arXiv subjects

Humberto Ramos Quoirin

Publications and source records attributed to Humberto Ramos Quoirin.

At least 19 recordsLinked to original sources

arXiv2025

Prescribed energy solutions of concave-convex type problems involving sign-changing or vanishing weights

We provide an abstract approach to find couples $(λ,u) \in \mathbb{R} \times X$ satisfying $$Φ_λ(u)=c \quad \mbox{and} \quad Φ'_λ(u)=0,$$ for some suitable values of $c \in \mathbb{R}$. Here $Φ_λ$ is a $C^1$ functional (set on a Banach space $X$) whose main prototype is the energy functional associated to a concave-convex problem with sign-changing or vanishing weights. This approach allows us to derive several existence, multiplicity and bifurcation type results for the equation $Φ'_λ(u)=0$ with $λ$ fixed.

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arXiv2025

Multiplicity of dead core solutions in indefinite elliptic problems

We investigate nonnegative solutions of indefinite elliptic problems which enjoy the dead core phenomenon. Our model is the subhomogeneous problem $$ -Δ_p u = (a^+(x) - μa^-(x))|u|^{q-2}u, \quad u \in W_0^{1,p}(Ω), $$ where $Ω$ is a bounded domain in $\mathbb{R}^N$, $1 0$. Thanks to a dead core formation property, we obtain multiple nonnegative dead core solutions of this problem for $μ$ large enough. This assertion, in combination with a uniqueness feature, yields an exact multiplicity result, which gives a precise description of the nonnegative solutions set of this problem for large values of $μ$. We also extend the multiplicity result to other classes of problems.

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arXiv2025

Further applications of the Nehari manifold method to functionals in $C^1(X \setminus \{0\})$

We proceed with the study of the Nehari manifold method for functionals in $C^1(X \setminus \{0\})$, where $X$ is a Banach space. We deal now with functionals whose fibering maps have two critical points (a minimiser followed by a maximiser). Under some additional conditions we show that the Nehari manifold method provides us with the ground state level and two sequences of critical values for these functionals. These results are applied to the class of {\it prescribed energy problems} as well as to the concave-convex problem for the {\it affine} $p$-Laplacian operator.

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arXiv2024

Some applications of the Nehari manifold method to functionals in $C^1(X \setminus \{0\})$

Given a real Banach space $X$, we show that the Nehari manifold method can be applied to functionals which are $C^1$ in $X \setminus \{0\}$. In particular we deal with functionals that can be unbounded near $0$, and prove the existence of a ground state and infinitely many critical points for such functionals. These results are then applied to three classes of problems: the {\it prescribed energy problem} for a family of functionals depending on a parameter, problems involving the {\it affine} $p$-Laplacian operator, and degenerate Kirchhoff type problems.

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arXiv2022

Local minimizers for a class of functionals over the Nehari set

We analyze the topological structure of the Nehari set for a class of functionals depending on a real parameter $λ$, and having two degrees of homogeneity. A special attention is paid to the extremal parameter $λ^*$, which is the threshold value for the Nehari set to be given by a {\it natural constraint}. The main difficulty arises when $λ>λ^*$, as the energy functional may be unbounded from below over the Nehari set. In such situation we prove the existence of local minimizers of the functional constrained to this set. We unify and extend previous existence and multiplicity results for critical points of indefinite, $(p,q)$-Laplacian, and Kirchhoff type problems.

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arXiv2022

Critical points with prescribed energy for a class of functionals depending on a parameter: existence, multiplicity and bifurcation results

We look for critical points with prescribed energy for the family of even functionals $Φ_μ=I_1-μI_2$, where $I_1,I_2$ are $C^1$ functionals on a Banach space $X$, and $μ\in \mathbb{R}$. For several classes of $Φ_μ$ we prove the existence of infinitely many couples $(μ_{n,c}, u_{n,c})$ such that $$Φ'_{μ_{n,c}}(\pm u_{n,c}) = 0 \quad \mbox{and} \quad Φ_{μ_{n,c}}( \pm u_{n,c}) = c \quad \forall n \in \mathbb{N}.$$ More generally, we analyze the structure of the solution set of the problem $$Φ_μ'(u)=0, \quad Φ_μ(u)=c$$ with respect to $μ$ and $c$. In particular, we show that the maps $c \mapsto μ_{n,c}$ are continuous, which gives rise to a family of {\it energy curves} for this problem. The analysis of these curves provide us with several bifurcation and multiplicity type results, which are then applied to some elliptic problems. Our approach is based on the {\it nonlinear generalized Rayleigh quotient} method developed in \cite{I1}.

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arXiv2021

Zero energy critical points of functionals depending on a parameter

We investigate zero energy critical points for a class of functionals $Φ_μ$ defined on a uniformly convex Banach space, and depending on a real parameter $μ$. More precisely, we show the existence of a sequence $(μ_n)$ such that $Φ_{μ_n}$ has a pair of critical points $\pm u_n$ satisfying $Φ_{μ_n}(\pm u_n)=0$, for every $n$. In addition, we provide some properties of $μ_n$ and $u_n$. This result, which is proved via a fibering map approach (based on the {\it nonlinear generalized Rayleigh quotient} method \cite{I1}) combined with the Ljusternik-Schnirelman theory, is then applied to several classes of elliptic pdes.

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arXiv2020

Ground states of elliptic problems over cones

Given a reflexive Banach space $X$, we consider a class of functionals $Φ\in C^1(X,\Re)$ that do not behave in a uniform way, in the sense that the map $t \mapsto Φ(tu)$, $t>0$, does not have a uniform geometry with respect to $u\in X$. Assuming instead such a uniform behavior within an open cone $Y \subset X \setminus \{0\}$, we show that $Φ$ has a ground state relative to $Y$. Some further conditions ensure that this relative ground state is the (absolute) ground state of $Φ$. Several applications to elliptic equations and systems are given.

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arXiv2020

Some uniqueness results in quasilinear subhomogeneous problems

We establish uniqueness results for quasilinear elliptic problems through the criterion recently provided in \cite{DFMST}. We apply it to generalized $p$-Laplacian subhomogeneous problems that may admit multiple nontrivial nonnegative solutions. Based on a generalized hidden convexity result, we show that uniqueness holds among strongly positive solutions and nonnegative global minimizers. Problems involving nonhomogeneous operators as the so-called $(p,r)$-Laplacian are also treated.

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arXiv2020

Uniqueness and positivity issues in a quasilinear indefinite problem

We consider the problem $$ (P_λ)\quad -Δ_{p}u=λu^{p-1}+a(x)u^{q-1},\quad u\geq0\quad\mbox{ in }Ω$$ under Dirichlet or Neumann boundary conditions. Here $Ω$ is a smooth bounded domain of $\mathbb{R}^{N}$ ($N\geq1$), $λ\in\mathbb{R}$, $1 0$). In particular, this problem has at most one positive solution for $λ<0$. Under some condition on $a$, the above uniqueness result fails for some values of $λ>0$ as we obtain, besides the ground state solution, a \textit{second} solution positive in $Ω_{a}^{+}$. We also provide conditions on $λ$, $a$ and $q$ such that these solutions become positive in $Ω$, and analyze the formation of dead cores for a generic solution.

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arXiv2020

Past and recent contributions to indefinite sublinear elliptic problems

We review the indefinite sublinear elliptic equation $-Δu=a(x)u^{q}$ in a smooth bounded domain $Ω\subset\mathbb{R}^{N}$, with Dirichlet or Neumann homogeneous boundary conditions. Here $0<q<1$ and $a$ is continuous and changes sign, in which case the strong maximum principle does not apply. As a consequence, the set of nonnegative solutions of these problems has a rich structure, featuring in particular both dead core and/or positive solutions. Overall, we are interested in sufficient and necessary conditions on $a$ and $q$ for the existence of positive solutions. We describe the main results from the past decades, and combine it with our recent contributions. The proofs are briefly sketched.

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arXiv2020

Non-existence of dead cores in fully nonlinear elliptic models

We investigate non-existence of nonnegative dead-core solutions for the problem $$|Du|^γF(x, D^2u)+a(x)u^q = 0 \quad \mbox{in} \quad Ω, \quad u=0 \quad \mbox{ on } \quad \partialΩ.$$ Here $Ω\subset \mathbb{R}^N$ is a bounded smooth domain, $F$ is a fully nonlinear elliptic operator, $a: Ω\to \mathbb{R}$ is a sign-changing weight, $γ\geq 0$, and $0<q<γ+1$. We show that this problem has no non-trivial dead core solutions if either $q$ is close enough to $γ+1$ or the negative part of $a$ is sufficiently small. In addition, we obtain the existence and uniqueness of a positive solution under these conditions on $q$ and $a$. Our results extend previous ones established in the semilinear case, and are new even for the simple model $|D u(x)|^γ \mathrm{Tr}(\mathrm{A}(x) D^2 u(x)) + a(x)u^{q}(x) = 0$, where $\mathrm{A} \in C^0(Ω;Sym(N))$ is a uniformly elliptic and non-negative matrix.

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arXiv2020

Uniqueness and sign properties of minimizers in a quasilinear indefinite problem

Let $1<q<p$ and $a\in C(\overlineΩ)$ be sign-changing, where $Ω$ is a bounded and smooth domain of $\mathbb{R}^{N}$. We show that the functional \[ I_{q}(u):=\int_Ω\left( \frac{1}{p}|\nabla u|^{p}-\frac{1}{q}a(x)|u|^{q}\right) , \] has exactly one nonnegative minimizer $U_{q}$ (in $W_{0}^{1,p}(Ω)$ or $W^{1,p}(Ω)$). In addition, we prove that $U_{q}$ is the only possible \textit{positive} solution of the associated Euler-Lagrange equation, which shows that this equation has at most one positive solution. Furthermore, we show that if $q$ is close enough to $p$ then $U_{q}$ is positive, which also guarantees that minimizers of $I_{q}$ do not change sign. Several of these results are new even for $p=2$.

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arXiv2020

Nonnegative solutions of an indefinite sublinear Robin problem II: local and global exactness results

We go further in the investigation of the Robin problem $(P_α)$: $-Δu=a(x)u^{q}$ in $Ω$, $u\geq0$ in $Ω$, $\partial_νu=αu$ on $\partial Ω$; on a bounded domain $Ω\subset\mathbb{R}^{N}$, with $a$ sign-changing and $0 0$. Moreover, strengthening the assumptions on $a$ and $q$ we provide a global (i.e. for every $α>0$) exactness result on the number of solutions of $(P_α)$ . Our approach also applies to the problem $(S_α)$: $-Δu=αu + a(x)u^{q}$ in $Ω$, $u\geq0$ in $Ω$, $\partial_νu=0$ on $\partial Ω$.

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arXiv2019

Nonnegative solutions of an indefinite sublinear Robin problem I: positivity, exact multiplicity, and existence of a subcontinuum

Let $Ω\subset\mathbb{R}^{N}$ ($N\geq1$) be a smooth bounded domain, $a\in C(\barΩ)$ a sign-changing function, and $0\leq q<1$. We investigate the Robin problem \[ \begin{cases} -Δu=a(x)u^{q} & \mbox{in $Ω$},\\ u\geq0 & \mbox{in $Ω$},\\ \partial_νu=αu & \mbox{on $\partial Ω$}, \end{cases} \] where $α\in\lbrack-\infty,\infty)$ and $ν$ is the unit outward normal to $\partialΩ$. Due to the lack of strong maximum principle structure, this problem may have \textit{dead core} solutions. However, for a large class of weights $a$ we recover a \textit{positivity} property when $q$ is close to $1$, which considerably simplifies the structure of the solution set. Such property, combined with a bifurcation analysis and a suitable change of variables, enables us to show the following exactness result for these values of $q$: $(P_α)$ has \textit{exactly} one nontrivial solution for $α\leq0$, \textit{exactly} two nontrivial solutions for $α>0$ small, and \textit{no} such solution for $α>0$ large. Assuming some further conditions on $a$, we show that these solutions lie on a subcontinuum. These results rely partially on (and extend) our previous work \cite{KRQU16}, where the cases $α=-\infty$ (Dirichlet) and $α=0$ (Neumann) have been considered. We also obtain some results for arbitrary $q\in\left[ 0,1\right) $. Our approach combines mainly bifurcation techniques, the sub-supersolutions method, and \textit{a priori} lower and upper bounds.

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arXiv2019

A curve of positive solutions for an indefinite sublinear Dirichlet problem

We investigate the existence of a curve $q\mapsto u_{q}$, with $q\in(0,1)$, of positive solutions for the problem $(P_{a,q})$: $-Δu=a(x)u^{q}$ in $Ω$, $u=0$ on $\partialΩ$, where $Ω$ is a bounded and smooth domain of $\mathbb{R}^{N}$ and $a:Ω\rightarrow\mathbb{R}$ is a sign-changing function (in which case the strong maximum principle does not hold). In addition, we analyze the asymptotic behavior of $u_{q}$ as $q\rightarrow0^{+}$ and $q\rightarrow1^{-}$. We also show that in some cases $u_{q}$ is the ground state solution of $(P_{a,q})$. As a byproduct, we obtain existence results for a singular and indefinite Dirichlet problem. Our results are mainly based on bifurcation and sub-supersolutions methods.

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arXiv2018

Loop type subcontinua of positive solutions for indefinite concave-convex problems

We establish the existence of loop type subcontinua of nonnegative solutions for a class of concave-convex type elliptic equations with indefinite weights, under Dirichlet and Neumann boundary conditions. Our approach depends on local and global bifurcation analysis from the zero solution in a non-regular setting, since the nonlinearities considered are not differentiable at zero, so that the standard bifurcation theory does not apply. To overcome this difficulty, we combine a regularization scheme with a priori bounds, and Whyburn's topological method. Furthermore, via a continuity argument we prove a positivity property for subcontinua of nonnegative solutions. These results are based on a positivity theorem for the associated concave problem proved in [15], and extend previous results established in the powerlike case.

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arXiv2018

A loop type component in the non-negative solutions set of an indefinite elliptic problem

We prove the existence of a loop type component of non-negative solutions for an indefinite elliptic equation with homogeneous Neumann boundary conditions. This result complements our previous results obtained in [12], where the existence of another loop type component was established in a different situation. Our proof combines local and global bifurcation theory, rescaling and regularizing arguments, a priori bounds, and Whyburn's topological method. A further investigation of the loop type component established in [12] is also provided.

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