Consider an L-2-normalized Laplace-Beltrami eigenfunction e(lambda) on a compact, boundary-less Riemannian manifold with Delta(e lambda) = -lambda(2)(e lambda). We study eigenfunction triple products = integral e(lambda)e(mu)(e(nu)) over bar dV. We show the overall l(2)-concentration of these triple products is determined by the measure of some set of configurations of triangles with side lengths equal to the frequencies lambda, mu, and nu. A rapidly vanishing proportion of this mass lies in the 'classically forbidden' regime where lambda, mu, and nu fail to satisfy the triangle inequality. As a consequence, we refine one result in a paper by Lu, Sogge, and Steinerberger 10. (c) 2022 Elsevier Inc. All rights reserved.
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