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Published online by Cambridge University Press: 22 January 2025
Let $\Omega \subset \mathbb{R}^d$ with
$d\geq 2$ be a bounded domain of class
${\mathcal C}^{1,\beta }$ for some
$\beta \in (0,1)$. For
$p\in (1, \infty )$ and
$s\in (0,1)$, let
$\Lambda ^s_{p}(\Omega )$ be the first eigenvalue of the mixed local–nonlocal operator
$-\Delta _p+(-\Delta _p)^s$ in Ω with the homogeneous nonlocal Dirichlet boundary condition. We establish a strict Faber–Krahn-type inequality for
$\Lambda _{p}^s(\cdot )$ under polarization. As an application of this strict inequality, we obtain the strict monotonicity of
$\Lambda _{p}^s(\cdot )$ over the annular domains and characterize the rigidity property of the balls in the classical Faber–Krahn inequality for
$-\Delta _p+(-\Delta _p)^s$.