Mark S. Ashbaugh, Rafael D. Benguria, “Universal Bounds for the Low Eigenvalues of Neumann Laplacians in $N$ Dimensions”, SIAM J Math Anal, 24:3 (1993), 557
Vincent Xiaosong Liu, “Instability for the Navier–Stokes equations on the 2-dimensional torus and a lower bound for the Hausdorff dimension of their global attractors”, Comm Math Phys, 147:2 (1992), 217
А. С. Братусь, А. Д. Мышкис, “К соотношению между первой и второй собственными частотами колебаний мембраны”, Ж. вычисл. матем. и матем. физ., 32:2 (1992), 320–325 ; A. S. Bratus', A. D. Myshkis, “The relation between the first and second natural frequencies of vibrations of a membrane”, Comput. Math. Math. Phys., 32:2 (1992), 264–269