\documentclass[reqno]{amsart} \usepackage{hyperref} \AtBeginDocument{{\noindent\small {\em Electronic Journal of Differential Equations}, Vol. 2006(2006), No. 31, pp. 1--7.\newline ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu \newline ftp ejde.math.txstate.edu (login: ftp)} \thanks{\copyright 2006 Texas State University - San Marcos.} \vspace{9mm}} \begin{document} \title[\hfilneg EJDE-2006/31\hfil Multiplicity of solutions] {Multiplicity of solutions for a quasilinear problem with supercritical growth} \author[G. M. Figueiredo\hfil EJDE-2006/31\hfilneg] {Giovany M. Figueiredo} \address{Giovany M. Figueiredo \newline Universidade Federal do Par\'{a}\\ CEP 66.075-110, Bel\'{e}m-Par\'{a}, Brazil} \email{giovany@ufpa.br} \date{} \thanks{Submitted October 26, 2005. Published March 16, 2006.} \subjclass[2000]{35A15, 35H30, 35B40} \keywords{Variational method; penalization method; Moser iteration method} \begin{abstract} The multiplicity and concentration of positive solutions are established for the equation $$ -\epsilon^{p}\Delta_{p}u+V(z)|u|^{p-2}u=|u|^{q-2}u +\lambda|u|^{s-2}u \quad\text{in }\mathbb{R}^N, $$ where $10$, $p 0, \quad\text{for } z \in \mathbb{R}^N, \end{gathered} \end{equation} $\epsilon > 0$, $p < q < p^{*} \leq s$, $p^{*}=\frac{Np}{N-p}$, $\lambda \geq 0$ and $\Delta_{p}u$ is the p-Laplacian operator; that is, $$ \Delta_{p}u=\sum_{i=1}^{N}{\frac{\partial}{\partial{x_{i}}} \Big(|\nabla{u}|^{p-2}\frac{\partial{u}}{\partial{x_{i}}}\Big)}. $$ We assume that $V$ is a continuous function satisfying \begin{equation} V(x)\geq V_{0}=\inf_{x \in \mathbb{R}^{N}} V(x)> 0\quad \mbox{for } x\in \mathbb{R}^{N}; \label{V1} \end{equation} Also assume that there exists an open and bounded domain $\Omega \subset \mathbb{R}^{N}$ such that \begin{equation} V_0 < \min_{\partial\Omega}V. \label{V2} \end{equation} In recent years, much attention has been paid to the existence and multiplicity of solutions for both subcritical and critical cases and to the concentration behavior of solutions for problem \begin{equation} -\epsilon^{2}\Delta u +V(z)u=f(u) \quad\text{in } \mathbb{R}^{N}, \label{P*} \end{equation} when $\epsilon$ is small. Interesting results may be found, for example, in \cite{Alves8,Bartsch,Benci1,Chabr1,Cingolani,Gui,Rabinow} and their references. Cingolani \& Lazzo \cite{Lazzo1}, using Lusternik-Schnirelman category and involving the sets \begin{gather*} M=\{x\in \Omega : V(x)=V_{0}\}, \\ M_{\delta}=\{x\in \mathbb{R}^{N}:dist(x,M)\leq \delta \}, \quad \delta>0, \end{gather*} showed a result of multiplicity of positive solutions for \eqref{P*}, where $\Omega=\mathbb{R}^{N}$, $f(u)=|u|^{q-2}u$ with $q \in (2, 2^{*})$, and \begin{equation} V_{\infty}=\liminf_{|x|\to\infty}V(x)>V_{0}=\inf_{\mathbb{R}^{N}}V(x)>0. \label{V} \end{equation} Recall that for a closed subset $Y$ of a topological space $X$, the Lusternik-Schnirelman category, denoted by $\mathop{\rm cat}_{X}Y$, is the least number of closed and contractible sets in $X$ which cover $Y$. Alves \& Souto \cite{Alves10} showed an existence and concentration result for \eqref{P*} with $f(u)=u^{q-1}+u^{2^{*}-1}$ assuming that condition \eqref{V} holds. Alves \& Figueiredo \cite{AlvesGio1} (see also \cite{giovany}) proved a multiplicity result for \begin{equation} -\epsilon^{p}\Delta_{p}u+V(z)|u|^{p-2}u=f(u) \ \ in \ \ \mathbb{R}^N \label{P**} \end{equation} using again Lusternik-Schinirelman category and assuming that condition \eqref{V} holds, $2\leq p0$, there exists $\overline{\epsilon}=\overline{\epsilon}(\delta)>0$ and $\lambda_{0}>0$ such that \eqref{Plamb} has at least $\mathop{\rm cat}_{M_{\delta}}M$ positive solutions for all $\epsilon\in(0,\overline{\epsilon})$ and for all $\lambda \in [0,\lambda_{0}]$. Moreover, if $u_{\epsilon}$ is a positive solution of \eqref{Plamb} and $\eta_{\epsilon}\in \mathbb{R}^{N}$ a global maximum point of $u_{\epsilon}$, then $$ \lim_{\epsilon\to 0}V(\eta_{\epsilon})=V_{0}. $$ \end{theorem} To solve problem \eqref{Plamb}, we first consider a truncated problem which involves only a subcritical Sobolev exponent. We show that any positive solution of truncated problem is a positive solution of \eqref{Plamb}. Hereafter, we will work with the following problem equivalent to \eqref{Plamb}, which is obtained under change of variable $z=\epsilon x$ \begin{equation} \label{Plambst} \begin{gathered} -\Delta_{p}u + V(\epsilon x)|u|^{p-2}u=|u|^{q-2}u + \lambda|u|^{s-2}u \quad\text{in } \mathbb{R}^N \\ u \in W^{1,p}(\mathbb{R}^N) \quad\text{with } 1 0,\quad \forall x \in \mathbb{R}^{N} . \end{gathered} \end{equation} \section{Truncated Problem} First of all, we have to note that because $f$ has supercritical growth we cannot use directly variational techniques because of the lack of compactness of the Sobolev immersions. So we construct a suitable truncation of $f$ in order to use variational methods or more precisely, the Mountain Pass Theorem. This truncation was used in \cite{rabino} (see also \cite{Chabr} and \cite{giovany}). Let $K>0$, be a constant to be determined later, and $\widehat{f}_{K}:\mathbb{R} \to \mathbb{R}$ given by \[ \widehat{f}_{K}(t) = \begin{cases} 0 &\text{if } t < 0 \\ t^{q-1}+\lambda t^{s-1} &\text{if } 0 \leq t < K \\ (1+\lambda K^{s - q})t^{q-1} &\text{if } t\geq K. \end{cases} \] Consider $\alpha,\gamma\in\mathbb{R}$ such that $\alpha<1<\gamma$ and $\eta \in C^{1}([\alpha K ,\gamma K])$ with $\alpha$ and $\gamma$ independent of $K$ and $\eta$ satisfying \begin{gather*} \eta(t)\leq\widehat{f}_{K}(t)\quad \mbox{for all } t\in [\alpha K ,\gamma K], \\ \eta(\alpha K)=\widehat{f}_{K}(\alpha K),\quad \eta(\gamma K)=\widehat{f}_{K}(\gamma K), \\ \eta'(\alpha K)=\widehat{f}'_{K}(\alpha K), \quad \eta'(\gamma K)=\widehat{f}'_{K}(\gamma K), \\ t\mapsto\frac{\eta(t)}{t^{p-1}} \mbox{ is increasing for all }t\in [\alpha K,\gamma K]. \end{gather*} Now using the functions $\eta$ and $\widehat{f}_{K}$, we define \[ f_{K}(t) =\begin{cases} \eta(t) &\text{if } t \in [\alpha K ,\gamma K], \\ \widehat{f}_{K}(t) &\text{if } t \not\in [\alpha K ,\gamma K] \end{cases} \] and the truncated problem \begin{equation} \label{Tlamb} \begin{gathered} -\Delta_{p}u + V(\epsilon x)|u|^{p-2}u = f_{K}(u) \\ u \in W^{1,p}(\mathbb{R}),\quad u > 0 \quad\text{in } \mathbb{R}^{N}. \end{gathered} \end{equation} It is easy to check that $f_{K} \in C^{1}(\mathbb{R})$, and that \begin{gather*} f_{K}(t)=0,\quad \mbox{for all } t < 0, \label{fK1} \\ f_{K}(t)\leq(1+\lambda K^{s-q})t^{q-1}\quad \mbox{for all } t\geq 0,\label{fK2} \\ F_{K}(t)\leq\frac{1}{q}(1+\lambda K^{s-q})t^{q}\quad \mbox{for all } t\geq 0, \quad F_{K}(t)=\int^{t}_{0}f_{K}(\xi)d\xi, \label{fK3} \end{gather*} there exists $\theta \in \mathbb{R}$ such that $p<\theta$ and \begin{equation} 0 < \theta F_{K}(t) \leq f_{K}(t)t \quad \mbox{for all } t>0, \label{fK4} \end{equation} the function \begin{gather} t\mapsto \frac{f_{K}(t)}{t^{p-1}} \mbox{ is increasing for all } t>0, \label{fK5} \\ f'_{K}(t)t^{2}-(p-1)f_{K}(t)t\geq (q -p )t^{q}. \label{fK6} \end{gather} \begin{remark}\label{final} \rm Note that if $u_{\epsilon,\lambda}$ is a positive solution of \eqref{Tlamb} such that there exists $K_{0} > 0$, where for each $K \geq K_{0}$, there exists $\lambda_{0}(K)>0$ such that $|u_{\epsilon,\lambda}|_{L^{\infty}(\mathbb{R}^{N})}\leq \alpha K$ for all $\epsilon \in (0, \bar{\epsilon})$ and for all $\lambda \in [0, \lambda_{0}]$, then $u_{\epsilon,\lambda}$ is a positive solution of \eqref{Plambst}. \end{remark} \section{Multiplicity and Concentration of positive solutions for Truncated Problem} The result below is related to the multiplicity and concentration of solutions for \eqref{Tlamb} and its proof can be found in \cite[Theorem 1.1]{AlvesGio2} or \cite{giovany}. \begin{theorem}\label{super11} Suppose that $V$ verify \eqref{V1}\eqref{V2}. Then, for any $\delta>0$, there exists $\overline{\epsilon}=\overline{\epsilon}(\delta,\lambda,K)>0$ such that $(T_\lambda)$ has at least $\mathop{\rm cat}_{M_{\delta}}M$ positive solutions for all $\epsilon\in(0,\overline{\epsilon})$ and for each $\lambda>0$. Moreover, if $u_{\epsilon,\lambda}$ is a positive solution of \eqref{Tlamb} and $\eta_{\epsilon}\in \mathbb{R}^{N}$ a global maximum point of $u_{\epsilon,\lambda}$, then $$ \lim_{\epsilon\to 0}V(\eta_{\epsilon})=V_{0}. $$ \end{theorem} \section{Multiplicity of positive solutions for \eqref{Plambst}} We recall that the weak solutions of \eqref{Tlamb} are the critical points of the functional $$ I_{\epsilon,\lambda}(u)=\frac{1}{p}\int_{\mathbb{R}^{N}}|\nabla u|^{p}+ \frac{1}{p}\int_{\mathbb{R}^{N}}V(\epsilon x)|u|^{p}-\int_{\mathbb{R}^{N}}F_{K}(u), $$ which is well defined for $u \in W_{\epsilon}$, where $$ W_{\epsilon}=\{u \in W^{1,p}(\mathbb{R}^{N}):\int_{\mathbb{R}^{N}}V(\epsilon x)|u|^{p}<\infty\} $$ endowed with the norm $$ \|u\|^{p}_{\epsilon}= \int_{\mathbb{R}^{N}}|\nabla u|^{p}+ \int_{\mathbb{R}^{N}}V(\epsilon x)|u|^{p}. $$ Let us also denote by $E_{V_{0},\lambda}$ the energy functional associated to the problem \begin{equation} \label{PV0lamb} \begin{gathered} -\Delta_{p}u + V_{0}|u|^{p-2}u = f_{K}(u) \\ u \in W^{1,p}(\mathbb{R}),\quad u > 0 \ in \ \mathbb{R}^{N}, \end{gathered} \end{equation} that is, $$ E_{V_{0},\lambda}(u) =\frac{1}{p}\int_{\mathbb{R}^{N}}|\nabla u|^p + \frac{1}{p}\int_{\mathbb{R}^{N}}V_{0}|u|^p - \int_{\mathbb{R}^N}F_{K}(u), $$ Here we will establish a preliminary estimative for $\|u_{\epsilon,\lambda}\|_{\epsilon}$. \begin{lemma}\label{super2} For any solution $u_{\epsilon,\lambda}$ of \eqref{Tlamb}, there exists $\bar{C}>0$, such that \begin{eqnarray*} \|u_{\epsilon,\lambda}\|_{\epsilon}\leq \bar{C}, \end{eqnarray*} for $\epsilon > 0$ sufficiently small and uniformly in $\lambda$. \end{lemma} \begin{proof} By \cite[Theorem 1.1]{AlvesGio2} (see \cite{giovany} too), we have that all solutions $u_{\epsilon,\lambda}$ from \eqref{Tlamb} verify the inequality \[ I_{\epsilon,\lambda}(u_{\epsilon,\lambda})\leq c_{V_{0},\lambda}+ h_{\lambda}(\epsilon), \] where $c_{V_{0},\lambda}$ is the level Mountain Pass related of functional $E_{V_{0},\lambda}$ and $h_{\lambda}(\epsilon)\to 0$ as $\epsilon\to 0$ for each $\lambda \geq 0$. In this case, we may suppose that \begin{eqnarray*} I_{\epsilon,\lambda}(u_{\epsilon,\lambda})\leq c_{V_{0},\lambda}+ 1, \end{eqnarray*} for all $\epsilon\in (0,\bar{\epsilon}(K,\lambda))$. Since $c_{V_{0},\lambda}\leq c_{V_{0},0}$, we have \begin{equation} \label{G} I_{\epsilon,\lambda}(u_{\epsilon,\lambda})\leq c_{V_{0},0}+ 1, \end{equation} for all $\epsilon\in (0,\bar{\epsilon}(K,\lambda))$ and for all $\lambda \geq 0$. Moreover, \begin{align*} I_{\epsilon,\lambda}(u_{\epsilon,\lambda}) &=I_{\epsilon,\lambda}(u_{\epsilon,\lambda}) - \frac{1}{\theta}I'_{\epsilon,\lambda}(u_{\epsilon,\lambda}) u_{\epsilon,\lambda}\\ &=\big(\frac{1}{p}-\frac{1}{\theta}\big) \|u_{\epsilon,\lambda}\|^{p}_{\epsilon}+ \int_{\mathbb{R}^{N}}\big[\frac{1}{\theta}f_{K}(u_{\epsilon,\lambda})u_{\epsilon,\lambda} -F_{K}(u_{\epsilon,\lambda})\big]. \end{align*} By \eqref{fK4}, \[ I_{\epsilon,\lambda}(u_\epsilon,\lambda)\geq \big(\frac{1}{p}-\frac{1}{\theta}\big)\|u_{\epsilon,\lambda}\|^{p}_{\epsilon} \] \noindent Therefore, by \eqref{G}, $\|u_{\epsilon,\lambda}\|_{\epsilon}\leq \bar{C}$, for $\epsilon\in (0,\bar{\epsilon}(K,\lambda))$ and for all $\lambda \geq 0$, where $$ \bar{C}=\Big[(c_{V_{0},0}+1)\big(\frac{\theta p}{\theta - p}\big)\Big]^{1/p}. $$ \end{proof} Now, we use the Moser iteration technique \cite{moser} (see also \cite{Chabr}) to prove that each solution found of \eqref{Tlamb} is a solution of \eqref{Plambst} \begin{proof}[Proof of Theorem \ref{thmD}] We use the notation $u_{\epsilon,\lambda}:=u$. For each $L>0$, we define \begin{gather*} u_{L} =\begin{cases} u &\text{if } u \leq L, \\ L &\text{if } u \geq L, \end{cases}\\ z_{L} = u_{L}^{p(\beta - 1)}u \quad \text{and} \quad w_{L} = uu_{L}^{\beta-1} \end{gather*} with $\beta > 1$ to be determined later. Taking $z_{L}$ as a test function, we obtain \begin{align*} \int_{\mathbb{R}^N}u_{L}^{p(\beta -1)}|\nabla u|^{p} &= -p(\beta -1)\int_{\mathbb{R}^N}u_{L}^{p\beta - p-1}u|\nabla u|^{p-2}\nabla u \nabla u_{L}\\ &\quad + \int_{\mathbb{R}^N}f_{K}(u)uu_{L}^{p(\beta -1)}- \int_{\mathbb{R}^N}V({\epsilon x})|u|^{p}u_{L}^{p(\beta - 1)}. \end{align*} By \eqref{fK2}, \begin{equation} \label{w1} \int_{\mathbb{R}^N}u_{L}^{p(\beta -1)}|\nabla u|^{p} \leq C_{\lambda,K} \int_{\mathbb{R}^N}u^{q }u_{L}^{p(\beta - 1)}, \end{equation} where $C_{\lambda,K}= (1+\lambda K^{s-q})$. From Sobolev imbedding, H\"{o}lder inequalities and \eqref{w1}, \[ %\label{3eqb} |w_{L}|^{p}_{p^{*}}\leq C_{1}\beta^{p}C_{\lambda,K}\Big(\int_{\mathbb{R}^N}u^{p^{*}} \Big)^{(q-p)/p^{*}} \Big(\int_{\mathbb{R}^N}w_{L}^{pp^{*}/[p^{*}-(q-p)]} \Big)^{[p^{*}-(q-p)]/p^{*}}, \] where $p < \frac{pp^{*}}{p^{*}-(q-p)} < p^{*}$. Recalling that $\|u_{\epsilon,\lambda}\|_{\epsilon}\leq \bar{C}$, we have \[ |w_{L}|^{p}_{p^{*}} \leq C_{2}\beta^{p}C_{\lambda,K}\bar{C}^{(q-p)/p^*} |w_{L}|^{p}_{\alpha^{*}} \] where $\alpha^{*}=\frac{pp^{*}}{p^{*}-(q-p)}$. Note that if $u^{\beta}\in L^{\alpha^{*}}(\mathbb{R}^{N})$, using the definition of $w_{L}$ and the fact that $u_{L}\leq u$, we obtain \[ \Big(\int_{\mathbb{R}^{N}}\mid uu_{L}^{\beta - 1}\mid^{p^{*}}\Big)^{p/p^*} \leq C_{3}\beta^{p}C_{\lambda,K}\Big(\int_{\mathbb{R}^{N}}u^{\beta \alpha^{*}}\Big)^{p/\alpha^{*}}< + \infty. \] By Fatou's Lemma on the variable $L$, we get \begin{equation}\label{w3} |u|_{\beta p^{*}} \leq (C_{4}C_{\lambda,K})^{1/\beta }\beta^{1/\beta}|u|_{\beta \alpha^{*}}. \end{equation} The assertion is obtained by iteration of estimative \eqref{w3}. Namely, let $\chi=\frac{p^{*}}{\alpha^{*}}$; i.e., $p^{*}=\chi\alpha^{*}$. Then \[ |u|_{\chi^{(m+1)}\alpha^{*}} \leq C_{5}(C_{4}C_{\lambda,K})^{\sum_{i=1}^{m}\frac{\chi^{-i}}{p}} \chi^{\sum_{i=1}^{m}i\chi^{-i}}\bar{C}. \] Passing to the limit as $m \to \infty$, we have \[ |u|_{L^{\infty}(\mathbb{R}^{N})} \leq C_{5}(C_{4}C_{\lambda,K})^{\sigma_{1}}\chi^{\sigma_{2}}\bar{C}, \] where $\sigma_{1}=\sum_{i=1}^{\infty}\frac{\chi^{-i}}{p}$ and $\sigma_{2}=\sum_{i=1}^{\infty}i\chi^{-i}$. To choose $\lambda_{0}$, we consider the inequality \[ \Big[C_{4}(1+\lambda K^{s-q})\Big]^{\sigma_{1}}\chi^{\sigma_{2}}C_{5}\bar{C}\leq \alpha K. \] We conclude that \[ (1+\lambda K^{s-q})^{\sigma_{1}}\leq \frac{\alpha K C_{6}}{C_{4}^{\sigma_{1}}\chi^{\sigma_{2}}\bar{C}}. \] We choose $\lambda_{0}$ verifying the inequality \[ \lambda_{0}\leq \Big[\frac{(\alpha K C_{6})^{\frac{1}{\sigma_{1}}}}{C_{4}\chi^\frac{\sigma_{2}}{\sigma_{1}} \bar{C}^{1/\sigma_1}} -1\Big] \frac{1}{K^{s-q}} \] and fixing $K$ such that \[ \Big[\frac{(\alpha K C_{6})^{1/\sigma_1}}{C_{4} \chi^\frac{\sigma_{2}}{\sigma_{1}}\bar{C}^{1/\sigma_1}} -1\Big] > 0, \] we have $|u_{\lambda,\epsilon}|_{L^{\infty}(\mathbb{R}^{N})} \leq \alpha K$ for all $\epsilon \in (0,\bar{\epsilon}(K,\lambda))$ and all $\lambda \in [0,\lambda_{0}]$. The result follows from Remark \ref{final}. \end{proof} \subsection*{Acknowledgements} We would like to thank to Professor C. O. 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