Why separate the complete Riemannian manifolds that admit and those that do not admit positive Green function? In summary, what is the motivation for studying parabolic and non-parabolic manifolds?
1 Answer
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One that comes to mind is that parabolicity of a Riemannian manifold is equivalent to recurrence of its Brownian motion, e.g. of the Markov process whose infinitesimal generator is $\frac12\Delta$.
See theorem 5.1 in
Grigor’yan, A. (1999). Analytic and geometric background of recurrence and non-explosion of the Brownian motion on Riemannian manifolds. Bulletin of the American Mathematical Society, 36(02), 135–250. https://doi.org/10.1090/S0273-0979-99-00776-4