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slug: oeis-a000680-schulte-half-factorial-prime-criterion bibkey: schulte2025a000680 doi: null url: https://oeis.org/A000680 triage: theorem motivation_gids:

  • D5/S3/Arith/Congruence/SchulteHalfFactorialPrimeCriterion

Schulte’s A000680 half-factorial primality criterion

Problem

OEIS A000680, NAME (%N, verbatim):

a(n) = (2n)!/2^n.

The settled COMMENT (%C, verbatim) is:

Conjecture: For n > 0 (a(n) + 2^n) is a multiple of (2n + 1) if and only if (2n + 1) is a prime number. - Werner Schulte, Oct 05 2025

The offset is %O 0,3, and the AUTHOR line (%A, verbatim) is:

N. J. A. Sloane

The full-quantifier reading is: for every n : Nat, if 0 < n, then (2 * n + 1) ∣ a n + 2 ^ n if and only if (2 * n + 1).Prime. The scope wall is exactly this biconditional for every positive natural n; no additional sequence values, implications, or historical claims are included.

Motivation

The independent question is Schulte’s named primality characterization of OEIS A000680 for every positive natural index. It is registered as an external open problem under pre-registration issue #8173. The formal result keeps the OEIS definition and the single Conjecture sentence as its public surface.

Gap

Readings of 2026-09-16: the OEIS entry still marks the line Conjecture (added 2025-10-05) with no proof line; OpenAlex "A000680" returns 33 hits, none about this criterion; Math.SE API 24 hits, none about it; arXiv was not searched (no API response). Wilson’s theorem and the odd-composite factorial divisibility are textbook facts; the named criterion itself was not found in these surfaces. Historical openness beyond them is ASSUMED-UNVERIFIED.

Repository prior art at origin/dev: git grep A000680 returned 0 hits. ZMod.wilsons_lemma is used by 2 D5 modules. The frozen D5/S3/Arith/Congruence/LaymanOddPowerFactorialResidue.factorial_dvd_triangular_of_not_odd_prime supplies the odd-composite factorial step and is REUSED; its direct frozen dependency is GID D5/S3/Arith/Congruence/LaymanOddPowerFactorialResidue.factorial_dvd_triangular_of_not_odd_prime, declaration statement_id sha256:e66be6c11321ef2a8bb90c32fb4a9755ab1468d5aece54699035c65784793600, with module pin sha256:0b53e5405adaa92296c5ed5a39e491fdddcc8d7f599adef8aa1f170222f074d4. That theorem does not by itself state the criterion because it only gives factorial divisibility for the odd-composite branch, not the equivalence with the A000680 expression.

Pinned Mathlib contains ZMod.wilsons_lemma, ZMod.pow_card_sub_one_eq_one, Nat.factorial_eq_mul_doubleFactorial, and Nat.doubleFactorial_two_mul; no statement of the criterion was found. Numerics for 1 ≤ n ≤ 600 checked both directions with zero exceptions; there are 196 primes 2n+1 in the range.

Pre-registration: issue #8173 (2026-09-15T19:06:58Z).

The public theorem is bind-only under the open-problem-resolution admission basis. The proof has no escape witness and no helper theorem declarations. Its direct frozen dependency is the LaymanOddPowerFactorialResidue declaration listed above; all remaining steps instantiate pinned Mathlib results and normalize their consequences.

Route

  1. Use 2^n * a(n) = (2n)! from the exact natural-number quotient.
  2. Multiply the divisibility statement by the unit 2^n modulo 2n+1.
  3. In the prime direction, use Wilson’s theorem for (2n)! and Fermat’s theorem for 2^(2n).
  4. In the composite direction, use the frozen odd-composite factorial divisibility and cancel the factorial term, leaving a coprime power.

Falsifier

A positive natural n for which the divisibility and primality sides differ would falsify the theorem. A composite 2n+1 satisfying the divisibility, or a prime 2n+1 failing it, is a direct counterexample. Finite numerical agreement cannot establish the universal claim.

Evidence

  • Final Lean module: D5/S3/Arith/Congruence/SchulteHalfFactorialPrimeCriterion.lean; 96 lines and 4320 UTF-8 bytes; its directory contains 44 Lean files.
  • lake env lean D5/S3/Arith/Congruence/SchulteHalfFactorialPrimeCriterion.lean: exit 0.
  • tools/scripts/agent/header-check.sh D5/S3/Arith/Congruence/SchulteHalfFactorialPrimeCriterion.lean: exit 0.
  • Final-source #print axioms for a and result: exit 0; a uses [propext] and result uses [propext, Classical.choice, Quot.sound].
  • Deleting Mathlib.Data.Nat.Factorial.DoubleFactorial from the direct imports: exit 1; deleting the frozen LaymanOddPowerFactorialResidue import: exit 1.
  • Numeric check of the final definition for every 1 ≤ n ≤ 600: exit 0; both directions, 196 prime cases, zero exceptions.

Triage

theorem; resolution proved for the OEIS Conjecture sentence at every positive natural index. The Scribe theorem node carries the matching OpenProblemResolutionClaim with ResolutionKind.Proved.

ASSUMED-UNVERIFIED

Historical openness beyond the OEIS, OpenAlex, and Math.SE surfaces described above is unverified. The arXiv surface was not searched because its API gave no response. No exhaustive novelty or priority claim is made.