Subcritical nonlocal problems with mixed boundary conditions

In this paper, by variational and topological arguments based on linking and $\nabla$-theorems, we prove the existence of multiple solutions for the following nonlocal problem with mixed Dirichlet-Neumann boundary data, $$\left\{ \begin{array}{lcl} (-\Delta)^su=\lambda u+f(x,u) & &\text{in } \Omega, \\[2pt] \mkern+39mu u=0& &\text{on } \Sigma_{\mathcal{D}}, \\[2pt] \mkern+26mu \displaystyle \frac{\partial u}{\partial \nu}=0& &\text{on } \Sigma_{\mathcal{N}}, \end{array} \right.$$ where $(-\Delta)^s$, $s\in (1/2,1)$, is the spectral fractional Laplacian operator, $\Omega\subset\mathbb{R}^N$, $N>2s$, is a smooth bounded domain, $\lambda>0$ is a real parameter, $\nu$ is the outward normal to $\partial\Omega$, $\Sigma_{\mathcal{D}}$, $\Sigma_{\mathcal{N}}$ are smooth $(N-1)$-dimensional submanifolds of $\partial\Omega$ such that $\Sigma_{\mathcal{D}}\cup\Sigma_{\mathcal{N}}=\partial\Omega$, $\Sigma_{\mathcal{D}}\cap\Sigma_{\mathcal{N}}=\emptyset$ and $\Sigma_{\mathcal{D}}\cap\overline{\Sigma}_{\mathcal{N}}=\Gamma$ is a smooth $(N-2)$-dimensional submanifold of $\partial\Omega$.