Deep holes of full-length Reed--Solomon codes: uniform bounds and exact classifications

Abstract

Let $u$ be a received word for the full-length Reed--Solomon code $\operatorname{RS}_q(k)$, with interpolation polynomial $f_u$ of degree $k+m$. For each $m\in\{3,4,5,6,7\}$, we classify all such deep holes over every finite field, without a field-size hypothesis. None occur for odd $m$ in this range. At $m=4$ and $m=6$, the only exceptions are, respectively, the Segre sextic and Frobenius-power families; these occur only in dimension two over fields of characteristic two. For arbitrary $m\geqslant3$, we also establish two uniform bounds. If $\sqrt q\geqslant12(m+1)\log(2m+2)$, no word of interpolation degree $k+m$ is a deep hole when $q$ is odd, $k\geqslant3$, or $m$ is odd. If $q>16(m+2)^4$, we obtain the corresponding complete classification at every admissible $k$, including the explicit dimension-two exceptions. Our proofs use an agreement criterion expressed in complete homogeneous symmetric functions, agreement-count estimates, finite geometry, permutation-polynomial restrictions, and subset constructions for solving the resulting symmetric-function equations. The remaining finite cases are settled by finite computations with supplied certificates and deterministic verification programs.