Curves of equiharmonic solutions, and problems at resonance

We consider the semilinear Dirichlet problem \Delta u+kg(u)=\mu _1 \varphi _1+\cdots +\mu _n \varphi _n+e(x) \;\; for $x \in \Omega$, \;\; u=0 \;\; on $\partial \Omega$, where ${\phi }_k$ is the $k$-th eigenfunction of the Laplacian on $\mathrm{\Omega }$ and $e(x)\perp {\phi }_k$, $k=1,\dots ,n$. Write the solution in the form $u(x)={\mathrm{\Sigma }}_{i=1}^n{\xi }_i{\phi }_i+U(x)$, with $U\perp {\phi }_k$, $k=1,\dots ,n$. Starting with $k=0$, when the problem is linear, we continue the solution in $k$ by keeping $\xi =(\xi _1, \ldots,\xi _n)$fixed,butallowingfor$\mu =(\mu _1, \ldots,\mu _n)$ to vary. Studying the map $\xi \to \mu$ provides us with the existence and multiplicity results for the above problem. We apply our results to problems at resonance, at both the principal and higher eigenvalues. Our approach is suitable for numerical calculations, which we implement, illustrating our results.