A sharp two-disk bound for the second positive Neumann eigenvalue under a curvature upper bound

Meiqi Liu, Zhouyu Long, Wenming Zou

Langford and Laugesen conjectured a sharp two-disk bound for the third Neumann eigenvalue under an upper Gaussian-curvature bound (Math. Ann. 386 (2023), 2255--2281, Conjecture 1.4…