Shub's conjecture on eigenvalue products and Grassmannian distortion

Let AGL(d,R)RIA \in GL(d,\mathbb{R}) \setminus \mathbb{R}I. For a Haar-random USO(d)U \in \mathrm{SO}(d), let λ1(UA),,λd(UA)\lambda_1(UA),\ldots,\lambda_d(UA) denote the eigenvalues ordered by decreasing modulus. Let gkg_k be a uniformly random kk-dimensional Grassmannian in Cd\mathbb{C}^d, represented by the span of the first kk columns of UU, and write UkU_k for the resulting d×kd \times k matrix and

detAgk:=det((AUk)(AUk)).\det A|g_k:=\det\bigl((AU_k)^*(AU_k)\bigr).

Shub's conjecture. There is a constant cd,kc_{d,k} such that

SO(d)i=1kλi(UA)dν(U)cd,kGrass(d,k)detAgkdμ(gk),\int_{\mathrm{SO}(d)}\prod_{i=1}^k|\lambda_i(UA)|\,d\nu(U)\geq c_{d,k}\int_{\mathrm{Grass}(d,k)}\det A|g_k\,d\mu(g_k), SO(d)i=1klogλi(UA)dν(U)cd,kGrass(d,k)logdetAgkdμ(gk),\int_{\mathrm{SO}(d)}\sum_{i=1}^k\log|\lambda_i(UA)|\,d\nu(U)\geq c_{d,k}\int_{\mathrm{Grass}(d,k)}\log\det A|g_k\,d\mu(g_k),

and one can choose cd,k=1c_{d,k}=1 for all dd and kk. The conjecture compares averaged products and logarithmic sums of the top eigenvalue moduli with averaged distortion on random Grassmannians. The full conjecture over SU(d)\mathrm{SU}(d) was proven by Dedieu and Shub, while the stated real orthogonal-group version is presented here as the question being addressed; the paper obtains asymptotic progress for the spectral radius.

Sources & referencesView supporting material

Primary source

Joshua Paik, “The Single Ring Theorem and a Question of Shub”, arXiv:2509.10358 (2025).

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