slug: oeis-a193351-alternating-divisor-sum-prime-square bibkey: lagneau2012a193351 doi: null url: https://oeis.org/A193351 triage: theorem motivation_gids:

• D5/S3/Arith/LagneauAlternatingDivisorSumPrimeSquare

Prime alternating divisor sums in OEIS A193351

Problem

OEIS A193351, NAME (%N, verbatim):

Numbers k such that A071324(k) is prime.

COMMENT (%C, verbatim):

Numbers k such that the alternating sum of all divisors of k (divisors nonincreasing, starting with k) is prime.

CONJECTURE (%C, verbatim):

For k > 3, if the alternating sum of all divisors of k (divisors nonincreasing, starting with k) is a prime number, then k is either a square or twice a square.

The entry also records:

This conjecture is verified up to k <= 10^8. - Shreyansh Jaiswal, Apr 25 2025

With T : ℕ → ℤ denoting that literal alternating sum, the formal claim is ∀ k : ℕ, 3 < k → Nat.Prime (T k).toNat → (∃ t : ℕ, k = t ^ 2) ∨ (∃ t : ℕ, k = 2 * t ^ 2). The use of .toNat makes the primality predicate explicit; the proof establishes the required nonnegativity before using it.

Motivation

The conjecture was recorded by Michel Lagneau in 2012. It gives a universal structural classification, rather than a finite extension of the sequence’s table, and its conclusion is strong enough to be checked directly by the Lean kernel.

Gap

The bounded search surfaces recorded in issue #7548 and its probe comment on 2026-09-13 found 46 OEIS revisions with no proof and zero arXiv hits for the identifiers and quoted phrases. The OEIS Open benchmark and google-deepmind/formal-conjectures did not contain the conjecture. MathOverflow and GitHub hits were implementations or mirrors only. Jaiswal’s 2025 note proves a lower bound, not this classification. A071324 %F records the parity classification without proof attribution. OpenAlex returned HTTP 429 and is therefore ASSUMED-UNVERIFIED. The mathematical core is the classical parity fact that sigma(n) is odd exactly for squares and twice-squares; no priority claim is made.

Route

(a) Pair consecutive terms of the decreasing divisor list to obtain 2*T(n) >= n. The case n=4 is a square; otherwise n>=5, and integrality gives T(n)>=3. (b) A prime value at least three is not two and hence is odd. (c) Alternating signs disappear modulo two, so T(n) is congruent to sigma(n) modulo two. (d) If sigma(n) is odd, every odd-prime exponent in the factorization of n is even. Thus n=2^a*u^2, which is a square when a is even and twice a square when a is odd.

Falsifier

One natural number k>3 for which (T k).toNat is prime while k is neither a square nor twice a square would contradict the theorem.

Evidence

• Lean module: D5/S3/Arith/LagneauAlternatingDivisorSumPrimeSquare.lean.
• Main theorem: lagneau_a193351.
• Axioms: std3 (propext, Classical.choice, Quot.sound).
• The orchestrator checked every n<=10^6: 213 prime values split into 118 squares and 95 twice-squares, with zero counterexamples and zero violations of T(n)>=n/2.
• The search seat and the independent probe reported the same bounded readings.

Triage

theorem. The formal result proves the full implication for every natural number greater than three.

ASSUMED-UNVERIFIED

OpenAlex coverage was not verified because its query returned HTTP 429. The other literature searches were bounded and do not establish exhaustive coverage. No first-publication or priority claim is made.