slug: oeis-a389000-digit-sum-square-least-witness bibkey: wu2025a389000 doi: null url: https://oeis.org/A389000 triage: theorem motivation_gids:

• D5/S1/Digit/Admissibility/DigitSumSquareLeastWitness

The least simultaneous digit-sum witness in A389000

Problem

Conjecture: a(9m+5) = 210^(2*m+1)-1 for m >= 0.

— Chai Wah Wu, Oct 01 2025. The OEIS rendering means 2·10^(2m+1) − 1. Here a(n) is the least positive integer k such that n divides both the base-10 digit sum of k and that of k^2. The entry was created Sep 22 2025.

Motivation

This is a first-tier recent OEIS conjecture selected by the search seat. KPI = open problems resolved. The target is the full unbounded formula, including leastness, for every natural index m.

Gap

The search seat reported no proof found after reading the OEIS entry and revision history on 2026-09-08 and searching by identifier on arXiv, MathOverflow, and GitHub. This Stage-B seat had no network access and did not independently repeat those searches. Absence of a proof in that scope does not establish exhaustive literature coverage or first-publication priority.

Route

Put n = 9m+5, L = 2m+1, and K = 2·10^L − 1. Both K and K^2 have digit sum 2n, so K satisfies the two divisibility conditions. For 0 < k < K, the leading-digit ceiling and its rigid equality case give 0 < digitSum k < 2n; divisibility by n therefore forces digitSum k = n. The mod-9 congruence gives digitSum(k^2) ≡ n^2 ≡ 7 (mod 9). Writing digitSum(k^2) = nt gives 5t ≡ 7 (mod 9), hence t ≥ 5 and digitSum(k^2) ≥ 5n. But k^2 < 4·10^(2L) gives the contrary ceiling digitSum(k^2) ≤ 18L+3 = 36m+21 < 5n. Thus K is least.

Falsifier

A counterexample index m ≥ 0 with a(9m+5) ≠ 2·10^(2m+1) − 1 would contradict the assertion. This could be witnessed by a smaller positive k satisfying both divisibilities, or by failure of the proposed candidate. The orchestrator’s exact check is supporting evidence only; it does not replace the universal proof and was not repeated by this seat.

Evidence

• Lean module: D5/S1/Digit/Admissibility/DigitSumSquareLeastWitness.lean.
• Main theorem: wu_conjecture.
• Public companions in that module: digitSum_witness, digitSum_witness_sq.
• The brief also requested companion labels wu_conjecture_one, altRowSum_succ_sub, and fib_cassini_two; none is declared in the target module, so they are not evidence for this resolution.
• The implementation-seat envelope reports all three public theorems have exactly the std3 axioms: propext, Classical.choice, Quot.sound.

Triage

theorem. The formal proof establishes the complete least-witness formula recorded in the OEIS conjecture, for every natural m.

ASSUMED-UNVERIFIED

The quotes, definition, attribution, and dates were supplied by the orchestrator. The OEIS revision history was read by the search seat, not by this seat. The reported literature scope was the OEIS entry/history and identifier searches on arXiv, MathOverflow, and GitHub on 2026-09-08; this seat performed no network verification. No exhaustive literature or priority claim follows. The std3 report and the orchestrator’s exact check were supplied evidence, not computations performed by Stage-B. Source-to-Lean identification is not itself a kernel-checked fact.