Abstract
We consider a metric graph consisting of two edges, one of which has length \(\varepsilon \) which we send to zero. On this graph we study the resolvent and spectrum of the Laplacian subject to a general vertex condition at the connecting vertex. Despite the singular nature of the perturbation (by a short edge), we find that the resolvent depends analytically on the parameter \(\varepsilon \). In contrast, the negative eigenvalues escape to minus infinity at rates that could be fractional, namely, \(\varepsilon ^0\), \(\varepsilon ^{-2/3}\) or \(\varepsilon ^{-1}\). These rates take place when the corresponding eigenfunction localizes, respectively, only on the long edge, on both edges, or only on the short edge.
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Notes
Intuitively, the derivative \(u'_\varepsilon \) does not change very much over a short edge; since \(u'_\varepsilon (0)=0\), we also expect \(u'_\varepsilon \) to be close to 0 on the other end.
The name “non-resonance” was chosen due to an analogy to Sommerfeld radiation condition for resonances, as it seeks to exclude the situation where non-zero values on the short edges occur in the absence of any input from the order 1 edges.
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Acknowledgements
The authors thank the anonymous referee for numerous improving suggestions. The research by D.I. Borisov was supported by Russian Science Foundation, grant no. 23-11-00009, https://rscf.ru/project/23-11-00009/.
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Berkolaiko, G., Borisov, D.I. & King, M. Exotic eigenvalues and analytic resolvent for a graph with a shrinking edge. Anal.Math.Phys. 13, 90 (2023). https://doi.org/10.1007/s13324-023-00853-3
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DOI: https://doi.org/10.1007/s13324-023-00853-3