Infinitely many global continua bifurcating from a single solution of an elliptic problem with concave-convex nonlinearity

We study the bifurcation of solutions of semilinear elliptic boundary value problems of the form \begin{align*} \begin{aligned} -Δu &= f_λ(|x|,u,|\nabla u|) &&\text{in }Ω, u &= 0 &&\text{on }\partialΩ, \end{aligned} \end{align*} on an annulus $Ω\subset\mathbb{R}^N$, with a concave-convex nonlinearity, a special case being the nonlinearity first considered by Ambrosetti, Brezis and Cerami: $f_λ(|x|,u,|\nabla u|)=λ|u|^{q-2}u + |u|^{p-2}u$ with $1<q<2<p$. Although the trivial solution $u_0\equiv0$ is nondegenerate if $λ=0$ we prove that $(λ_0,u_0)=(0,0)$ is a bifurcation point. In fact, the bifurcation scenario is very singular: We show that there are infinitely many global continua of radial solutions $\mathcal{C}_j^\pm\subset\mathbb{R}\times\mathcal{C}^1(Ω)$, $j\in\mathbb{N}_0$ which bifurcate from the trivial branch $\mathbb{R}\times\{0\}$ at $(λ_0,u_0)=(0,0)$ and consist of solutions having precisely $j$ nodal annuli. A detailed study of these continua shows that they accumulate at $\mathbb{R}_{\ge0}\times\{0\}$ so that every $(λ,0)$ with $λ\ge0$ is a bifurcation point. Moreover, adding a point at infinity to $\mathcal{C}^1(Ω)$ they also accumulate at $\mathbb{R}\times\{\infty\}$, so there is bifurcation from infinity at every $λ\in\mathbb{R}$.

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