Restricted Binomial GCDs at Primes Congruent to -1

Abstract

For integers $m\ge 2$ and $m\mid N$, let $G(N;m)=\gcd \{(\genfrac{}{}{0ex}{}{N}{k}):0<k<N,m\mid k\}$. We prove a complete $p$-adic valuation formula for $G(N;m)$ at primes $p\equiv -1(\mathrm{mod}m)$, under the hypotheses $m\ge 3$, $m\mid N$, and $m<N$. Writing $N={\sum }_i{d}_i{p}^i$, put $A={\sum }_{i\text{ even}}{d}_i$ and $B={\sum }_{i\text{ odd}}{d}_i$. Then $v_p(G(N;m))$ is $2$ in the exceptional mixed case $(A,B)=(1,1)$ with $p=m-1$, is $1$ in the mixed case $(A,B)=(1,1)$ with $m<p$, is $1$ in the one-parity cases $(A,B)=(m,0)$ and $(A,B)=(0,m)$, and is $0$ otherwise. The proof uses Kummer's theorem to translate the problem into digitwise borrow counts and a minimal signed zero-sum classification. A source-level literature audit through 25 September 2026 located no equivalent prior theorem for the full minus-one classification: McTague's corrected same-residue extension covers the one-parity $p>m$ subcases, but not the mixed-parity branch or the $p=m-1$ regime. The theorem, its plus-one companion, a scaling reduction, and the $m=3,4,6$ specializations have been formalized in Lean 4 / Mathlib.

8 pages, no figures. Lean 4 / Mathlib formalization: https://github.com/jfairfaxball-348/pascal-minus-one . Palomar record PALOMAR-2026-09-25-000017, version 1