On Courant's nodal domain property for linear combinations of eigenfunctions, Part II
arXiv:1803.00449 · doi:10.1007/978-3-030-68490-7_4
Abstract
Generalizing Courant's nodal domain theorem, the "Extended Courant property" is the statement that a linear combination of the first eigenfunctions has at most nodal domains. In a previous paper (Documenta Mathematica, 2018, Vol. 23, pp. 1561--1585), we gave simple counterexamples to this property, including convex domains. In the present paper, using some input from numerical computations, we pursue the investigation of the Extended Courant property with two new examples, the equilateral rhombus and the regular hexagon.
This version differs notably from arXiv 1803.00449v2 (15 Jun 2018). The part concerning Gelfand's approach to Sturm's 1D theorem has been removed (it is developed in arXiv:1807.03990 and will appear in the Moscow Math. Journal). The treatment of Sturm's 1D theorem in the periodic case has been removed. The short paragraph on product-like constructions has been removed as well. A section concerning the hexagon has been added. Minor changes with respect to hal-01718768, version 3 (7 Jun 2019). The paper focuses on the Extended Courant property for the equilateral rhombus and the regular hexagon. Accepted for publication in a forthcoming ''Erick Balslev Memorial Volume'' edited by Sergio Albeverio and Ricardo Weder