The effect of the domain's configuration space on the number of nodal solutions of singularly perturbed elliptic equations

Thomas Bartsch, Tobias Weth



We prove a new multiplicity result for nodal solutions of the
Dirichlet problem for the singularly perturbed equation
$-\varepsilon^2 \Delta u+u =f(u)$ for $\varepsilon> 0$ small on a bounded domain
$\Omega\subset{\mathbb R}^N$. The nonlinearity $f$ grows superlinearly and
subcritically. We relate the topology of the configuration space
$C\Omega=\{(x,y)\in\Omega\times\Omega:x\not=y\}$ of ordered pairs in
the domain to the number of solutions with exactly two nodal domains.
More precisely, we show that there exist at least $\text{\rm cupl}(C\Omega)+2$
nodal solutions, where $\text{\rm cupl}$ denotes the cuplength of a topological
space. We furthermore show that $\text{\rm cupl}(C\Omega)+1$ of these solutions
have precisely two nodal domains, and the last one has at most three nodal


Singularly perturbed problem; sign-changing solution; configuration space; Lusternik-Schnirelmn category

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