1. G. Alessandrini, Nodal lines of eigenfunctions of the fixed membrane problem in general convex domains. Comment. Math. Helvetici 69, 142–154 (1994)
2. V. Arnold, The topology of real algebraic curves (the works of Petrovskii and their development). Uspekhi Math. Nauk. 28(5), 260–262 (1973). English translation in [4] 3
3. V. Arnold, Topological properties of eigenoscillations in mathematical physics. Proc. Steklov Inst. Math. 273, 25–34 (2011)
4. V. Arnold, Topology of real algebraic curves (Works of I.G. Petrovskii and their development). Translated by Oleg Viro. in A.B. Givental, B.A. Khesin, A.N. Varchenko, V.A. Vassilev, O.Y. Viro (Eds.) Collected Works, Volume II. Hydrodynamics, Bifurcation Theory and Algebraic Geometry, 1965–1972 (Springer, 2014)
5. L. Bauer, E.L. Reiss, Cutoff wavenumbers and modes of hexagonal waveguides. SIAM J. Appl. Math. 35(3), 508–514 (1978)