Abstract
The Liouville property of a complete Riemannian manifold M (i.e., the question whether there exist non-trivial bounded harmonic functions on M) attracted a lot of attention. For Cartan–Hadamard manifolds the role of lower curvature bounds is still an open problem. We discuss examples of Cartan–Hadamard manifolds of unbounded curvature where the limiting angle of Brownian motion degenerates to a single point on the sphere at infinity, but where nevertheless the space of bounded harmonic functions is as rich as in the non-degenerate case. To see the full boundary the point at infinity has to be blown up in a non-trivial way. Such examples indicate that the situation concerning the famous conjecture of Greene and Wu about existence of non-trivial bounded harmonic functions on Cartan–Hadamard manifolds is much more complicated than one might have expected.
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Arnaudon, M., Thalmaier, A. & Ulsamer, S. Existence of non-trivial harmonic functions on Cartan–Hadamard manifolds of unbounded curvature. Math. Z. 263, 369–409 (2009). https://doi.org/10.1007/s00209-008-0422-6
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DOI: https://doi.org/10.1007/s00209-008-0422-6
Keywords
- Harmonic function
- Poisson boundary
- Cartan–Hadamard manifold
- Conjecture of Greene–Wu
- Dirichlet problem at infinity