Nodal statistics from quantum graphs to random matrices: universality and phase transitions

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• Lior Alon (Massachusetts Institute of Technology)
• Thursday 15 January 2026, 15:30-16:30
• Seminar Room 1, Newton Institute.

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GSTW01 - High energy spectral theory: geometry and dynamics

Motivated by Berry’s random wave model for chaotic domains, high-energy eigenfunctions are expected, at the scale of the wavelength, to behave like random waves. Accordingly, the size of nodal sets is determined to first order by the Weyl law, while its second-order fluctuations are expected to be universal. In analogy with spectral statistics, we refer to these fluctuations of the nodal set size as nodal statistics.   Beginning with work of Berry, and later of Blum–Gnutzmann–Smilansky and Bogomolny–Schmit, it was conjectured that in chaotic systems nodal statistics are asymptotically Gaussian. While striking mathematical results support this picture on the sphere (Nazarov–Sodin), arithmetic symmetries on the torus prevent chaoticity and lead to a breakdown of universality, as shown by Wigman and collaborators and by Marinucci, Rossi, and Peccati.   In this talk I will follow Smilansky’s insight and focus on nodal statistics in graph-based models. For quantum graphs and discrete operators on graphs, Berkolaiko’s nodal magnetic theorem relates nodal statistics to the stability of eigenvalues under magnetic perturbations. This connection yields Gaussian limiting statistics for quantum graphs with disjoint cycles (Alon–Band–Berkolaiko), and, combined with Morse inequalities, extends to random discrete operators on finite graphs with disjoint cycles and to complete graphs with strong on-site disorder (Alon–Goresky).   I will conclude with recent joint work on nodal statistics for random matrices (Alon–Mikulincer–Urschel), showing that nodal statistics for GOE matrices obey a semicircle law rather than the conjectured Gaussian behaviour. If time permits, I will briefly discuss ongoing work on the Rosenzweig–Porter model, where adding a random on-site potential to a GOE matrix leads to nodal statistics interpolating from semicircle behaviour at low disorder to Gaussian behaviour at strong disorder, suggesting a phase transition between universality classes and a connection to localization–delocalization phenomena.

This talk is part of the Isaac Newton Institute Seminar Series series.

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