Bialy's conjecture on spectral rigidity among ellipses

Let ρ0\rho_0 and ρ1\rho_1 be two distinct rotation numbers in (0,1/2](0,1/2]. For ellipses E\mathscr E and E\mathscr E', let βE(ρ)\beta_{\mathscr E}(\rho) denote the Mather beta function at rotation number ρ\rho. Bialy's conjecture. If

βE(ρ0)=βE(ρ0),βE(ρ1)=βE(ρ1),\beta_{\mathscr E}(\rho_0)=\beta_{\mathscr E'}(\rho_0),\qquad \beta_{\mathscr E}(\rho_1)=\beta_{\mathscr E'}(\rho_1),

then E\mathscr E and E\mathscr E' are the same up to isometries. This conjecture asks whether two values of Mather's beta function determine an ellipse up to isometry; the source gives no resolution, so the conjecture remains open.

Sources & referencesView supporting material

Primary source

Corentin Fierobe, “Spectral rigidity among ellipses, Bialy's conjecture and local extrema of Mather's beta function”, arXiv:2603.09439 (2026).

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