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# Unilateral global bifurcation for *p*-Laplacian with singular weight

- Xin Liao
^{1}and - Guowei Dai
^{1}Email author

**2013**:577

https://doi.org/10.1186/1029-242X-2013-577

© Liao and Dai; licensee Springer. 2013

**Received:**4 February 2013**Accepted:**19 November 2013**Published:**11 December 2013

## Abstract

In this paper, we establish a Dancer-type unilateral global bifurcation theorem for the one-dimensional *p*-Laplacian with a singular weight which may not be in ${L}^{1}$. As the applications of this theorem, we prove the existence of nodal solutions for *p*-Laplacian with ${f}_{0}\in [0,+\mathrm{\infty}]$ or ${f}_{\mathrm{\infty}}\in [0,+\mathrm{\infty}]$, where $f(s)/({|s|}^{p-2}s)$ approaches ${f}_{0}$ and ${f}_{\mathrm{\infty}}$ as *s* approaches 0 and ∞, respectively.

**MSC:**34B16, 34C10, 34C23.

## Keywords

- unilateral bifurcation
- nodal solutions
- singular weight

## 1 Introduction

*p*-Laplacian problem

*λ*is a positive parameter, $m(x)$ and $f\in C(\mathbb{R},\mathbb{R})$ satisfy the following assumptions:

- (A1)$m(x)?\mathcal{A}$, $m(x)=0$ and $m(x)?0$ on any subinterval of $(0,1)$, where$\mathcal{A}=\{m(x)?{L}_{\mathrm{loc}}^{1}(0,1)|{?}_{0}^{1}{x}^{p-1}{(1-x)}^{p-1}m(x)\phantom{\rule{0.2em}{0ex}}dx<+\mathrm{8}\};$
- (A2)
$f(s)s>0$ for $s?0$.

*k*th eigenvalue which is obtained in [[1], Theorem 2.1] of the following problem

Let ${f}_{0}:={lim}_{s\to 0}f(s)/{\phi}_{p}(s)$. By an argument similar to Rabinowitz’s unilateral global bifurcation theory [[2], Theorem 1.27], Kajikiya *et al.* [3] established the following result.

**Theorem 1.1** *Assume that* (A1)-(A2) *hold and* ${f}_{0}\in (0,+\mathrm{\infty})$. *Then*, *for each* $k\in \mathbb{N}$, *there exist two unbounded sub*-*continua* ${\mathcal{C}}_{k}^{\pm}$ *in*
*bifurcating from* $({\lambda}_{k}/{f}_{0},0)$. *Furthermore*, ${\mathcal{C}}_{k}^{\pm}\cap (\mathbb{R}\times \{0\})=\{({\lambda}_{k}/{f}_{0},0)\}$ *and if* $(\lambda ,u)\in {\mathcal{C}}_{k}^{+}\setminus \{({\lambda}_{k}/{f}_{0},0)\}$ (${\mathcal{C}}_{k}^{-}\setminus \{({\lambda}_{k}/{f}_{0},0)\}$), *then* *u* *is a* $(k-1)$-*nodal solution in* $(0,1)$ *satisfying* ${u}^{\prime}(0)>0$ (${u}^{\prime}(0)<0$), *respectively*.

However, as pointed out by Dancer [4, 5], López-Gómez [6] and Shi and Wang [7], the original statement of Theorem 1.27 of [2] is stronger than what one can actually prove so far. In [4], Dancer gave a corrected version of the unilateral global bifurcation theorem for a linear operator which has been extended to the one-dimensional *p*-Laplacian problem by Dai and Ma [8]. The first purpose of the present work is to repair the proof of Theorem 1.1 by the methods which we used in [8].

*et al.*[3] studied the existence of positive solutions as well as sign-changing solutions of problem (1.1) with ${f}_{0}\in (0,+\mathrm{\infty})$ and ${f}_{\mathrm{\infty}}=0$. Later, they [9] again considered the case of ${f}_{0}\in (0,+\mathrm{\infty})$ and ${f}_{\mathrm{\infty}}=+\mathrm{\infty}$. Another aim of this paper is to investigate the existence of nodal solutions for problem (1.1) with all of the following six cases:

- (1)
${f}_{0}\in (0,+\mathrm{\infty})$ and ${f}_{\mathrm{\infty}}\in (0,+\mathrm{\infty})$;

- (2)
${f}_{0}=0$ and ${f}_{\mathrm{\infty}}\in (0,+\mathrm{\infty})$;

- (3)
${f}_{0}=+\mathrm{\infty}$ and ${f}_{\mathrm{\infty}}\in (0,+\mathrm{\infty})$;

- (4)
${f}_{0}=0$ and ${f}_{\mathrm{\infty}}=+\mathrm{\infty}$;

- (5)
${f}_{0}=\mathrm{\infty}$ and ${f}_{\mathrm{\infty}}=+\mathrm{\infty}$;

- (6)
${f}_{0}=0$ and ${f}_{\mathrm{\infty}}=0$.

When $p=2$, $m(x)\in C[0,1]$, Ma and Thompson [10] considered the interval of *λ*, in which there exist nodal solutions of problem (1.1) under some suitable assumptions on *f*. In [11], Ma extended the above results to the case of $m\in C(0,1)$ satisfying $0<{\int}_{0}^{1}x(1-x)m(x)\phantom{\rule{0.2em}{0ex}}dx<+\mathrm{\infty}$. For $p\ne 2$, Del Pino *et al.* [12] investigated the existence of solutions for problem (1.1) with $m\equiv 1$ using the Leray-Schauder degree by the deformation along *p*. By the upper and lower solutions method, fixed point index theory on cones and the shooting method, the authors of [13–16] studied the existence of positive solutions or sign-changing solutions for problem (1.1) under some suitable assumptions on *m* and *f*. In [17, 18], Lee and Sim studied the existence of positive solutions as well as sign-changing solutions for problem (1.1) when $m\in {L}^{1}(0,1)$. Recently, Dai [19] studied the existence of nodal solutions for problem (1.1) when $m\in C[0,1]$ and ${f}_{0}\notin (0,+\mathrm{\infty})$ or ${f}_{\mathrm{\infty}}\notin (0,+\mathrm{\infty})$. In this paper, we extend the corresponding results of [19] to the case of *m* satisfying (A1). Clearly, the above six cases for problem (1.1) have not been studied by now.

The main results of the present paper are the following two theorems.

**Theorem 1.2** *Let* (A1)-(A2) *hold and* ${f}_{0}\in (0,+\mathrm{\infty})$. *Then from each* $({\lambda}_{k}/{f}_{0},0)$ *it bifurcates an unbounded continuum* ${\mathcal{C}}_{k}$ *of solutions to problem* (1.1), *with exactly* $k-1$ *simple zeros*.

**Theorem 1.3** *Let* (A1)-(A2) *hold and* ${f}_{0},{f}_{\mathrm{\infty}}\in (0,+\mathrm{\infty})$. *If* $\lambda \in ({\lambda}_{k}/{f}_{\mathrm{\infty}},{\lambda}_{k}/{f}_{0})\cup ({\lambda}_{k}/{f}_{0},{\lambda}_{k}/{f}_{\mathrm{\infty}})$, *then problem* (1.1) *has at least two solutions* ${u}_{k}^{+}$ *and* ${u}_{k}^{-}$ *such that* ${u}_{k}^{+}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is positive near* 0, *and* ${u}_{k}^{-}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is negative near* 0.

The rest of this paper is arranged as follows. In Section 2, we establish the unilateral global bifurcation theory for problem (1.1). In Section 3, we prove the existence of nodal solutions for problem (1.1) with any one of the above six cases.

## 2 Unilateral global bifurcation

*E*be the Banach space ${C}_{0}^{1}[0,1]$ with the norm $\parallel u\parallel ={\parallel u\parallel}_{\mathrm{\infty}}+{\parallel {u}^{\prime}\parallel}_{\mathrm{\infty}}$, where ${\parallel u\parallel}_{\mathrm{\infty}}={max}_{x\in [0,1]}|u|$. Consider the following auxiliary problem

It is well known that ${G}_{p}:{L}^{1}(0,1)\to E$ is continuous and maps equi-integrable sets of ${L}^{1}(0,1)$ into relatively compacts of *E*. One may refer to Lee and Sim [17] and Manásevich and Mawhin [20] for details.

*f*satisfying (A2). Hence, for $(\lambda ,u)\in \mathbb{R}\times E$, we can define

Lemma 2.4 of [3] has shown that ${T}_{\lambda}$ and *F* are completely continuous from $\mathbb{R}\times E$ to *E*. So $I-{T}_{\lambda}$ is a completely continuous vector field in ${C}^{1}[0,1]$. Thus the Leray-Schauder degree ${\mathrm{d}}_{\mathrm{LS}}(I-{T}_{\lambda},{B}_{r}(0),0)$ is well defined for an arbitrary *r*-ball ${B}_{r}(0)$ and $\lambda \ne {\lambda}_{k}$, $k\in \mathbb{N}$.

**Lemma 2.1** ([[3], Theorem 3.2])

*Assume that*(A1)

*holds and let*${\{{\lambda}_{k}\}}_{k\in \mathbb{N}}$

*be the sequence of eigenvalues of problem*(1.2).

*Let*

*λ*

*be a constant with*$\lambda \ne {\lambda}_{k}$

*for all*$k\in \mathbb{N}$.

*Then*,

*for arbitrary*$r>0$,

*where* *β* *is the number of eigenvalues* ${\lambda}_{k}$ *of problem* (1.2) *less than* *λ*.

Using Lemma 2.1 and the famous global interval bifurcation theorem due to Schmitt and Thompson [21], the authors of [3] established the following result.

**Lemma 2.2** ([[3], Lemma 4.4])

*Assume that* (A1)-(A2) *hold and* ${f}_{0}\in (0,+\mathrm{\infty})$. *Then* $({\lambda}_{k}/{f}_{0},0)$ *is a bifurcation point of*
*and the associated bifurcation branch* ${\mathcal{C}}_{k}$ *in* $\mathbb{R}\times E$ *whose closure contains* $({\lambda}_{k}/{f}_{0},0)$ *is either unbounded or contains a pair* $({\lambda}_{j}/{f}_{0},0)$ *with* $j\ne k$.

Next, we shall prove that the first choice of the alternative of Lemma 2.2 is the only possibility. Let ${S}_{k}^{+}$ denote the set of functions in *E* which have exactly $k-1$ interior nodal zeros in $(0,1)$ and are positive near $x=0$, and set ${S}_{k}^{-}=-{S}_{k}^{+}$, and ${S}_{k}={S}_{k}^{+}\cup {S}_{k}^{-}$. It is clear that ${S}_{k}^{+}$ and ${S}_{k}^{-}$ are disjoint and open in *E*. Finally, let ${\mathrm{\Phi}}_{k}^{\pm}=\mathbb{R}\times {S}_{k}^{\pm}$ and ${\mathrm{\Phi}}_{k}=\mathbb{R}\times {S}_{k}$ under the product topology.

**Lemma 2.3** *Under the assumptions of Lemma * 2.2, *the last alternative of Lemma * 2.2 *is impossible if* ${\mathcal{C}}_{k}\subset ({\mathrm{\Phi}}_{k}\cup \{({\lambda}_{k}/{f}_{0},0)\})$.

*Proof*Suppose on the contrary that if there exists $({\lambda}_{m},{u}_{m})\to ({\lambda}_{j}/{f}_{0},0)$ when $m\to +\mathrm{\infty}$ with $({\lambda}_{m},{u}_{m})\in {\mathcal{C}}_{k}$, ${u}_{m}\not\equiv 0$ and $j\ne k$. Let $f(s)={f}_{0}{\phi}_{p}(s)+\xi (s)$ with $\xi (s)/{\phi}_{p}(s)\to 0$ as $s\to 0$. Set ${v}_{m}:={u}_{m}/\parallel {u}_{m}\parallel $, then ${v}_{m}$ should be a solution of the problem

*u*and

and $\parallel {v}_{0}\parallel =1$. Hence ${v}_{0}\in {S}_{j}$ which is an open set in *E*, and as a consequence for some *m* large enough, ${v}_{m}\in {S}_{j}$, and this is a contradiction. □

*Proof of Theorem 1.2*Taking into account Lemma 2.2 and Lemma 2.3, we only need to prove that ${\mathcal{C}}_{k}\subset ({\mathrm{\Phi}}_{k}\cup \{({\lambda}_{k}/{f}_{0},0)\})$. By an argument similar to that of Lemma 2.3, we can show that there exists a neighborhood $\mathcal{O}$ of $({\lambda}_{k}/{f}_{0},0)$ such that $\mathcal{O}\cap {\mathcal{C}}_{k}\subset ({\mathrm{\Phi}}_{k}\cup \{({\lambda}_{k}/{f}_{0},0)\})$. Suppose ${\mathcal{C}}_{k}\not\subset ({\mathrm{\Phi}}_{k}\cup \{({\lambda}_{k}/{f}_{0},0)\})$. Then there exists $(\lambda ,u)\in {\mathcal{C}}_{k}\cap (\mathbb{R}\times \partial {S}_{k})$ such that $(\lambda ,u)\ne ({\lambda}_{k}/{f}_{0},0)$, $u\notin {S}_{k}$, and $({\lambda}_{n},{u}_{n})\to (\lambda ,u)$ with $({\lambda}_{n},{u}_{n})\in {\mathcal{C}}_{k}\cap (\mathbb{R}\times {S}_{k})$. Since $u\in \partial {S}_{k}$, by Lemma 4.1 of [3], $u\equiv 0$. Let ${w}_{n}:={u}_{n}/\parallel {u}_{n}\parallel $, then ${w}_{n}$ should be a solution of the problem

and $\parallel {w}_{0}\parallel =1$. Hence $\lambda {f}_{0}={\lambda}_{j}$ for some $j\ne k$. Therefore, $({\lambda}_{n},{u}_{n})\to ({\lambda}_{j}/{f}_{0},0)$ with $({\lambda}_{n},{u}_{n})\in {\mathcal{C}}_{k}\cap (\mathbb{R}\times {S}_{k})$. This contradicts Lemma 2.3. □

*Proof of Theorem 1.1* Applying a similar method to prove [[8], Theorem 3.2] with obvious changes (we only need to replace $g(t,u;\mu )$ with $\xi (u)$ in the proof of Lemmas 3.1, 3.2 and 3.4 of [8]), we can obtain the result of Theorem 1.1. □

## 3 Nodal solutions

In this section, we use Theorem 1.1 to prove the existence of nodal solutions for problem (1.1) with all of the six cases introduced at the start.

*Proof of Theorem 1.3*Applying Theorem 1.1 to problem (1.1), we have that there are two distinct unbounded continua ${\mathcal{C}}_{k}^{+}$ and ${\mathcal{C}}_{k}^{-}$, consisting of the bifurcation branch ${\mathcal{C}}_{k}$ emanating from $({\lambda}_{k}/{f}_{0},0)$, such that

To complete the proof of this theorem, it will be enough to show that ${\mathcal{C}}_{k}^{\nu}$ joins $({\lambda}_{k}/{f}_{0},0)$ to $({\lambda}_{k}/{f}_{\mathrm{\infty}},+\mathrm{\infty})$. Let $({\lambda}_{n},{u}_{n})\in {\mathcal{C}}_{k}^{\nu}$ satisfy ${\lambda}_{n}+\parallel {u}_{n}\parallel \to +\mathrm{\infty}$. We note that ${\lambda}_{n}>0$ for all $n\in \mathbb{N}$ since $(0,0)$ is the only solution of problem (1.1) for $\lambda =0$ and ${\mathcal{C}}_{k}^{\nu}\cap (\{0\}\times E)=\mathrm{\varnothing}$.

We divide the rest of the proof into two steps.

Step 1. We show that there exists a constant *M* such that ${\lambda}_{n}\in (0,M]$ for $n\in \mathbb{N}$ large enough.

The signum condition implies that there exists a positive constant *ϱ* such that ${\tilde{f}}_{n}(x)\ge \varrho $ for any $x\in [0,1]$. By Theorem 2.1 of [1], we get ${u}_{n}$ must change its sign more than $k-1$ times in $(0,1)$ for *n* large enough, and this contradicts the fact that ${u}_{n}\in {\mathcal{C}}_{k}^{\nu}$.

Step 2. We show that ${\mathcal{C}}_{k}^{\nu}$ joins $({\lambda}_{k}/{f}_{0},0)$ to $({\lambda}_{k}/{f}_{\mathrm{\infty}},+\mathrm{\infty})$.

*E*, after taking a subsequence if necessary, we have that ${v}_{n}\rightharpoonup \overline{v}$ for some $\overline{v}\in E$. Moreover, from (3.1) and the fact that $\tilde{\eta}$ is nondecreasing, we have that

*F*, it follows that

where $\overline{\lambda}=\underset{n\to +\mathrm{\infty}}{lim}{\lambda}_{n}$, again choosing a subsequence and relabeling it if necessary.

It is clear that $\parallel \overline{v}\parallel =1$ and $\overline{v}\in \overline{{\mathcal{C}}_{k}^{\nu}}\subseteq {\mathcal{C}}_{k}^{\nu}$ since ${\mathcal{C}}_{k}^{\nu}$ is closed in $\mathbb{R}\times E$. Therefore, $\overline{\lambda}{f}_{\mathrm{\infty}}={\lambda}_{k}$, so that $\overline{\lambda}={\lambda}_{k}/{f}_{\mathrm{\infty}}$. Therefore, ${\mathcal{C}}_{k}^{\nu}$ joins $({\lambda}_{k}/{f}_{0},0)$ to $({\lambda}_{k}/{f}_{\mathrm{\infty}},+\mathrm{\infty})$. □

**Theorem 3.1** *Let* (A1) *and* (A2) *hold*. *If* ${f}_{0}=0$ *and* ${f}_{\mathrm{\infty}}\in (0,+\mathrm{\infty})$, *then for any* $\lambda \in ({\lambda}_{k}/{f}_{\mathrm{\infty}},+\mathrm{\infty})$, *problem* (1.1) *has at least two solutions* ${u}_{k}^{+}$ *and* ${u}_{k}^{-}$ *such that* ${u}_{k}^{+}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is positive near* 0, *and* ${u}_{k}^{-}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is negative near* 0.

*Proof*If $(\lambda ,u)$ is any solution of problem (1.1) with ${\parallel u\parallel}_{\mathrm{\infty}}\ne 0$, dividing problem (1.1) by ${\parallel u\parallel}_{\mathrm{\infty}}^{2(p-1)}$ and setting $w=u/{\parallel u\parallel}_{\mathrm{\infty}}^{2}$ yields

Similarly, we can also show that ${\tilde{f}}_{\mathrm{\infty}}={f}_{0}$.

Now, applying Theorem 5.1 of [3] and the inversion $w\to w/{\parallel w\parallel}_{\mathrm{\infty}}^{2}=u$, we can achieve the conclusion. □

The following result is a direct corollary of Theorem 2.4 of [9].

**Theorem 3.2** *Let* (A1) *and* (A2) *hold*. *If* ${f}_{0}=0$ *and* ${f}_{\mathrm{\infty}}=+\mathrm{\infty}$, *then for any* $\lambda \in (0,+\mathrm{\infty})$, *problem* (1.1) *has two solutions* ${u}_{k}^{+}$ *and* ${u}_{k}^{-}$ *such that* ${u}_{k}^{+}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is positive near* 0, *and* ${u}_{k}^{-}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is negative near* 0.

**Theorem 3.3** *Let* (A1) *and* (A2) *hold*. *If* ${f}_{0}=+\mathrm{\infty}$ *and* ${f}_{\mathrm{\infty}}\in (0,+\mathrm{\infty})$, *then for any* $\lambda \in (0,{\lambda}_{1}/{f}_{\mathrm{\infty}})$, *problem* (1.1) *has two solutions* ${u}_{k}^{+}$ *and* ${u}_{k}^{-}$ *such that* ${u}_{k}^{+}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is positive near* 0, *and* ${u}_{k}^{-}$ *has exactly* $k-1$ *simple zeros in* $(0,1)$ *and is negative near* 0.

*Proof* By an argument similar to Theorem 3.1 and the conclusion of [[9], Theorem 2.1], we can obtain the conclusion. □

Next, we shall need the following topological lemma.

**Lemma 3.1** (see [22])

*Let*

*X*

*be a Banach space and let*${C}_{n}$

*be a family of closed connected subsets of*

*X*.

*Assume that*:

- (i)
*there exist*${z}_{n}\in {C}_{n}$, $n=1,2,\dots $ ,*and*${z}^{\ast}\in X$*such that*${z}_{n}\to {z}^{\ast}$; - (ii)
${r}_{n}=sup\{\parallel x\parallel |x\in {C}_{n}\}=+\mathrm{\infty}$;

- (iii)
*for every*$R>0$, $({\bigcup}_{n=1}^{+\mathrm{\infty}}{C}_{n})\cap {B}_{R}$*is a relatively compact set of**X*,*where*${B}_{R}=\{x\in X|\parallel x\parallel \le R\}.$

Using Theorem 1.1, Lemma 3.1 and a similar method to prove [[19], Theorems 2.7 and 2.8] with obvious changes, we may obtain the following two theorems.

**Theorem 3.4** *Let* (A1) *and* (A2) *hold*. *If* ${f}_{0}=+\mathrm{\infty}$ *and* ${f}_{\mathrm{\infty}}=+\mathrm{\infty}$, *then there exists* ${\lambda}^{+}>0$ *such that for any* $\lambda \in (0,{\lambda}^{+})$, *problem* (1.1) *has two solutions* ${u}_{k,1}^{+}$ *and* ${u}_{k,2}^{+}$ *such that they have exactly* $k-1$ *simple zeros in* $(0,1)$ *and are positive near* 0. *Similarly*, *there exists* ${\lambda}^{-}>0$ *such that for any* $\lambda \in (0,{\lambda}^{-})$, *problem* (1.1) *has two solutions* ${u}_{k,1}^{-}$ *and* ${u}_{k,2}^{-}$ *such that they have exactly* $k-1$ *simple zeros in* $(0,1)$ *and are negative near* 0.

**Theorem 3.5** *Let* (A1) *and* (A2) *hold*. *If* ${f}_{0}=0$ *and* ${f}_{\mathrm{\infty}}=0$, *then there exists* ${\lambda}_{\ast}^{+}>0$ *such that for any* $\lambda \in ({\lambda}_{\ast}^{+},+\mathrm{\infty})$, *problem* (1.1) *has two solutions* ${u}_{k,1}^{+}$ *and* ${u}_{k,2}^{+}$ *such that they have exactly* $k-1$ *simple zeros in* $(0,1)$ *and are positive near* 0. *Similarly*, *there exists* ${\lambda}_{\ast}^{-}>0$ *such that for any* $\lambda \in ({\lambda}_{\ast}^{-},+\mathrm{\infty})$, *problem* (1.1) *has two solutions* ${u}_{k,1}^{-}$ *and* ${u}_{k,2}^{-}$ *such that they have exactly* $k-1$ *simple zeros in* $(0,1)$ *and are negative near* 0.

## Declarations

### Acknowledgements

The authors are very grateful to the anonymous referee for his or her valuable suggestions. Research supported by the NSFC (No. 11261052).

## Authors’ Affiliations

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