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\begin{document}
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%\title[Elliptic equations with indefinite concave nonlinearity]{Existence and multiplicity of positive %solutions for nonlinear elliptic equations with indefinite concave nonlinearity}

\title[Combined effects in some elliptic problems]{Combined effects of concave-convex nonlinearities and indefinite potential in some elliptic problems}


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\author[N.S. Papageorgiou]{Nikolaos S. Papageorgiou}
\address{National Technical University, Department of Mathematics,
				Zografou Campus, Athens 15780, Greece}
\email{\tt npapg@math.ntua.gr}

\author[V.D. R\u{a}dulescu]{Vicen\c{t}iu D. R\u{a}dulescu}
\address{King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia \&
           Institute of Mathematics ``Simion Stoilow" of the Romanian Academy, P.O. Box 1-764,
          014700 Bucharest, Romania}
\email{\tt vicentiu.radulescu@math.cnrs.fr}

\keywords{Indefinite concave term, superlinear perturbation, $p$-Laplacian, nonlinear regularity, positive solutions. \\
\phantom{aa} 2010 AMS Subject Classification:
35J66, 35J70, 35J92.}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\begin{abstract}
We consider a nonlinear Dirichlet problem driven by the $p$-Laplacian and a reaction which exhibits the combined effects of concave (that is, sublinear) terms and of convex (that is, superlinear) terms. The concave term is indefinite and the convex term need not satisfy the usual in such cases Ambrosetti-Rabinowitz condition. We prove a bifurcation-type result describing the set of positive solutions as the positive parameter $\lambda$ varies.
\end{abstract}

\maketitle

%\tableofcontents

\section{Introduction}\label{sec1}
Let $\Omega\subseteq\RR^{N}$ be a bounded domain with a $C^{2}$-boundary $\partial\Omega$. In this paper, we
study the following nonlinear parametric elliptic problem
\begin{center}
$\left\{  \begin{array}{ll}
    -\Delta_p u(z)=\vartheta(z) u(z)^{q-1}+f(z,u(z),\lambda)\ \mbox{in}\ \Omega\\
  u=0\ \mbox{on}\ \partial\Omega\\  u>0\ \mbox{in}\ \Omega\,,
    \end{array}\right. \hspace{4cm}(P_{\lambda})$
\end{center}
where $\lambda>0$ and  $1<q<p$.
Here $\Delta_{p}$ denotes the $p$-Laplace differential operator defined by
\begin{eqnarray*}
\Delta_{p}u=\mbox{div}\,(|Du|^{p-2}Du)\ \mbox{for all}\ u\in W_{0}^{1,p}(\Omega),\ 1<p<\infty.
\end{eqnarray*}
The perturbation $(z,x)\longmapsto f(z,x,\lambda)$ is a Carath\'eodory function (that is, for all $x\in\RR,\ z\longmapsto f(z,x,\lambda)$ is measurable and for a.a. $z\in\Omega$, $x\longmapsto f(z,x,\lambda)$ is continuous), which exhibits $(p-1)$-superlinear growth near $+\infty$, without satisfying the usual in such cases (unilateral) Ambrosetti-Rabinowitz condition (AR-condition for short). So, in problem $(P_{\lambda})$ we have the combined effects of a concave (that is, of a $(p-1)$-sublinear) nonlinearity which is expressed by the term $\vartheta(z)u^{q-1}$ (recall $1<q<p$) and of a convex (that is, of a $(p-1)$-superlinear) nonlinearity, expressed by the term $f(z,u,\lambda)$. Hence, we are dealing with a ``concave-convex problem". The interesting feature of our work here, is that the concave term $\vartheta(z)u^{q-1}$ is {\it indefinite}, namely the weight function $\vartheta(\cdot)$ may change sign.

Problems with combined nonlinearities, were first investigated by Ambrosetti, Brezis and Cerami \cite{2}, where $p=2$ (semilinear problem) and the parametric reaction has the form
\begin{eqnarray*}
\lambda x^{q-1}+x^{r-1}\ \mbox{for all}\ x\geq 0,\ \mbox{with}\ 1<q<r<2^*=\left\{  \begin{array}{ll}
    \frac{2N}{N-2}\ &\mbox{if}\ 2<N,\\
    +\infty\ &\mbox{if}\ N=1,2.
    \end{array}\right.
\end{eqnarray*}
They proved bifurcation type results describing the dependence of the set of positive solutions on the parameter $\lambda>0$. Their work was extended to nonlinear problems driven by the $p$-Laplacian, by Garcia Azorero, Manfredi and Peral Alonso \cite{7} and Guo and Zhang \cite{9}. Problems with more general reactions, were studied by Hu and Papageorgiou \cite{10} and Marano and Papageorgiou \cite{13}. Problems with indefinite concave nonlinearities were investigated by de Paiva \cite{5}, Li, Wu and Zhou \cite{12},  and Papageorgiou and R\u adulescu \cite{16}
only in the context of semilinear equations (that is, $p=2$) and with a particular reaction of the form $x\longmapsto \vartheta(z)x^{q-1}+\lambda x^{r-1}$ for all $x\geq 0$, with $\vartheta\in L^{\infty}(\Omega)$ and $1<q<2<r<2^*$. We also refer to the related papers by Figueiredo, Gossez and Ubilla \cite{fgu} and
Narukawa and Takajo \cite{nt}.

Using variational methods based on the critical point theory, combined with suitable truncation and comparison techniques, we establish the existence, nonexistence and multiplicity of positive solutions for problem $(P_{\lambda})$ as the parameter $\lambda>0$ varies.

\section{Mathematical Background}\label{sec2}
Let $X$ be a Banach space and $X^{*}$ be its topological dual. By $\langle\cdot,\cdot\rangle$ we denote the duality
brackets for the pair $(X^{*},X)$. Given $\varphi\in C^{1}(X)$, we say that $\varphi$ satisfies the ``Cerami
condition" (the ``$C$-condition" for short), if the following is true:

\smallskip
\noindent ``Every sequence $\{u_{n}\}_{n\geq1}\subseteq X$ such that  $\{\varphi(u_{n})\}_{n\geq1}\subseteq\RR$ is bounded and
$$(1+\|u_{n}\|)\varphi'(u_{n})\rightarrow 0\ \mbox{in}\ X^{*}\ \mbox{as}\ n\to\infty,$$
 admits a strongly convergent subsequence".

\smallskip
This is a compactness type condition on the functional $\varphi$, which is needed since the ambient space $X$ need not be locally compact (since, in general $X$ is infinite dimensional). The $C$-condition is the main tool in proving a deformation theorem, from which one can
derive the minimax theory for the critical values of $\varphi$. One of the main results in this theory, is the so-called
``mountain pass theorem" due to Ambrosetti and Rabinowitz \cite{3}, stated here is a slightly more general form (see Gasinski and Papageorgiou
\cite{8}).
\begin{theorem}\label{th1}
Assume that $\varphi\in C^{1}(X)$ satisfies the C-condition, $u_{0},u_{1}\in X$, $\|u_{1}-u_{0}\|>\rho>0$,
\begin{eqnarray*}
&&\max\{\varphi(u_{0}),\varphi(u_{1})\}<\inf[\varphi(u): \|u-u_{0}\|=\rho]=\eta_{\rho}\\
&\mbox{and}& c=\displaystyle\inf_{\gamma\in\Gamma}\max_{0\leq t\leq1}\varphi(\gamma(t))\ \mbox{with}\ \Gamma=\{\gamma\in
C([0,1],X): \gamma(0)=u_{0}, \gamma(1)=u_{1}\}.
\end{eqnarray*}
Then $c\geq \eta_{\rho}$ and $c$ is a critical value of $\varphi$.
\end{theorem}
In the analysis of problem $(P_{\lambda})$, in addition to the Sobolev space $W^{1,p}_0(\Omega)$ we will also use the Banach space $C_0^1(\overline{\Omega})=\{u\in C^1(\overline{\Omega}):\ u|_{\partial\Omega}=0\}$. This is an ordered Banach space with positive cone $C_{+}=\{u\in C_0^1(\overline{\Omega}):\ u(z)\geq 0\ \mbox{for all}\ z\in\overline{\Omega}\}$. This cone has a nonempty interior given by
$$\mbox{int}\, C_{+}=\left\{u\in C_{+}:u(z)>0\ \mbox{for all}\ z\in\Omega,\ \frac{\partial u}{\partial n}|_{\partial\Omega}<0\right\}.$$
Here by $n(\cdot)$ we denote the outward unit normal on $\partial\Omega$.

Let $f_0:\Omega\times\RR\to\RR$ be a Carath\'eodory function with subcritical growth in $x\in\RR$, that is,
\begin{eqnarray*}
&&|f_0(z,x)|\leq a_0(z)(1+|x|^{r-1})\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\in\RR,\\
&\mbox{with}&\ a_0\in L^{\infty}(\Omega)_{+}\ \mbox{and}\ 1<r<p^*=\left\{  \begin{array}{ll}
    \frac{Np}{N-p}\ &\mbox{if}\ p<N,\\
    +\infty\ &\mbox{if}\ N\leq p.
    \end{array}\right.
\end{eqnarray*}

We set $F_0(z,x)=\int\limits_0^x f_0(z,s)ds$ and consider the $C^1$-functional $\varphi_0:W^{1,p}_0(\Omega)\to\RR$ defined by
$$\varphi_0(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}F_0(z,u(z))dz\ \mbox{for all}\ u\in W^{1,p}_0(\Omega).$$

The next result can be found in Garcia Azorero, Manfredi and Peral Alonso \cite{7} and essentially is a consequence of the nonlinear regularity theory of Lieberman \cite{11}.

\begin{prop}\label{prop2}
 Assume that $u_0\in W^{1,p}(\Omega)$ is a local $C_0^1(\overline{\Omega})$-minimizer of $\varphi_0$, that is, there exists $\rho_0>0$ such that
$$\varphi_0(u_0)\leq \varphi(u_0+h)\ \mbox{for all}\ h\in C_0^1(\overline{\Omega})\ \mbox{with}\ \|h\|_{C_0^1(\overline{\Omega})}\leq\rho_0.$$
Then $u_0\in C^{1,\alpha}_0(\overline{\Omega})$ for some $\alpha\in (0,1)$ and it is also a local $W^{1,p}_0(\Omega)$-minimizer of $\varphi_0$, that is, there exists $\rho_1>0$ such that
$$\varphi_0(u_0)\leq \varphi(u_0+h)\ \mbox{for all}\ h\in W^{1,p}_0(\Omega)\ \mbox{with}\ \|h\|\leq\rho_1.$$
\end{prop}
Hereafter by $\|\cdot\|$ we denote the norm of $W_{0}^{1,p}(\Omega)$. By virtue of the Poincar\'e inequality, we have
$$\|u\|=\|Du\|_{p}\ \mbox{for all}\ \ u\in W_{0}^{1,p}(\Omega).$$

Let $A:W_{0}^{1,p}(\Omega)\rightarrow W^{-1,p'}(\Omega)=W_{0}^{1,p}(\Omega)^{*}$ $\left(\frac{1}{p}+\frac{1}{p'}=1\right)$ be the nonlinear map defined by
\begin{eqnarray*}
\langle A(u),y\rangle=\int\limits_{\Omega}|Du|^{p-2}(Du,Dy)_{\RR^{N}}dz\ \ \mbox{for all}\ u,y\in W_{0}^{1,p}(\Omega).
\end{eqnarray*}

The next proposition summarizes the main properties of this map (see, for example, Papageorgiou and Kyritsi \cite[p. 314]{15}).
\begin{prop}\label{prop3}
The map $A:W_{0}^{1,p}(\Omega)\rightarrow W^{-1,p'}(\Omega)$ is bounded (that is, maps bounded sets to bounded
sets), demicontinuous, strictly monotone, hence maximal monotone too and of type $(S)_{+}$ (that is, if
$u_{n}\overset{w}\rightarrow u$ in $W_{0}^{1,p}(\Omega)$ and
\begin{eqnarray*}
\limsup_{n\rightarrow\infty}\langle A(u_{n}), u_{n}-u\rangle\leq 0,
\end{eqnarray*}
then $u_{n}\rightarrow u$ in $W_{0}^{1,p}(\Omega)$ as $n\to\infty$).
\end{prop}
We recall that the Dirichlet $p$-Laplacian ($-\Delta_p,W_{0}^{1,p}(\Omega)$) admits a smallest eigenvalue $\hat{\lambda}_1>0$. Sometimes we write $\hat{\lambda}_1(\Omega)>0$ to emphasize the domain $\Omega$. This eigenvalue is isolated, simple with eigenfunctions of constant sign. The nonlinear regularity theory and the nonlinear maximum principle (see, for example, Gasinski and Papageorgiou \cite[pp. 737-738]{8}), imply that every positive eigenfunction corresponding to $\hat{\lambda}_1>0$ belongs in $\mbox{int}\, C_{+}$.

Finally let us fix our notation. So, for $x\in\RR$, we set $x^{\pm}=\max\{\pm x,0\}$. Then given $u\in W_{0}^{1,p}(\Omega)$, we set $u^{\pm}(\cdot)=u(\cdot)^{\pm}$. We have
$$u^{\pm}\in W_{0}^{1,p}(\Omega),\ u=u^{+}-u^{-},\ |u|=u^{+}+u^{-}.$$
Given any measurable function $h:\Omega\times\RR\to\RR$ (for example, a Carath\'eodory function), we define
$$N_h(u)(\cdot)=h(\cdot,u(\cdot))\ \mbox{for all}\ u\in W_{0}^{1,p}(\Omega)$$
(the Nemytski map corresponding to $h$). Evidently $z\longmapsto N_h(u)(z)$ is measurable on $\Omega$. By $|\cdot|_N$ we denote the Lebesgue measure on $\RR^N$.


\section{Positive Solutions}\label{sec3}
The hypotheses on the data of problem $(P_{\lambda})$ are the following:

\smallskip
$H_1$: $\vartheta\in L^{\infty}(\Omega)$ and if $D_{+}=\{z\in\Omega: \vartheta(z)>0\}$, then $D_{+}\neq\emptyset$ and there exists an open set $\Omega_{+}$ such that  $\overline{\Omega}_{+}\subseteq D_{+}$, $\partial\Omega_{+}$ is $C^2$ and $\operatorname*{ess\,inf}\limits_{\Omega_{+}}\vartheta=m_{+}>0$.

$H_{2}$: $f:\Omega\times\RR\times (0,\infty)\rightarrow\RR$ is a function such that  for all $\lambda>0$, $(z,x)\longmapsto f(z,x,\lambda)$ is Carath\'eodory,  $f(z,0,\lambda)=0$ for a.a. $z\in\Omega$ and
\begin{itemize}
\item[(i)] $f(z,x,\lambda)\leq a(z,\lambda)(1+x^{r(\lambda)-1})$ for a.a. $z\in\Omega$ with
 $$r(\lambda)\in (p,p^*),\ a(\cdot,\lambda)\in L^{\infty}(\Omega)_{+},\ \|a(\cdot,\lambda)\|_{\infty}\to 0\ \mbox{as}\ \lambda\to 0^{+};$$
\item[(ii)] if $F(z,x,\lambda)=\int\limits_0^x f(z,s,\lambda)ds$, then
$$\displaystyle\lim_{x\rightarrow+\infty}\frac{F(z,x,\lambda)}{x^{p}}=+\infty\ \mbox{uniformly for a.a.}\ z\in\Omega$$
and there exist $\tau\in\left((r(\lambda)-p)\max\{\frac{N}{p},1\},p^*\right)$, $\tau>q$ and $\eta_0(\lambda)>0$ with $\lambda\longmapsto\eta_0(\lambda)$ nondecreasing such that
$$\eta_0(\lambda)\leq\liminf_{x\to+\infty}\frac{f(z,x,\lambda)x-p F(z,x,\lambda)}{x^{\tau}}\ \mbox{uniformly for a.a.}\ z\in\Omega;$$
\item[(iii)] for every $\rho>0$, there exists $m_{\rho}(\lambda)>0$ such that  $m_{\rho}(\lambda)\to+\infty$ as $\lambda\to+\infty$,
$$\inf[f(z,x,\lambda):\ x\geq\rho]=m_{\rho}(\lambda)>0,$$
for a.a. $z\in\Omega$, all $x\geq 0$, the map $\lambda\longmapsto f(z,x,\lambda)$ is nondecreasing and for every $\xi>0$, for a.a. $z\in\Omega$, all $x\geq\xi$, all $\lambda'>\lambda>0$, we have $f(z,x,\lambda)\geq m_{\xi}(\lambda)$ with $m_{\xi}(\cdot)$ nondecreasing, $m_{\xi}(\lambda)\to+\infty$ as $\lambda\to\infty$ and $f(z,x,\lambda')-f(z,x,\lambda)\geq\eta_{\xi}>0$;
\item[(iv)] for every $\rho>0$, there exists $\xi_{\rho}(\lambda)>0$ such that  for a.a. $z\in\Omega$, the function
    $$x\longmapsto f(z,x,\lambda)+\xi_{\rho}(\lambda)x^{p-1}$$
    is nondecreasing on $[0,\rho]$.
\end{itemize}

\begin{remark}\label{remark}
Since we are interested on positive solutions and all the above hypotheses concern the positive semiaxis $\RR_{+}=[0,+\infty)$, without any loss of generality, we may assume that $f(z,x,\lambda)=0$ for a.a. $z\in\Omega$, all $x\leq 0$, all $\lambda>0$. Hypothesis $H_2(ii)$ implies that for a.a. $z\in\Omega$ and all $\lambda>0$, the perturbation $x\longmapsto f(z,x,\lambda)$ is $(p-1)$-superlinear near $+\infty$. However, we do not employ the usual in such cases $AR$-condition. We recall that the $AR$-condition (unilateral version, since we assume that $f(z,x,\lambda)=0$ for a.a. $z\in\Omega$, all $x\leq 0$, all $\lambda>0$), says that there exist $\mu=\mu(\lambda)>p$ and $M=M(\lambda)>0$ such that
\begin{itemize}
\item [(a)] $0<\mu F(z,x,\lambda)\leq f(z,x,\lambda)x\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\geq M$;
\item [(b)] $\operatorname*{ess\,inf}\limits_{\Omega}F(\cdot,M,\lambda)>0$.
\end{itemize}
(see Ambrosetti and Rabinowitz \cite{3} and Mugnai \cite{14}). Integrating $(a)$ and using $(b)$, we obtain the weaker condition
$$c_1x^{\mu}\leq F(z,x,\lambda)\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\geq M\ \mbox{and some}\ c_1=c_1(\lambda)>0.$$
Evidently this unilateral growth estimate implies the much weaker condition
$$\lim\limits_{x\to+\infty}\frac{F(z,x,\lambda)}{x^p}=+\infty\ \mbox{uniformly for a.a.}\ z\in\Omega.$$
\end{remark}

Note that our hypothesis $H_2(ii)$ is weaker than the $AR$-condition. Indeed, suppose that the $AR$-condition holds (see $(a)$ and $(b)$ above). We may assume that $\mu>(r(\lambda)-p)\max\{\frac{N}{p},1\}$. We have
\begin{eqnarray*}
\frac{f(z,x,\lambda)x-p F(z,x,\lambda)}{x^{\mu}}&=&\frac{f(z,x,\lambda)x-\mu F(z,x,\lambda)}{x^{\mu}}+(\mu-p)\frac{F(z,x,\lambda)}{x^{\mu}}\\
&\geq&(\mu-p)\frac{F(z,x,\lambda)}{x^{\mu}}\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\geq M\ (\mbox{see}\ (a))\\
&\geq&(\mu-p)c_1\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\geq M,
\end{eqnarray*}
\begin{eqnarray*}
&\Rightarrow&\liminf\limits_{x\to+\infty}\frac{f(z,x,\lambda)x-p F(z,x,\lambda)}{x^{\mu}}\geq(\mu-p)c_1(\lambda)=\eta_0(\lambda)\ \mbox{uniformly for a.a.}\ z\in\Omega.
\end{eqnarray*}
So, hypothesis $H_2(ii)$ holds. See the examples that follow for functions which satisfy our hypothesis $H_2(ii)$ but not the $AR$-condition.
\begin{ex}
The following functions satisfy hypotheses $H_{2}$. For the sake of simplicity we drop the $z$-dependence:
\begin{eqnarray*}
&&f_1(x,\lambda)=\lambda x^{r-1}\ \mbox{for all}\ x\geq 0\ \mbox{with}\ p<r<p^*,\\
&&f_2(x,\lambda)=\xi(\lambda) x^{p-1}\left(\ln (1+x)+\frac{x}{p(x+1)}\right)\ \mbox{for all}\ x\geq 0
\end{eqnarray*}
with $\lambda\longmapsto\xi(\lambda)$  strictly increasing on $(0,+\infty)$, $\lim\limits_{\lambda\to\infty}\xi(\lambda)=+\infty$, $\lim\limits_{\lambda\to 0^{+}}\xi(\lambda)=0$.

Note that $f_2(\cdot,\lambda)$ does not satisfy the $AR$-condition.
\end{ex}
Let $$\mathcal{L}=\{\lambda>0:\ \mbox{problem}\ (P_{\lambda})\ \mbox{admits a positive solution}\}$$
$$S(\lambda)=\mbox{set of positive solutions of problem}\ (P_{\lambda}).$$

First we establish the nonemptiness and a structural property of the set $\mathcal{L}$ of admissible parameters.
\begin{prop}\label{prop4}
If hypotheses $H_1$ and $H_2$ hold, then $\mathcal{L}\neq\emptyset$, for every $u\in S(\lambda)$ we have $u(z)>0$ for all $z\in\Omega$ and $\lambda\in\mathcal{L}$ implies $(0,\lambda]\subseteq\mathcal{L}$.
\end{prop}
\begin{proof}
We consider the following auxiliary Dirichlet problem
\begin{eqnarray}\label{eq1}
-\Delta_p e(z)=1\ \mbox{in}\ \Omega,\ e|_{\partial\Omega}=0.
\end{eqnarray}
Recalling that $A$ is maximal monotone, strictly monotone and coercive (by virtue of the Poincar\'e inequality), we see that problem (\ref{eq1}) has a unique solution $e\in W^{1,p}_0(\Omega)\setminus\{0\}$ (see, for example, Gasinski and Papageorgiou \cite[p. 319]{8}). Acting on (\ref{eq1}) with $-e^{-}\in W^{1,p}_0(\Omega)$, we obtain
\begin{eqnarray*}
&&\|De^{-}\|_p^p=-\int\limits_{\Omega}e^{-}dz\leq 0,\\
&\Rightarrow& e\geq 0,\ e\neq 0.
\end{eqnarray*}
From the nonlinear regularity theory and the nonlinear maximum principle (see, for example, Gasinski and Papageorgiou \cite[pp. 737-738]{8}), we have $e\in\mbox{int}\, C_{+}$.
\begin{claim}
There exists $\overline{\lambda}>0$ such that  for all $\lambda\in (0,\overline{\lambda})$, we can find $\xi=\xi(\lambda)>0$ for which we have
$$\xi^{q-1}\|\vartheta\|_{\infty}\|e\|_{\infty}^{q-1}+\|a(\cdot,\lambda)\|_{\infty}(1+\xi^{r-1}\|e\|_{\infty}^{r-1})<\xi^{p-1}.$$
\end{claim}
Arguing by contradiction, suppose that we can find $\lambda_n\downarrow 0$ such that
$$\xi^{p-1}\leq \xi^{q-1}\|\vartheta\|_{\infty}\|e\|_{\infty}^{q-1}+\|a(\cdot,\lambda_n)\|_{\infty}(1+\xi^{r-1}\|e\|_{\infty}^{r-1})$$
\hspace{8cm} for all $\xi>0$, all $n\geq 1$.

Passing to the limit as $n\to\infty$ and using hypothesis $H_2(i)$, we obtain
$$\xi^{p-q}\leq \|\vartheta\|_{\infty}\|e\|_{\infty}^{q-1}\ \mbox{for all}\ \xi>0,$$
a contradiction. This proves the Claim.

Let $\overline{u}=\xi e\in\mbox{int}\, C_{+}$. Then we have
\begin{eqnarray}\label{eq2}
-\Delta_p \overline{u}(z)=\xi^{p-1}>\vartheta(z)\overline{u}(z)^{q-1}+f(z,\overline{u}(z),\lambda)\ \mbox{for a.a.}\ z\in\Omega\\
\hspace{5cm} (\mbox{see the Claim and hypothesis}\ H_2(i)).\nonumber
\end{eqnarray}
Fix $\lambda\in (0,\overline{\lambda})$ and consider the following Carath\'eodory function
\begin{eqnarray}\label{eq3}
g_{\lambda}(z,x)=\left\{  \begin{array}{ll}
    0\ &\mbox{if}\ x<0\\
    \vartheta(z)x^{q-1}+f(z,x,\lambda)\ &\mbox{if}\ 0\leq x\leq\overline{u}(z)\\
    \vartheta(z)\overline{u}(z)^{q-1}+f(z,\overline{u}(z),\lambda)\ &\mbox{if}\ \overline{u}(z)<x.
    \end{array}\right.
\end{eqnarray}

We set $G_{\lambda}(z,x)=\int\limits_0^x g_{\lambda}(z,s)ds$ and consider the $C^1$-functional $\psi_{\lambda}:W_0^{1,p}(\Omega)\to\RR$ defined by
$$\psi_{\lambda}(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}G_{\lambda}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega).$$
From (\ref{eq3}) it is clear that $\psi_{\lambda}$ is coercive. Also, using the Sobolev embedding theorem, we can see that $\psi_{\lambda}$ is sequentially weakly lower semicontinuous. So, by the Weierstrass theorem, we can find $u_{\lambda}\in W_0^{1,p}(\Omega)$ such that
\begin{eqnarray}\label{eq4}
\psi_{\lambda}(u_{\lambda})=\inf[\psi_{\lambda}(u):\ u\in W_0^{1,p}(\Omega)].
\end{eqnarray}
Let $u\in C_{+}\setminus\{0\}$ with $\mbox{supp}\ u\subseteq\Omega_{+}$. Recall that $\overline{u}\in\mbox{int}\, C_{+}$. Hence, we have that $\overline{u}|_{\overline{\Omega}_{+}}>0$ (see hypothesis $H_1$). Therefore, we can find $t\in (0,1)$ small such that  $tu\leq\overline{u}$. So, using hypothesis $H_2(iii)$ and (\ref{eq3}), we have
$$\psi_{\lambda}(tu)\leq\frac{t^p}{p}\|Du\|_p^p-\frac{t^q}{q}\int\limits_{\Omega}\vartheta(z)u^qdz.$$
Since $\mbox{supp}\ u\subseteq\Omega_{+}$ and $q<p$, choosing $t\in (0,1)$ even smaller if necessary, we infer that
\begin{eqnarray*}
&&\psi_{\lambda}(tu)<0,\\
&\Rightarrow&\psi_{\lambda}(u_{\lambda})<0=\psi_{\lambda}(0)\ (\mbox{see}\ (\ref{eq4})),\ \mbox{hence}\ u_{\lambda}\neq 0.
\end{eqnarray*}
From (\ref{eq4}), we have
\begin{eqnarray}\label{eq5}
&&\psi_{\lambda}'(u_{\lambda})=0,\nonumber\\
&\Rightarrow& A(u_{\lambda})=N_{g_{\lambda}}(u_{\lambda}).
\end{eqnarray}
On (\ref{eq5}) first we act with $-u_{\lambda}^{-}\in W_0^{1,p}(\Omega)$ and obtain
\begin{eqnarray*}
&&\|Du_{\lambda}^{-}\|_p^p=0\ (\mbox{see}\ (\ref{eq3})),\\
&\Rightarrow& u_{\lambda}\geq 0,\ u_{\lambda}\neq 0.
\end{eqnarray*}
Also, on (\ref{eq5}) we act with $(u_{\lambda}-\overline{u})^{+}\in W_0^{1,p}(\Omega)$ and obtain
\begin{eqnarray*}
\langle A(u_{\lambda}),(u_{\lambda}-\overline{u})^{+}\rangle&=&\int\limits_{\Omega}g_{\lambda}(z,u_{\lambda})(u_{\lambda}-\overline{u})^{+}dz\\
&=&\int\limits_{\Omega}[\vartheta(z)\overline{u}^{q-1}+f(z,\overline{u},\lambda)](u_{\lambda}-\overline{u})^{+}dz\ (\mbox{see}\ (\ref{eq3}))\\
&\leq&\int\limits_{\Omega}\xi^{p-1}(u_{\lambda}-\overline{u})^{+}dz\ (\mbox{see the Claim})\\
&=&\langle A(\overline{u}),(u_{\lambda}-\overline{u})^{+}\rangle\ (\mbox{see}\ (\ref{eq1})),
\end{eqnarray*}
\begin{eqnarray*}
&\Rightarrow&\int\limits_{\{u_{\lambda}>\overline{u}\}}(|Du_{\lambda}|^{p-2}Du_{\lambda}-|D\overline{u}|^{p-2}D\overline{u},Du_{\lambda}- D\overline{u})_{\RR^N}dz\leq 0,\\
&\Rightarrow&|\{u_{\lambda}>\overline{u}\}|_N=0,\ \mbox{hence}\ u_{\lambda}\leq\overline{u}.
\end{eqnarray*}
So, we have proved that
$$u_{\lambda}\in [0,\overline{u}]=\{u\in W_0^{1,p}(\Omega):\ 0\leq u(z)\leq\overline{u}(z)\ \mbox{for a.a.}\ z\in\Omega\}.$$
This fact and (\ref{eq3}) imply that $u_{\lambda}\in S(\lambda)$. The nonlinear regularity theory (see Lieberman \cite{11}), implies that $u_{\lambda}\in C_{+}\setminus\{0\}$. Invoking Harnack's inequality (see Pucci and Serrin \cite[p. 163]{17}), we infer that $u_{\lambda}(z)>0$ for all $z\in\Omega$.

Now let $\lambda\in\mathcal{L}$ and let $\mu\in (0,\lambda)$, $u_{\lambda}\in S(\lambda)$. We have
\begin{eqnarray}\label{eq6}
-\Delta_p u_{\lambda}(z)&=&\vartheta(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\lambda)\nonumber\\
&\geq& \vartheta(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\mu)\ \mbox{for}\ \mu\ \mbox{a.a.}\ z\in\Omega\\
&&\hspace{4cm} (\mbox{see hypothesis}\ H_2(iii))\nonumber.
\end{eqnarray}
We introduce the Carath\'eodory function $\gamma_{\mu}(z,x)$ defined by
\begin{eqnarray}\label{eq7}
\gamma_{\mu}(z,x)=\left\{  \begin{array}{ll}
    0\ &\mbox{if}\ x<0\\
    \vartheta(z)x^{q-1}+f(z,x,\mu)\ &\mbox{if}\ 0\leq x\leq u_{\lambda}(z)\\
    \vartheta(z)u_{\lambda}(z)+f(z,u_{\lambda}(z),\mu)\ &\mbox{if}\ u_{\lambda}(z)<x.
    \end{array}\right.
\end{eqnarray}
We set $\Gamma_{\mu}(z,x)=\int\limits_0^x\gamma_{\mu}(z,s)ds$ and consider the $C^1$-functional $\tau_{\mu}:W_0^{1,p}(\Omega)\to\RR$ defined by
$$\tau_{\mu}(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}\Gamma_{\mu}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega).$$
From (\ref{eq7}) it is clear that $\tau_{\mu}$ is coercive. Also, it is sequentially weakly lower semicontinuous. So, we can find $u_{\mu}\in W_0^{1,p}(\Omega)$ such that
$$\tau_{\mu}(u_{\mu})=\inf[\tau_{\mu}(u):\ u\in W_0^{1,p}(\Omega)].$$
As before, we can show that
$$\tau_{\mu}(u_{\mu})<0=\tau_{\mu}(0),\ \mbox{hence}\ u_{\mu}\neq 0.$$
Also, we have
\begin{eqnarray*}
&&\tau_{\mu}'(u_{\mu})=0,\\
&\Rightarrow& A(u_{\mu})=N_{\gamma_{\mu}}(u_{\mu}).
\end{eqnarray*}
Acting with $-u_{\mu}^{-}\in  W_0^{1,p}(\Omega)$ and with $(u_{\mu}-u_{\lambda})^{+}\in W_0^{1,p}(\Omega)$, as before, we show that
\begin{eqnarray*}
&&u_{\mu}\in [0,u_{\lambda}]=\{u\in W_0^{1,p}(\Omega):\ 0\leq u(z)\leq u_{\lambda}(z)\ \mbox{for a.a.}\ z\in\Omega\},\\
&\Rightarrow& u_{\mu}\in S(\mu)\ (\mbox{see}\ (\ref{eq7})),\\
&\Rightarrow& \mu\in \mathcal{L}\ \mbox{and so}\ (0,\lambda]\subseteq\mathcal{L}.
\end{eqnarray*}
This completes the proof.
\end{proof}

Let $\lambda^*=\sup\mathcal{L}$.
\begin{prop}\label{prop5}
If hypotheses $H_1$ and $H_2$ hold, then $\lambda^*<\infty$.
\end{prop}
\begin{proof}
Let $\hat{\lambda}_1^{+}=\hat{\lambda}_1(\Omega_{+})$ (see Section \ref{sec2}) and take $\beta>\hat{\lambda}_1^{+}$.
\begin{claim}
There exists $\lambda_0>0$ big such that
$$\vartheta(z)x^{q-1}+f(z,x,\lambda)\geq\beta x^{p-1}\ \mbox{for a.a.}\ z\in\Omega_{+},\ \mbox{all}\ x\geq 0,\ \mbox{all}\ \lambda\geq\lambda_0.$$
\end{claim}
Since $q<p$, we can find $\delta>0$ small such that
\begin{eqnarray}\label{eq8}
\vartheta(z)x^{q-1}\geq\beta x^{p-1}\ \mbox{for a.a.}\ z\in\Omega_{+},\ \mbox{all}\ 0\leq x\leq\delta\ (\mbox{see hypothesis}\ H_1).
\end{eqnarray}
Note that hypothesis $H_2(ii)$ implies that for all $\lambda>0$, we have
$$\lim\limits_{x\to+\infty}\frac{f(z,x,\lambda)}{x^{p-1}}=+\infty\ \mbox{uniformly for a.a.}\ z\in\Omega.$$
So, we can find $M=M(\lambda)>0$ such that
\begin{eqnarray}\label{eq9}
f(z,x,\lambda)\geq\beta x^{p-1}\ \mbox{for a.a.}\ z\in\Omega\ \mbox{all}\ x\geq M.
\end{eqnarray}
Let $\delta>0$ be as in (\ref{eq8}) and $\lambda>0$. By virtue of hypothesis $H_2(iii)$, we have
$$\inf[f(z,x,\lambda):\ x\geq\delta]=m_{\delta}(\lambda)>0,$$
with $\lambda\longmapsto m_{\delta}(\lambda)$ nondecreasing and $m_{\delta}(\lambda)\to+\infty$ as $\lambda\to+\infty$. So, we can choose $\lambda_0\geq 1$ big such that
\begin{eqnarray}\label{eq10}
&&m_{\delta}(\lambda)\geq\beta M^{p-1}\ \mbox{for all}\ \lambda\geq\lambda_0,\nonumber\\
&\Rightarrow& f(z,x,\lambda)\geq\beta x^{p-1}\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ \delta\leq x\leq M,\ \mbox{all}\ \lambda\geq\lambda_0.
\end{eqnarray}
Combining (\ref{eq8}), (\ref{eq9}), (\ref{eq10}), we conclude that the Claim holds.

Take $\lambda>\lambda_0$ and assume that $\lambda\in\mathcal{L}$. Then we can find $u_{\lambda}\in S(\lambda)$ such that
$$-\Delta_p u_{\lambda}(z)=\vartheta(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\lambda)\geq\beta u_{\lambda}(z)^{q-1}\ \mbox{for a.a.}\ z\in\Omega_{+}$$
\hspace{8,5cm} (see the Claim).

Let $\hat{u}_{+}\in\mbox{int}\, C_{+}(\Omega_{+})$ be the principal, $L^p$-normalized (that is, $\|\hat{u}_{+}\|_{L^p(\Omega_+)}=1$) positive eigenfunction for $(-\Delta_p,W_0^{1,p}(\Omega_{+}))$. From Proposition \ref{prop4}, we know that
$$u_{\lambda}|_{\overline{\Omega}_{+}}>0.$$
So, we can find $t\in (0,1)$ small such that
\begin{eqnarray}\label{eq11}
t\hat{u}_{+}(z)\leq u_{\lambda}(z)\ \mbox{for all}\ z\in\overline{\Omega}_{+}.
\end{eqnarray}
We have
\begin{eqnarray}\label{eq12}
-\Delta_p(t\hat{u}_{+})(z)=\hat{\lambda}_1^{+}(t\hat{u}_{+})(z)^{p-1}<\beta(t\hat{u}_{+})(z)^{p-1}\ \mbox{for a.a.}\ z\in\Omega_{+}\\
\hspace{6cm} (\mbox{recall that}\ \hat{\lambda}_1^{+}<\beta).\nonumber
\end{eqnarray}
We consider the following Carath\'eodory function
\begin{eqnarray}\label{eq13}
k_{\beta}(z,x)=\left\{  \begin{array}{ll}
    \beta(t\hat{u}_{+})(z)^{p-1}\ &\mbox{if}\ x<t\hat{u}_{+}(z)\\
    \beta x^{p-1}\ &\mbox{if}\ t\hat{u}_{+}(z)\leq x\leq u_{\lambda}(z),\ (z,x)\in\Omega_{+}\times\RR,\\
    \beta u_{\lambda}(z)^{p-1}\ &\mbox{if}\ u_{\lambda}(z)<x.
    \end{array}\right.
\end{eqnarray}
(see (\ref{eq11})).

We set $K_{\beta}(z,x)=\int\limits_0^x k_{\beta}(z,s)ds$ and consider the $C^1$-functional $\chi_{\beta}:W_0^{1,p}(\Omega_{+})\to\RR$ defined by
$$\chi_{\beta}(u)=\frac{1}{p}\|Du\|_{L^p(\Omega_{+},\RR^N)}-\int\limits_{\Omega_{+}}K_{\beta}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega_{+}).$$
From (\ref{eq13}) it is clear that $\chi_{\beta}$ is coercive and also it is sequentially weakly lower semicontinuous. So, by the Weierstrass theorem, we can find $\tilde{u}_{\beta}\in W_0^{1,p}(\Omega_{+})$ such that
\begin{eqnarray}\label{eq14}
&&\chi_{\beta}(\tilde{u}_{\beta})=\inf[\chi_{\beta}(u):\ u\in W_0^{1,p}(\Omega_{+})],\nonumber\\
&\Rightarrow& \chi_{\beta}'(\tilde{u}_{\beta})=0,\nonumber\\
&\Rightarrow& \hat{A}(\tilde{u}_{\beta})=N_{k_{\beta}}(\tilde{u}_{\beta})\ \left(\mbox{with}\ \hat{A}=A|_{W_0^{1,p}(\Omega_{+})}\right).
\end{eqnarray}
On (\ref{eq14}), first we act with $(t\hat{u}_{+}-\tilde{u}_{\beta})^{+}\in W_0^{1,p}(\Omega_{+})$. Then
\begin{eqnarray*}
\langle A(\tilde{u}_{\beta}),(t\hat{u}_{+}-\tilde{u}_{\beta})^{+}\rangle&=&\int\limits_{\Omega_{+}}k_{\beta}(z,\tilde{u}_{\beta}) (t\hat{u}_{+}-\tilde{u}_{\beta})^{+}dz\\
&=&\int\limits_{\Omega_{+}}\beta (t\hat{u}_{+})^{p-1} (t\hat{u}_{+}-\tilde{u}_{\beta})^{+}dz\\
&\geq&\int\limits_{\Omega_{+}}\hat{\lambda}_1^{+} (t\hat{u}_{+})^{p-1} (t\hat{u}_{+}-\tilde{u}_{\beta})^{+}dz\ (\mbox{see}\ (\ref{eq12}))\\
&=&\langle \hat{A}(t\hat{u}_{+}),(t\hat{u}_{+}-\tilde{u}_{\beta})^{+}\rangle,
\end{eqnarray*}
\begin{eqnarray*}
&\Rightarrow&\langle \hat{A}(t\hat{u}_{+})-\hat{A}(\tilde{u}_{\beta}),(t\hat{u}_{+}-\tilde{u}_{\beta})^{+}\rangle\leq 0,\\
&\Rightarrow&|\{t\hat{u}_{+}>\tilde{u}_{\beta}\}_{\Omega_{+}}|_N=0,\ \mbox{hence}\ t\hat{u}_{+}\leq \tilde{u}_{\beta}\ \mbox{in}\ \Omega_{+}.
\end{eqnarray*}
Next, on (\ref{eq14}) we act with $(\tilde{u}_{\beta}-\tilde{u}_{\lambda})^{+}\in W_0^{1,p}(\Omega_{+})$ where $\tilde{u}_{\lambda}=u_{\lambda}|_{\Omega_{+}}$ (recall that $u_{\lambda}|_{\overline{\Omega}_{+}}>0$). We have
\begin{eqnarray*}
& &\langle\hat{A}(\tilde{u}_{\beta}),(\tilde{u}_{\beta}-\tilde{u}_{\lambda})^{+}\rangle\\
&=&\int\limits_{\Omega_{+}}k_{\beta}(z,\tilde{u}_{\beta})(\tilde{u}_{\beta}-\tilde{u}_{\lambda})^{+}dz\\
&=&\int\limits_{\Omega_{+}}\beta\tilde{u}_{\lambda}^{p-1}(\tilde{u}_{\beta}-\tilde{u}_{\lambda})^{+}dz\\
&\leq&\int\limits_{\Omega_{+}}[\vartheta(z)\tilde{u}_{\lambda}^{q-1}+f(z,\tilde{u}_{\lambda},\lambda)] (\tilde{u}_{\beta}-\tilde{u}_{\lambda})^{+}dz\ (\mbox{see the Claim})\\
&=&\langle\hat{A}(\tilde{u}_{\beta})-\hat{A}(\tilde{u}_{\lambda}),(\tilde{u}_{\beta}-\tilde{u}_{\lambda})^{+}\rangle\ (\mbox{since}\ u_{\lambda}\in S(\lambda)),
\end{eqnarray*}
\begin{eqnarray*}
&\Rightarrow&|\{\tilde{u}_{\beta}>\tilde{u}_{\lambda}\}_{\Omega_{+}}|_N=0,\ \mbox{hence}\ \tilde{u}_{\beta}\leq\tilde{u}_{\lambda}\ \mbox{in}\ \Omega_{+}.
\end{eqnarray*}
So, finally we have
$$t\hat{u}_{+}(z)\leq\tilde{u}_{\beta}(z)\leq u_{\lambda}(z)\ \mbox{for all}\ z\in\overline{\Omega}_{+}.$$
From (\ref{eq13}) and (\ref{eq14}) it follows that
$$-\Delta_p\tilde{u}_{\beta}(z)=\beta\tilde{u}_{\beta}(z)^{p-1}\ \mbox{for a.a.}\ z\in\Omega_{+},\ \tilde{u}_{\beta}|_{\Omega_{+}}=0,\ \tilde{u}_{\beta}\geq 0,$$
a contradiction since $\beta>\hat{\lambda}_1^{+}$ (recall that every nonprincipal eigenvalue of $(-\Delta_p, W_0^{1,p}(\Omega_{+}))$), has nodal (that is, sign changing) eigenfunctions, see \cite{8}.

This means that $\lambda^*\leq\lambda_0<\infty$.
\end{proof}

In what follows, for every $\lambda>0$, by $\varphi_{\lambda}:W_0^{1,p}(\Omega)\to\RR$ we denote the energy functional for problem $(P_{\lambda})$ defined by
$$\varphi_{\lambda}(u)=\frac{1}{p}\|Du\|_p^p-\frac{1}{q}\int\limits_{\Omega}\vartheta(z)u^{+}(z)^qdz-\int\limits_{\Omega}F(z,u(z),\lambda)dz$$
\hspace{8,5cm} $\mbox{for all}\ u\in W_0^{1,p}(\Omega)$

Evidently $\varphi_{\lambda}\in C^1(W_0^{1,p}(\Omega))$.
\begin{prop}\label{prop6}
If hypotheses $H_1$ and $H_2$ hold, then $\lambda^*\in\mathcal{L}$.
\end{prop}
\begin{proof}
Let $\{\lambda_n\}_{n\geq 1}\subseteq\mathcal{L}$ such that  $\lambda_n\to (\lambda^*)^{-}$ as $n\to\infty$ and for every $n\geq 1$, let $u_n\in S(\lambda_n)$. We may assume that
\begin{eqnarray}\label{eq15}
\varphi_{\lambda_n}(u_n)<0\ \mbox{for all}\ n\geq 1.
\end{eqnarray}
Indeed, if $\lambda<\tilde{\lambda}<\lambda^*$ and $\tilde{u}\in S(\tilde{\lambda})$, then by virtue of hypothesis $H_2(iii)$, we have
\begin{eqnarray}\label{eq16}
-\Delta_p\tilde{u}(z)&=&\vartheta(z)\tilde{u}(z)^{q-1}+f(z,\tilde{u}(z),\tilde{\lambda})\nonumber\\
&\geq&\vartheta(z)\tilde{u}(z)^{q-1}+f(z,\tilde{u}(z),\lambda)\ \mbox{for a.a.}\ z\in\Omega\\
&&\hspace{3cm} (\mbox{see hypothesis}\ H_2(iii))\nonumber.
\end{eqnarray}
Then reasoning as in the proof of Proposition \ref{prop4}, we introduce the following truncation of the reaction of problem $(P_{\lambda})$:
\begin{eqnarray}\label{eq17}
w_{\lambda}(z,x)=\left\{  \begin{array}{ll}
     0\ &\mbox{if}\ x<0\\
     \vartheta(z)x^{q-1}+f(z,x,\lambda)\ &\mbox{if}\ 0\leq x\leq\tilde{u}(z),\\
     \vartheta(z)\tilde{u}(z)^{q-1}+f(z,\tilde{u}(z),\lambda)\ &\mbox{if}\ \tilde{u}(z)<x.
    \end{array}\right.
\end{eqnarray}
This is a Carath\'eodory function. We set $W_{\lambda}(z,x)=\int\limits_0^x w_{\lambda}(z,s)ds$ and consider the $C^1$-functional $\hat{\psi_{\lambda}}:W_0^{1,p}(\Omega)\to\RR$ defined by
$$\hat{\psi_{\lambda}}(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}W_{\lambda}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega).$$
Again, $\hat{\psi_{\lambda}}$ is coercive (see (\ref{eq17})) and sequentially weakly lower semicontinuous. So, we can find $u_{\lambda}\in W_0^{1,p}(\Omega)$ such that
\begin{eqnarray}\label{eq18}
\hat{\psi_{\lambda}}(u_{\lambda})=\inf[\hat{\psi_{\lambda}}(u):\ u\in W_0^{1,p}(\Omega)].
\end{eqnarray}
As in the proof of Proposition \ref{prop4}, we show that
$$\hat{\psi_{\lambda}}(u_{\lambda})<0=\hat{\psi_{\lambda}}(0),\ \mbox{hence}\ u_{\lambda}\neq 0.$$
From (\ref{eq18}), we have
\begin{eqnarray}\label{eq19}
& &\hat{\psi_{\lambda}}'(u_{\lambda})=0,\nonumber\\
&\Rightarrow& A(u_{\lambda})=N_{w_{\lambda}}(u_{\lambda}).
\end{eqnarray}
On (\ref{eq19}) first we act with $-u_{\lambda}^{-}\in W_0^{1,p}(\Omega)$ and obtain $u_{\lambda}\geq 0$, $u_{\lambda}\neq 0$ (see (\ref{eq17})). Then we act with $(u_{\lambda}-\tilde{u})^{+}\in W_0^{1,p}(\Omega)$. We obtain
\begin{eqnarray*}
& &\langle A(u_{\lambda}),(u_{\lambda}-\tilde{u})^{+}\rangle\\
&=&\int\limits_{\Omega_{+}}w_{\lambda}(z,u_{\lambda})(u_{\lambda}-\tilde{u})^{+}dz\\
&=&\int\limits_{\Omega_{+}}[\vartheta(z)\tilde{u}^{q-1}+f(z,\tilde{u},\lambda)](u_{\lambda}-\tilde{u})^{+}dz\ (\mbox{see}\ (\ref{eq17}))\\
&\leq& \langle A(\tilde{u}),(u_{\lambda}-\tilde{u})^{+}\rangle\ (\mbox{see}\ (\ref{eq16})),
\end{eqnarray*}
\begin{eqnarray*}
&\Rightarrow&\langle A(u_{\lambda})-A(\tilde{u}),(u_{\lambda}-\tilde{u})^{+}\rangle\leq 0,\\
&\Rightarrow& |\{u_{\lambda}>\tilde{u}\}|_N=0,\ \mbox{hence}\ u_{\lambda}\leq\tilde{u}.
\end{eqnarray*}
So, we have
\begin{eqnarray*}
& &u_{\lambda}\in [0,\tilde{u}]=\{u\in W_0^{1,p}(\Omega):\ 0\leq u(z)\leq\tilde{u}(z)\ \mbox{for a.a.}\ z\in\Omega\},\\
&\Rightarrow& u_{\lambda}\in S(\lambda)\ (\mbox{see}\ (\ref{eq17}))\ \mbox{and}\ \varphi_{\lambda}(u_{\lambda})<0\ (\mbox{since}\ \varphi_{\lambda}|_{[0,\tilde{u}]}=\hat{\psi}_{\lambda}|_{[0,\tilde{u}]}).
\end{eqnarray*}
This proves that we can always assume that (\ref{eq15}) holds.

From (\ref{eq15}) we have
\begin{eqnarray}\label{eq20}
\|Du_n\|_p^p-\frac{p}{q}\int\limits_{\Omega}\vartheta(z)u_n^q dz-\int\limits_{\Omega}p F(z,u_n,\lambda_n)dz<0\ \mbox{for all}\ n\geq 1.
\end{eqnarray}
Also, since $u_n\in S(\lambda_n)$ for all $n\geq 1$, we have
\begin{eqnarray}\label{eq21}
A(u_n)=\vartheta(z)u_n^{q-1}+N_{f_{\lambda_n}}(u_n),\ n\geq 1,
\end{eqnarray}
where $f_{\lambda_n}(z,x)=f(z,x,\lambda_n)$. On (\ref{eq21}) we act with $u_n\in W_0^{1,p}(\Omega)$ and obtain
\begin{eqnarray}\label{eq22}
-\|Du_n\|_p^p+\int\limits_{\Omega}\vartheta(z)u_n^q dz+\int\limits_{\Omega} f(z,u_n,\lambda_n)u_n dz=0\ \mbox{for all}\ n\geq 1.
\end{eqnarray}
Adding (\ref{eq20}) and (\ref{eq22}), we obtain
\begin{eqnarray}\label{eq23}
\int\limits_{\Omega}[f(z,u_n,\lambda_n)u_n-p F(z,u_n,\lambda_n)]dz\leq\left(\frac{p}{q}-1\right)\int\limits_{\Omega}\vartheta(z)u_n^q dz\ \mbox{for all}\ n\geq 1.
\end{eqnarray}
It is clear that in hypothesis $H_2(i)$ without any loss of generality, we may assume that $\lambda\to r(\lambda)$ and $\lambda\to \|a(\cdot,\lambda)\|_{\infty}$ are both nondecreasing in $(0,+\infty)$. Then hypotheses $H_2(i)(ii)$, imply that we find $c_2,c_3>0$ such that
\begin{eqnarray}\label{eq24}
&&c_3 x^{\tau}-c_4\leq f(z,x,\lambda_n)x-p F(z,x,\lambda_n)\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\geq 0,\ \mbox{all}\ n\geq 1.
\end{eqnarray}
Using (\ref{eq24}) in (\ref{eq23}) and recalling that $\tau>q$ (see hypothesis $H_2(ii)$), we infer that
\begin{eqnarray}\label{eq25}
\{u_n\}_{n\geq 1}\subseteq L^{\tau}(\Omega)\ \mbox{is bounded}.
\end{eqnarray}
It is clear from hypothesis $H_2(iii)$ that without any loss of generality, we may assume that $\tau<r_1=r(\lambda_1)\leq r(\lambda_n)$ for all $n\geq 1$ (see hypothesis $H_1(i)$). Suppose $N\neq p$. Then we can find $t\in (0,1)$ such that
\begin{eqnarray}\label{eq26}
\frac{1}{r_1}=\frac{1-t}{\tau}+\frac{t}{p^*}.
\end{eqnarray}
Invoking the interpolation inequality (see, for example, Gasinski and Papageorgiou \cite[p. 905]{8}), we have
\begin{eqnarray}\label{eq27}
& &\|u_n\|_r\leq\|u_n\|_{\tau}^{1-t}\|u_n\|_{p^*}^t\ \mbox{for all}\ n\geq 1,\nonumber\\
&\Rightarrow&\|u_n\|_r^r\leq M_1\|u_n\|^{tr}\ \mbox{for some}\ M_1>0,\ \mbox{all}\ n\geq 1\\
&&\hspace{1,5cm}(\mbox{see}\ (\ref{eq25})\ \mbox{and use the Sobolev embedding theorem}).\nonumber
\end{eqnarray}
From hypothesis $H_2(i)$, we have
$$f(z,x,\lambda_n)x\leq c_4 (1+x^r)\ \mbox{for a.a.}\ z\in\Omega,\ \mbox{all}\ x\geq 0,\ \mbox{some}\ c_4>0.$$
From (\ref{eq22}), (\ref{eq25}) and since $\tau>q$ (see $H_2(ii)$) and $\vartheta\in L^{\infty}(\Omega)$ (see $H_1$)
\begin{eqnarray}\label{eq28}
\|Du_n\|_p^p&\leq&c_5(1+\|u_n\|_r^r)\ \mbox{for some}\ c_5>0,\ \mbox{all}\ n\geq 1\nonumber\\
&\leq& c_6(1+\|u_n\|^{tr})\ \mbox{for some}\ c_6>0,\ \mbox{all}\ n\geq 1\ (\mbox{see}\ (\ref{eq27})).
\end{eqnarray}
From (\ref{eq26}) and our hypothesis on $\tau$ (see $H_2(iii)$), it follows that $tr<p$. Hence from (\ref{eq28}) we infer that
$$\{u_n\}_{n\geq 1}\subseteq W_0^{1,p}(\Omega)\ \mbox{is bounded}.$$
If $N=p$, then since $p^*=+\infty$ and $W_0^{1,p}(\Omega)$ is compactly embedded into $L^q(\Omega)$ for all $q\in [1,\infty)$, then the above argument works, if we replace $p^*$ by $\eta>r>p$ big. Then again we reach the same conclusion.

So, we may assume that
\begin{eqnarray}\label{eq29}
u_n\overset{w}{\to}u\ \mbox{in}\ W_0^{1,p}(\Omega)\ \mbox{and}\ u_n\to u\ \mbox{in}\ L^{r_*}(\Omega)\ (r_*=r(\lambda^*))\ \mbox{as}\ n\to\infty.
\end{eqnarray}
On (\ref{eq21}) we act with $u_n-u\in W_0^{1,p}(\Omega)$, pass to the limit as $n\to\infty$ and use (\ref{eq29}). Then
\begin{eqnarray}\label{eq30}
& &\lim\limits_{n\to\infty}\langle A(u_n),u_n-u\rangle=0\nonumber,\\
&\Rightarrow& u_n\to u\ \mbox{in}\ W_0^{1,p}(\Omega)\ \mbox{as}\ n\to\infty\\
&&\hspace{2cm}(\mbox{see Proposition}\ \ref{prop3}).\nonumber
\end{eqnarray}
Passing to the limit as $n\to\infty$ in (\ref{eq21}) and using (\ref{eq30}), we obtain
\begin{eqnarray}\label{eq31}
A(u)=\vartheta(z)u^{q-1}+N_{f_{\lambda^*}}(u).
\end{eqnarray}
We need to show that $u\neq 0$, because then $u\in S(\lambda^*)$, that is $\lambda^*\in\mathcal{L}$. To this end, we consider the following auxiliary Dirichlet problem
\begin{eqnarray}\label{eq32}
-\Delta_p u(z)=\vartheta(z)u(z)^{q-1}\ \mbox{in}\ \Omega_{+},\ u|_{\partial\Omega_{+}}=0,\ u>0.
\end{eqnarray}
Since $q<p$, a straightforward application of the direct method (as before), establishes that problem (\ref{eq32}) admits a nontrivial solution $\overline{u}\in W_0^{1,p}(\Omega),\ \overline{u}\geq 0$. The nonlinear regularity theory and the nonlinear maximum principle (see, for example, Gasinski and Papageorgiou \cite[pp. 737-738]{8}), imply that $\overline{u}\in\mbox{int}\, C_{+}$. Moreover, Theorem $2$ of Diaz and Saa \cite{6}, implies that $\overline{u}\in\mbox{int}\, C_{+}$ is the unique positive solution of (\ref{eq32}).

Now, let $\lambda\in\mathcal{L}$ and $u_{\lambda}\in S(\lambda)$. We introduce the following Carath\'eodory function
\begin{eqnarray}\label{eq33}
j(z,x)=\left\{  \begin{array}{ll}
     0\ &\mbox{if}\ x<0\\
     \vartheta(z)x^{q-1}\ &\mbox{if}\ 0\leq x\leq u_{\lambda}(z),\ (z,x)\in\Omega_{+}\times\RR\\
     \vartheta(z)u_{\lambda}(z)^{q-1}\ &\mbox{if}\ u_{\lambda}(z)<x.
    \end{array}\right.
\end{eqnarray}
Let $J(z,x)=\int\limits_0^x j(z,s)ds$ and consider the $C^1$-functional $J:W_0^{1,p}(\Omega)\to\RR$ defined by
$$J(u)=\frac{1}{p}\|Du\|_{L^p(\Omega_{+},\RR^N)}^p-\int\limits_{\Omega_{+}}J(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega_{+}).$$
As before, (\ref{eq33}) implies that $J(\cdot)$ is coercive. Also, it is sequentially weakly lower semicontinuous. So, we can find $\tilde{u}\in W^{1,p}(\Omega_{+})$ such that
\begin{eqnarray}\label{eq34}
J(\tilde{u})=\inf[J(u):\ u\in W_0^{1,p}(\Omega_{+})].
\end{eqnarray}
Since $q<p$ and $u_{\lambda}|_{\overline{\Omega}_{+}}>0$ (recall $u_{\lambda}\in\mbox{int}\, C_{+}$),as in previous similar cases, we have
$$J(\tilde{u})<0=J(0),\ \mbox{hence}\ \tilde{u}\neq 0.$$
From (\ref{eq34}), we have
\begin{eqnarray}\label{eq35}
& & J'(\tilde{u})=0,\nonumber\\
&\Rightarrow& A(\tilde{u})=N_j(\tilde{u}).
\end{eqnarray}
On (\ref{eq35}) we act with $-\tilde{u}^{-}\in W_0^{1,p}(\Omega_{+})$ and with $(\tilde{u}-u_{\lambda})^{+}\in W_0^{1,p}(\Omega_{+})$ (here we use the fact that $u_{\lambda}|_{\overline{\Omega}_{+}}>0$) and we obtain
\begin{eqnarray*}
& & 0<\tilde{u}(z)\leq u_{\lambda}(z)\ \mbox{for all}\ z\in\overline{\Omega}_{+},\ \tilde{u}\neq 0,\\
&\Rightarrow& \tilde{u}\ \mbox{is a positive solution of}\ (\ref{eq32}),\\
&\Rightarrow& \tilde{u}=\overline{u}\ (\mbox{recall that}\ \overline{u}\ \mbox{is the unique positive solution of}\ (\ref{eq32})),\\
&\Rightarrow& \overline{u}\leq u_{\lambda}\ \mbox{in}\ \Omega_{+}.
\end{eqnarray*}
So, we have
\begin{eqnarray*}
& &\overline{u}(z)\leq u_n(z)\ \mbox{for all}\ z\in\overline{\Omega}_{+},\ \mbox{all}\ n\geq 1,\\
&\Rightarrow& \overline{u}(z)\leq u(z)\  \mbox{for all}\ z\in\Omega_{+}\ (\mbox{see}\ (\ref{eq30})),\\
&\Rightarrow& u\neq 0.
\end{eqnarray*}
Therefore, $u\in S(\lambda^*)$ and so $\lambda^*\in\mathcal{L}$.
\end{proof}

Next, we look for additional positive solutions for problem ($P_{\lambda}$). To this end, we consider the following auxiliary Dirichlet problem:
\begin{center}
$\left\{  \begin{array}{ll}
     -\Delta_p u(z)=\vartheta^{+}(z) u(z)^{q-1}+f(z,u(z),\lambda)\ \mbox{in}\ \Omega\\
  u=0\ \mbox{on} \  \partial\Omega\\
   u>0\ \mbox{in}\ \Omega.
     \end{array}\right. \hspace{4cm}(Au_{\lambda})$
\end{center}
Reasoning as in the proofs of Propositions \ref{prop4}, \ref{prop5} and \ref{prop6} (with $\Omega_{+}$ replaced by $\Omega$) we obtain.
\begin{prop}\label{prop7}
If hypotheses $H_1$ and $H_2$ hold, then there exists $\lambda_0^*\in (0,\lambda^*]$ such that  for all $\lambda\in (0,\lambda_0^*]$ problem $(Au_{\lambda})$ has at least one positive solution $\tilde{u}_{\lambda}\in\mbox{int}\, C_{+}$.
\end{prop}
\begin{remark}
Note that in this case, the solution $\tilde{u}_{\lambda}\in C_{+}\setminus\{0\}$ satisfies
\begin{eqnarray*}
& &-\Delta_p \tilde{u}_{\lambda}(z)=\vartheta^{+}(z)\tilde{u}_{\lambda}(z)^{q-1}+f(z,\tilde{u}_{\lambda}(z),\lambda)\geq 0\ \mbox{for a.a.}\ z\in\Omega\\
&&\hspace{5cm}(\mbox{see hypotheses}\ H_2(iii))\\
&\Rightarrow&\tilde{u}_{\lambda}\in\mbox{int}\, C_{+}\ (\mbox{see Gasinski and Papageorgiou \cite[p. 738]{8}}).
\end{eqnarray*}
\end{remark}
We can use Proposition \ref{prop7}, to produce a multiplicity result for the positive solutions of problem $(P_{\lambda})$ with $\lambda\in (0,\lambda_0^*]$.
\begin{prop}\label{prop8}
If hypotheses $H_1$ and $H_2$ hold and $\lambda\in (0,\lambda_0^*]$, then problem $(P_{\lambda})$ has at least two positive solutions
\begin{eqnarray*}
&&u_{\lambda},\ \hat{u}_{\lambda}\in C_{+}\setminus\{0\},\ u_{\lambda}\neq\hat{u}_{\lambda}\\
&&0<u_{\lambda}(z)\leq\hat{u}_{\lambda}(z)\ \mbox{for all}\ z\in\Omega.
\end{eqnarray*}
\end{prop}
\begin{proof}
Let $\mu\in (\lambda,\lambda_0^*)$. From Proposition \ref{prop4}, we know that $\lambda,\mu\in\mathcal{L}$ and we can find $u_{\lambda}\in S(\lambda)\subseteq C_{+}$ and $\hat{u}_{\lambda}\in\mbox{int}\, C_{+}$ solution of $(Au_{\mu})$. We claim that we can have
\begin{eqnarray}\label{eq36}
u_{\lambda}\leq\tilde{u}_{\mu}.
\end{eqnarray}
Indeed note that
\begin{eqnarray}\label{eq37}
-\Delta_p \tilde{u}_{\mu}(z)&=&\vartheta^{+}(z)\tilde{u}_{\mu}(z)^{q-1}+f(z,\tilde{u}_{\mu}(z),\mu)\nonumber\\
&\geq&\vartheta(z)\tilde{u}_{\mu}(z)^{q-1}+f(z,\tilde{u}_{\mu}(z),\lambda)\ \mbox{for a.a.}\ z\in\Omega\\
&&\hspace{3cm}(\mbox{see hypothesis}\ H_2(iii)).\nonumber
\end{eqnarray}
We consider the following truncation of the reaction of problem $(P_{\lambda})$.
\begin{eqnarray}\label{eq38}
\hat{g}_{\lambda}(z,x)=\left\{  \begin{array}{ll}
      0,\ &\mbox{if}\ x<0\\
      \vartheta(z)x^{q-1}+f(z,x,\lambda)\ &\mbox{if}\ 0\leq x\leq\tilde{u}_{\mu}(z),\\
      \vartheta(z)\tilde{u}_{\mu}(z)^{q-1}+f(z,\tilde{u}_{\mu}(z),\lambda)\ &\mbox{if}\ \tilde{u}_{\mu}(z)<x.
     \end{array}\right.
\end{eqnarray}
This is a Carath\'eodory function.

We set $\hat{G}_{\lambda}(z,x)=\int\limits_0^x\hat{g}_{\lambda}(z,s)ds$ and introduce the $C^1$-functional $\hat{\psi}_{\lambda}: W_0^{1,p}(\Omega)\to\RR$ defined by
$$\hat{\psi}_{\lambda}(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}\hat{G}_{\lambda}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega).$$
Evidently $\hat{\psi}_{\lambda}$ is coercive (see (\ref{eq38})) and sequentially weakly lower semicontinuous. So, we can find $u_{\lambda}\in W_0^{1,p}(\Omega)$ such that
\begin{eqnarray}\label{eq39}
\hat{\psi}_{\lambda}(u_{\lambda})=\inf[\hat{\psi}_{\lambda}(u):\ u\in W_0^{1,p}(\Omega)].
\end{eqnarray}
Since $q<p$, as before (see, for example, the proof of Proposition \ref{prop4}), we have
$$\hat{\psi}_{\lambda}(u_{\lambda})<0=\hat{\psi}_{\lambda}(0),\ \mbox{hence}\ u_{\lambda}\neq 0.$$
From (\ref{eq39}) we have
\begin{eqnarray}\label{eq40}
& &\hat{\psi}_{\lambda}'(u_{\lambda})=0,\nonumber\\
&\Rightarrow& A(u_{\lambda})=N_{\hat{g}_{\lambda}}(u_{\lambda}).
\end{eqnarray}
On (\ref{eq40}) we act with $-u_{\lambda}^{-}\in W_0^{1,p}(\Omega)$ and with $(u_{\lambda}-\tilde{u}_{\mu})^{+}\in W_0^{1,p}(\Omega)$. As in the proof of Proposition \ref{prop4}, using this time (\ref{eq37}), we show that
$$u_{\lambda}\in [0,\tilde{u}_{\mu}]=\{u\in W_0^{1,p}(\Omega):\ 0\leq u(z)\leq\tilde{u}_{\mu}(z)\ \mbox{for a.a.}\ z\in\Omega\}.$$
Hence $u_{\lambda}\in S(\lambda)\subseteq C_{+}$ (see (\ref{eq38})) and $u_{\lambda}(z)>0$ for all $z\in\Omega$ (by Harnack's inequality, see Pucci and Serrin \cite[p. 163]{17}). Therefore (\ref{eq36}) holds.

We introduce the following truncation of the reaction of problem $(P_{\lambda})$:
\begin{eqnarray}\label{eq41}
k_{\lambda}(z,x)=\left\{  \begin{array}{ll}
      \vartheta(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\lambda)\ &\mbox{if}\ x\leq u_{\lambda}(z)\\
      \vartheta(z)x^{q-1}+f(z,x,\lambda)\ &\mbox{if}\ u_{\lambda}(z)<x.
     \end{array}\right.
\end{eqnarray}
This is a Carath\'eodory function.

We set $K_{\lambda}(z,x)=\int\limits_0^x k_{\lambda}(z,s)ds$ and consider the $C^1$-functional $\tau_{\lambda}: W_0^{1,p}(\Omega)\to\RR$ defined by
$$\tau_{\lambda}(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}K_{\lambda}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega).$$
From the proof of Proposition \ref{prop6}, we know that
\begin{eqnarray}\label{eq42}
\tau_{\lambda}\ \mbox{satisfies the}\ C- \mbox{condition}.
\end{eqnarray}
We truncate $k_{\lambda}(z,\cdot)$ as follows:
\begin{eqnarray}\label{eq43}
\hat{k}_{\lambda}(z,x)=\left\{  \begin{array}{ll}
      k_{\lambda}(z,x)\ &\mbox{if}\ x\leq \tilde{u}_{\mu}(z)\\
      k_{\lambda}(z,\tilde{u}_{\mu}(z))\ &\mbox{if}\ \tilde{u}_{\mu}(z)<x.
     \end{array}\right.
\end{eqnarray}
This too is a Carath\'eodory function. We set $\hat{K}_{\lambda}(z,x)=\int\limits_0^x \hat{k}_{\lambda}(z,s)ds$ and consider the $C^1$-functional $\hat{\tau}_{\lambda}:W_0^{1,p}(\Omega)\to\RR$ defined by
$$\hat{\tau}_{\lambda}(u)=\frac{1}{p}\|Du\|_p^p-\int\limits_{\Omega}\hat{K}_{\lambda}(z,u(z))dz\ \mbox{for all}\ u\in W_0^{1,p}(\Omega).$$
From (\ref{eq43}) it is clear that $\hat{\tau}_{\lambda}$ is coercive. Also, it is sequentially weakly lower semicontinuous. So, we can find $\overline{u}_{\lambda}\in W_0^{1,p}(\Omega)$ such that
\begin{eqnarray}\label{eq44}
& & \hat{\tau}_{\lambda}(\overline{u}_{\lambda})=\inf[\hat{\tau}_{\lambda}(u):\ u\in W_0^{1,p}(\Omega)],\nonumber\\
&\Rightarrow&\hat{\tau}_{\lambda}'(\overline{u}_{\lambda})=0,\nonumber\\
&\Rightarrow& A(\overline{u}_{\lambda})=N_{\hat{k}_{\lambda}}(\overline{u}_{\lambda}).
\end{eqnarray}
On (\ref{eq44}) we act, first with $(u_{\lambda}-\overline{u}_{\lambda})\in W_0^{1,p}(\Omega)$ and then with $(\overline{u}_{\lambda}-\tilde{u}_{\mu})^{+}\in W_0^{1,p}(\Omega)$. As in the proof of Proposition \ref{prop5}, we show that
$$\overline{u}_{\lambda}\in [u_{\lambda},\tilde{u}_{\mu}]=\{u\in W_0^{1,p}(\Omega):\ u_{\lambda}(z)\leq u(z)\leq\tilde{u}_{\mu}(z)\ \mbox{for a.a.}\ z\in\Omega\}.$$
If $\overline{u}_{\lambda}\neq u_{\lambda}$, then this is the desired second positive solution of problem $(P_{\lambda})$ and $u_{\lambda}\leq\overline{u}_{\lambda}=\hat{u}_{\lambda}$.

So, we may assume that $\overline{u}_{\lambda}=u_{\lambda}$. Let $\rho=\|\tilde{u}_{\mu}\|_{\infty}$ (recall that $\tilde{u}\in\mbox{int}\, C_{+}$, see Proposition \ref{prop7}) and let $\xi_{\rho}=\xi_{\rho}(\mu)>0$ be as postulated by hypothesis $H_2(iv)$ for the perturbation $f(z,\cdot,\mu)$. We have
\begin{eqnarray}\label{eq45}
& &-\Delta_p u_{\lambda}(z)+\xi_{\rho}u_{\lambda}(z)^{p-1}\nonumber\\
&=&\vartheta(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\lambda)+\xi_{\rho}u_{\lambda}(z)^{p-1}\nonumber\\
&=&\vartheta(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\mu)+\xi_{\rho}u_{\lambda}(z)^{p-1}+[f(z,u_{\lambda}(z),\lambda)-f(z,u_{\lambda}(z),\mu)] \nonumber\\
&\leq&\vartheta^{+}(z)u_{\lambda}(z)^{q-1}+f(z,u_{\lambda}(z),\mu)+\xi_{\rho}u_{\lambda}(z)^{p-1}-\delta_{(\lambda',\lambda)}(z)\nonumber\\
&&\hspace{2cm}\mbox{with}\ \delta_{(\lambda',\lambda)}(z)=f(z,u_{\lambda}(z),\mu)-f(z,u_{\lambda}(z),\lambda)\nonumber\\
&\leq&\vartheta^{+}(z)\tilde{u}_{\mu}(z)+f(z,\tilde{u}_{\mu}(z),\mu)+\xi_{\rho}\tilde{u}_{\mu}(z)^{p-1}\ (\mbox{see hypothesis}\ H_2(iv))\nonumber\\
&=&-\Delta_p\tilde{u}_{\mu}(z)+\xi_{\rho}\tilde{u}_{\mu}(z)^{p-1}\ \mbox{for a.a.}\ z\in\Omega.
\end{eqnarray}
For every $K\subseteq\Omega$ compact, we have $u_{\lambda}|_{K}\geq\xi>0$ (recall $u_{\lambda}(z)>0$ for all $z\in\Omega$, see Proposition \ref{prop4}). So, by hypothesis $H_2(iii)$ we have
$$\delta_{\lambda,\mu}|_{K}\geq m_{\xi}>0.$$
Then from (\ref{eq45}) and Proposition $2.6$ of Arcoya and Ruiz \cite{4} (recall that $\tilde{u}_{\mu}\in\mbox{int}\, C_{+}$), we infer that
\begin{eqnarray}\label{eq46}
\tilde{u}_{\mu}-u_{\lambda}\in\mbox{int}\, C_{+}.
\end{eqnarray}
We claim that $u_{\lambda}$ is a local $C_0^1(\overline{\Omega})$-minimizer of the functional $\tau_{\lambda}$. Indeed, if this is not the case, we can find $\{u_n\}_{n\geq 1}\subseteq C_0^1(\overline{\Omega})$ such that
\begin{eqnarray}\label{eq47}
u_n\to u_{\lambda}\ \mbox{in}\ C_0^1(\overline{\Omega})\ \mbox{and}\ \tau_{\lambda}(u_n)<\tau_{\lambda}(u_{\lambda})\ \mbox{for all}\ n\geq 1.
\end{eqnarray}
From (\ref{eq46}) and (\ref{eq47}) it follows that we can find $n_0\in\NN$ such that
\begin{eqnarray*}
& & u_n\leq\tilde{u}_{\mu}\ \mbox{for all}\ n\geq n_0,\\
&\Rightarrow& u_n^{+}\leq\tilde{u}_{\mu}\ \mbox{for all}\ n\geq n_0\ (\mbox{recall}\ \tilde{u}_{\mu}\in\mbox{int}\, C_{+}).
\end{eqnarray*}
Also, we have
$$\hat{\tau}_{\lambda}(u_n^{+})=\tau_{\lambda}(u_n^{+})\leq\tau_{\lambda}(u_n)<\tau_{\lambda}(u_{\lambda})\ \mbox{for all}\ n\geq n_0$$
which contradicts the fact that $u_{\lambda}=\overline{u}_{\lambda}$ is a global minimizer of $\hat{\tau}_{\lambda}$.

Since $u_{\lambda}$ is a local $C_0^1(\overline{\Omega})$-minimizer of $\tau_{\lambda}$. We may assume that $u_{\lambda}$ is an isolated critical point of $\tau_{\lambda}$, or otherwise we already have a sequence of distinct positive solutions, since the critical set of $\tau_{\lambda}$ is in $[u_{\lambda})=\{u\in W_0^{1,p}(\Omega):\ u_{\lambda}(z)\leq u(z)\ \mbox{for a.a}\ z\in\Omega\}$ (see (\ref{eq41})). Therefore, we can find $\rho\in (0,1)$ small such that
\begin{eqnarray}\label{eq48}
\tau_{\lambda}(u_{\lambda})<\inf[\tau_{\lambda}(u):\ \|u-u_{\lambda}\|=\rho]=m_{\lambda}
\end{eqnarray}
(see Aizicovici, Papageorgiou and Staicu \cite{1} (proof of Proposition $29$)).

Moreover, by virtue of hypothesis $H_2(ii)$, if $u\in\mbox{int}\, C_{+}$, then
\begin{eqnarray}\label{eq49}
\tau_{\lambda}(tu)\to-\infty\ \mbox{as}\ t\to+\infty.
\end{eqnarray}
Because of (\ref{eq42}), (\ref{eq48}) and (\ref{eq49}), we can apply Theorem \ref{th1} (the mountain pass theorem) and find $\hat{u}_{\lambda}\in W_0^{1,p}(\Omega)$ such that
\begin{eqnarray}\label{eq50}
\tau_{\lambda}'(\hat{u}_{\lambda})=0\ \mbox{and}\ m_{\lambda}\leq\tau(\hat{u}_{\lambda}).
\end{eqnarray}
From (\ref{eq49}), (\ref{eq50}) and since the critical set of $\tau_{\lambda}$ is in $[u_{\lambda})$, it follows that
$$\hat{u}_{\lambda}\in S(\lambda)\subseteq C_{+}\ \mbox{and}\ \hat{u}_{\lambda}\neq u_{\lambda},\ u_{\lambda}\leq\hat{u}_{\lambda}.$$
Finally from Proposition \ref{prop4}, we have
$$0<u_{\lambda}(z)\leq\hat{u}_{\lambda}(z)\ \mbox{for all}\ z\in\Omega.$$
The proof is now complete.
\end{proof}

So, summarizing the situation for problem $(P_{\lambda})$, we can state the following result describing the set of positive solutions as the parameter $\lambda>0$ varies.
\begin{theorem}\label{th9}
If hypotheses $H_1$ and $H_2$ hold, then
\begin{itemize}
\item[(a)] there exists $\lambda^*>0$ such that  for all $\lambda\in (0,\lambda^*]$ problem $(P_{\lambda})$ has at least one positive solution $u_{\lambda}\in C_{+}$ with $u_{\lambda}(z)>0$ for all $z\in\Omega$ and for $\lambda>\lambda^*$ there are no positive solutions;
 \item[(b)] there exists $\lambda_0^* \in (0,\lambda^*]$ such that  for all $\lambda\in (0,\lambda_0^*)$ problem $(P_{\lambda})$ has at least two positive solutions
     $$u_{\lambda},\ \hat{u}_{\lambda}\in C_{+},\ u_{\lambda}\leq \hat{u}_{\lambda},\ u_{\lambda}\neq\hat{u}_{\lambda},\ u_{\lambda}(z)>0\ \mbox{for all}\ z\in\Omega.$$
\end{itemize}
\end{theorem}
\begin{remark}
It will be interesting to know if $\lambda_0^*=\lambda^*$. Also, it is not clear to us if this result can be extended to Neumann problems. A careful inspection of the proofs reveals that they fail in the Neumann case.
\end{remark}

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\end{document}


