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Any three eigenvalues do not determine a triangle

Despite the moduli space of triangles being three dimensional, we prove the existence of two triangles which are not isometric to each other for which the first, second and fourth Dirichlet eigenvalues coincide, establishing a numerical observation from Antunes-Freitas [P. R. S. Antunes and P. Freitas. Proc. R. Soc. Lond.Ser. A Math. Phys. Eng. Sci., 467(2130):1546-1562, 2011]. The two triangles are far from any known, explicit cases. To do so, we develop new tools to rigorously enclose eigenvalues to a very high precision, as well as their position in the spectrum. This result is also mentioned as (the negative part of) Conjecture 6.46 in [R. Laugesen, B. Siudeja, Shape optimization and spectral theory, 149-200. De Gruyter Open, Warsaw, 2017], Open Problem 1 in [D. Grieser, S. Maronna, Notices Amer. Math. Soc., 60(11):1440-1447, 2013] and Conjecture 3 in [Z. Lu, J. Rowlett. Amer. Math. Monthly, 122(9):815-835, 2015.].