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Seminar

Lower bounds on the Lyapunov exponent of linear PDEs driven by random velocity September 29, 2025 (11:00 AM PDT - 12:00 PM PDT)
Parent Program:
Location: SLMath: Eisenbud Auditorium, Online/Virtual
Speaker(s) Jaeyun Yi (École Polytechnique Fédérale de Lausanne (EPFL))
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Lower bounds on the Lyapunov exponent of linear PDEs driven by random velocity

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Lyapunov exponents have been extensively studied in the context of fluid mechanics, particularly for the Euler and Navier-Stokes equations. In this regard, the lower bounds of Lyapunov exponents are known to serve as a benchmark for chaos and turbulence. We introduce two primary models in fluid mechanics: passive scalar advection and linearized stochastic Navier-Stokes equations, and prove the lower bounds of their Lyapunov exponents, which are quantitative in the diffusion parameter. Our result provides a typical filamentation length scale, which partially answers the Bachelor conjecture.  The proof relies on the high-frequency stochastic instability of the projective process via a detailed analysis of its spectral median. This is joint work with Martin Hairer, Sam Punshon-Smith, and Tommaso Rosati. 

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Lower bounds on the Lyapunov exponent of linear PDEs driven by random velocity