94 research outputs found

    Variational principle for Hamiltonians with degenerate bottom

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    We consider perturbations of Hamiltonians whose Fourier symbol attains its minimum along a hypersurface. Such operators arise in several domains, like spintronics, theory of supercondictivity, or theory of superfluidity. Variational estimates for the number of eigenvalues below the essential spectrum in terms of the perturbation potential are provided. In particular, we provide an elementary proof that negative potentials lead to an infinite discrete spectrum.Comment: 9 page

    On the Robin eigenvalues of the Laplacian in the exterior of a convex polygon

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    Let Ξ©βŠ‚R2\Omega\subset \mathbb{R}^2 be the exterior of a convex polygon whose side lengths are β„“1,...,β„“M\ell_1,...,\ell_M. For Ξ±>0\alpha>0, let HΞ±Ξ©H^\Omega_\alpha denote the Laplacian in Ξ©\Omega, uβ†¦βˆ’Ξ”uu\mapsto -\Delta u, with the Robin boundary conditions βˆ‚u/βˆ‚Ξ½=Ξ±u\partial u/\partial\nu =\alpha u, where Ξ½\nu is the exterior unit normal at the boundary of Ξ©\Omega. We show that, for any fixed m∈Nm\in\mathbb{N}, the mmth eigenvalue EmΞ©(Ξ±)E^\Omega_m(\alpha) of HΞ±Ξ©H^\Omega_\alpha behaves as E^\Omega_m(\alpha)=-\alpha^2+\mu^D_m +\mathcal{O}\Big(\dfrac{1}{\sqrt\alpha}\Big) \quad {as $\alpha$ tends to $+\infty$}, where ΞΌmD\mu^D_m stands for the mmth eigenvalue of the operator D1βŠ•...βŠ•DMD_1\oplus...\oplus D_M and DnD_n denotes the one-dimensional Laplacian fβ†¦βˆ’f"f\mapsto -f" on (0,β„“n)(0,\ell_n) with the Dirichlet boundary conditions.Comment: 10 pages. To appear in Nanosystems: Physics, Chemistry, Mathematics. Minor revision: misprints corrected, references update

    Resolvents of self-adjoint extensions with mixed boundary conditions

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    We prove a variant of Krein's resolvent formula expressing the resolvents of self-adjoint extensions through the associated boundary conditions. Applications to solvable quantum-mechanical problems are discussed.Comment: 15 pages, rewritten almost completel

    An inequality for the maximum curvature through a geometric flow

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    We provide a new proof of the following inequality: the maximum curvature kmaxk_\mathrm{max} and the enclosed area AA of a smooth Jordan curve satisfy kmaxβ‰₯Ο€/Ak_\mathrm{max}\ge \sqrt{\pi/A}. The feature of our proof is the use of the curve shortening flow.Comment: 2 page
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