slug: oeis-a376317-kurkov-periodic-boundary-schroder
bibkey: oeis2024a376317
doi: null
url: https://oeis.org/A376317
triage: theorem
motivation_gids:
  - D5/S1/Recurrence/Algebraic/PeriodicBoundarySchroder.result

Kurkov’s periodic boundary recurrences and the large Schroder diagonal

Problem

Mikhail Kurkov’s OEIS A376317, revision 25, states:

Conjecture: cases f(n) = n mod 2 and f(n) = [(n mod 3) > 0] both gives A006318.

The source array is the natural recursion with P(0,m)=1, the positive-row branch P(n,m)=P(n-1,m) for m<=n, and P(n,m)=P(n,m-1)+P(n-1,m+f(m-n)) otherwise. The zeroth-row clause has priority. The settled statement keeps both shifts in one proposition:

∀ n : ℕ,
  P f2 (n + 1) (n + 1) = Nat.largeSchroder n ∧
  P f3 (n + 1) (n + 1) = Nat.largeSchroder n

Here f2 d = d % 2 and f3 d = 0 exactly when d % 3 = 0, and Nat.largeSchroder is A006318. The statement includes n=0; no alternate array, truncation, integrality premise, or second count is substituted. A376388 repeats this same conjecture and is not a second resolution.

Motivation

The two periodic source shifts have the same diagonal sequence. The result answers the complete unbounded source question, rather than a finite prefix. Its admission basis is open-problem-resolution; utility is none.

Gap

The source reading and bounded literature screen are recorded in Library/RecurrenceComposition/oeis2024a376317.md. That note records the exact OEIS revision, the repeated A376388 wording, the arXiv metadata queries, the pinned LeanOpenProblems corpus screen, and the targeted third-party Lean readings. No exact proof was found in that scope. The scope does not establish worldwide openness, priority, or exhaustive absence of an unpublished proof.

The preregistration and settlement record remain in issue #23318. Its sole proved terminal comment is #6481166931; the source reconstruction disclosure is #6480851425. No new mathematical terminal issue is introduced by this dossier.

Route

The Lean definition preserves the source’s lexicographic recursion, including the below-diagonal branch. Writing C_f(n,d)=P_f(n,n+d) yields the boundary and step equations used by the proof. A coefficient-identification induction over rational formal power series then reduces the diagonal to the coefficient of a family H.

For the period-two shift, the family alternates between (1+xS)S^j and S^(j+1), where S is Mathlib’s large Schroder series. For the period-three shift, R=(S^2+S)/2 supplies the three residue branches. The quadratic Schroder-series identity verifies both systems, and coefficient extraction gives the two natural equalities. All exponents are natural; no integrality assumption on R is used.

Falsifier

Any natural n for which either diagonal differs from Nat.largeSchroder n refutes the conjunction. Changing the zeroth-row priority, the positive-row branch, or either periodic shift changes the source question and is not a counterexample to this statement.

Evidence

The frozen declaration is D5/S1/Recurrence/Algebraic/PeriodicBoundarySchroder.result, with canonical exposition in the corresponding Blueprint Scribe. Its scoped Lean report records the standard closure {propext, Classical.choice, Quot.sound} and no additional axiom or sorry. The Reg mirror compiles, but its current audit readout remains declared_unresolved (IE-C050, unclassified_form, source.statement_reconstruction) under the disclosure in #23318. That audit limitation does not alter the kernel-checked theorem or claim an audit validation that is not present.

Triage

theorem; resolution proved for the exact joint A376317/A376388 conjecture quoted above. The result settles both periodic shifts for every natural index. Other A006318 identities, the source entries’ main sequences, and claims of global literature priority remain outside this dossier.

ASSUMED-UNVERIFIED

The literature scope is bounded by the cited Library note. Global priority, exhaustive absence of prior proofs, and the status of inaccessible or unpublished work remain unverified. The typed escape audit is unresolved as disclosed above; no declared_validated registration is claimed.