A gradient estimate for harmonic functions sharing the same zeros
Dan Mangoubi
Let $u, v$ be two harmonic functions in $\{|z|<2\}\subset\mathbb{C}$ which have exactly the same set $Z$ of zeros. We observe that $\big|\nabla\log |u/v|\big|$ is bounded in the unit disk by a constant which depends on $Z$ only. In case $Z=\emptyset$ this goes back to Li-Yau's gradient estimate for positive harmonic functions. The general boundary Harnack principle gives only Hölder estimates on $\log |u/v|$.
keywords: gradient estimates nodal set Harnack Li-Yau harmonic functions boundary Harnack principle.
Nodal geometry of graphs on surfaces
Yong Lin Gábor Lippner Dan Mangoubi Shing-Tung Yau
We prove two mixed versions of the Discrete Nodal Theorem of Davies et. al. [3] for bounded degree graphs, and for three-connected graphs of fixed genus $g$. Using this we can show that for a three-connected graph satisfying a certain volume-growth condition, the multiplicity of the $n$th Laplacian eigenvalue is at most $2[ 6(n-1) + 15(2g-2)]^2$. Our results hold for any Schrödinger operator, not just the Laplacian.
keywords: genus.test multiplicity of eigenvalues Nodal domain

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