Multiple solutions for resonant elliptic equations via local linking theory and Morse theory (Q5929876)

The authors deal with the multiple solutions of the problem NEWLINE\[NEWLINE \begin{cases} -\Delta u=g(x,u) \quad &\text{in }\Omega\\ u=0\quad &\text{on } \partial \Omega\end{cases}NEWLINE\]NEWLINE where \(g\in C^1(\overline \Omega\times \mathbb{R}, \mathbb{R})\). They consider two classes of elliptic resonant problems, namely:NEWLINENEWLINENEWLINEa) double-double resonant case, b) the authors introduce some new conditions and compute the critical groups both at zero and at infinity precisely. Combined local linking theory and Morse theory, the authors present three solutions for the completely resonant case.