{t\rightarrow\infty} \frac{f(t)f"(t)}{f'(t)^{2}}\leq τ_{+}:=\limsup_{t\rightarrow\infty} \frac{f(t)f"(t)}{f'(t)^{2}}\leq 2,$$ we show that the extremal solution $(u^*, v^*)$ associated to the above system is smooth provided\\ $N<\frac{2α_{*}(2-τ_{+})+2τ_{+}}{τ_{+}}\max\{1,τ_{+}\}$, where $α_{*}>1$ denotes the largest root of the $2^{nd}$ order polynomial $$P_{f}(α,τ_{-},τ_{+}):=(2-τ_{-})^{2} α^{2}- 4(2-τ_{+})α+4(1-τ_{+}).$$ As a consequences, $u^*, v^*\in L^\infty(Ω)$ for $N<5$. Moreover, if $τ_{-}=τ_{+}$, then $u^*, v^*\in L^\infty(Ω)$ for $N<10$.