Rationality conjecture for limiting invariants of Fermat hypersurface rings

For d2d\geq2, let

Sp,n,d=Fp[[x0,,xn]]/(x0d++xnd)S_{p,n,d}=\mathbb{F}_p[[x_0,\ldots,x_n]]/(x_0^d+\cdots+x_n^d)

be the degree-dd Fermat hypersurface ring, and define cnc_n and cnc'_n by

limpeHK(Sp,n,d)=cn,\lim_{p\to\infty}e_{HK}(S_{p,n,d})=c_n, limps(Sp,n,d)=cn.\lim_{p\to\infty}s(S_{p,n,d})=c'_n.

Here ξ2d\xi_{2d} is a primitive 2d2d-th root of unity. Rationality conjecture. The generating functions

n0cnαnandn0cnαn\sum_{n\geq0}c_n\alpha^n\qquad\text{and}\qquad\sum_{n\geq0}c'_n\alpha^n

are rational functions in α\alpha, cos(λiα)\cos(\lambda_i\alpha), and sin(λiα)\sin(\lambda_i\alpha), with the coefficients of the rational functions and the λi\lambda_i lying in Q(ξ2d)\mathbb{Q}(\xi_{2d}). This conjecture seeks a uniform description of the limiting Hilbert–Kunz multiplicities and FF-signatures for Fermat hypersurface rings across dimensions. The supplied source gives no resolution status.

Sources & referencesView supporting material

Primary source

Cheng Meng, “Limits of F-invariants and Riemann-Stieltjes integral”, arXiv:2507.13898 (2026).

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