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slug: oeis-a380678-quadratic-square-reversion-dyadic-support-parity bibkey: hanna2025a380678 doi: null url: https://oeis.org/A380678 triage: theorem motivation_gids:

  • D5/S1/Recurrence/Parity/QuadraticSquareReversionDyadicSupportParity

Dyadic support of the odd coefficients of A380678

Problem

Paul D. Hanna, OEIS A380678, February 2025. The Library note records the month and year but not the exact %A line date. The following NAME and COMMENT are copied verbatim from Library/Recurrence/hanna2025a380678.md.

NAME:

G.f. A(x) satisfies A( x - A(x)^2/(1 - A(x)^2) ) = x.

COMMENT:

Conjecture: for n>= 1, a(n) is odd iff n is a term of A027383, where A027383(2m) = 32^m - 2 and A027383(2m+1) = 42^m - 2.

Motivation

This is a first-tier OEIS conjecture from the 2025 entry. KPI = open problems resolved. The target is a parity classification for every index n >= 1.

Gap

The search seat reported no proof found after reading the OEIS entry and revision history on 2026-09-09 and searching by identifier on arXiv, MathOverflow, and GitHub. This Stage-B seat had no network access and did not independently repeat those searches. This is a bounded search report, not a claim of exhaustive literature coverage or publication priority.

Route

Construct A over the integers as the stabilised fixed point of the reversion-type contraction f + X - f.subst (X - f^2 * invOfUnit (1-f^2) 1). The square-denominator argument gains one degree of coefficient agreement: its difference factors as (f-g)*(f+g) times the two unit inverses. The inner argument has linear coefficient one, so substitution has multiplier 1 on a(n) at the first differing coefficient. Thus generating_equation states A(0) = 0, a(1) = 1, and exactly the NAME equation, represented by A.subst (X - A^2 * invOfUnit (1 - A^2) 1) = X. generating_unique covers every normalised integral solution.

Over ZMod 2, write Abar for the coefficientwise reduction of A. Characteristic two gives 1 - Abar^2 = (1 - Abar)^2. The reduced equation is therefore exactly the equation that the frozen QuadraticReversionDyadicSupportParity.lacunary_reversion proves for lacunarySeries. Reduced uniqueness gives mod_two_identity, namely Abar = lacunarySeries. Its coefficient formula gives hanna_conjecture: for n >= 1, Odd (a n) iff there exists a natural m with n + 2 = 3 * 2^m or n + 2 = 4 * 2^m. These are precisely the COMMENT’s A027383 support families: A027383(2*m) = 3*2^m - 2 and A027383(2*m+1) = 4*2^m - 2.

Target generality is I because the module imports the frozen D5/S1/Recurrence/Invariants/QuadraticReversionDyadicSupportParity, itself generality G. Its statement_id is sha256:64c91825be7bdc54e67d6c12e38a5e78833d3aa76de1009a7dfea7dd81ebe781.

Falsifier

A counterexample index n >= 1 where the coefficient of the normalised NAME solution is odd without belonging to either family, or even while belonging to either family, would falsify the assertion. The orchestrator’s exact numerical check is supporting evidence only; no finite prefix replaces the universal proof.

Evidence

  • Lean module: D5/S1/Recurrence/Parity/QuadraticSquareReversionDyadicSupportParity.lean.
  • Main theorem: hanna_conjecture.
  • Companions: generating_equation, generating_unique, mod_two_identity.
  • Axioms: std3 (propext, Classical.choice, Quot.sound) for each public theorem, as reported by the implementation seat’s single-file Lean check.

Triage

theorem. The formal proof establishes the universal parity assertion for the constructed series and proves its NAME equation and normalised uniqueness.

ASSUMED-UNVERIFIED

The OEIS quotes were supplied by the orchestrator and copied from the Library note. February 2025 is the supplied entry date at month precision; the exact %A line date was absent from that note and was not independently verified. The OEIS revision history was read by the search seat, not by this seat. The reported literature scope was the OEIS entry and revision history plus identifier searches on arXiv, MathOverflow, and GitHub on 2026-09-09; it was not exhaustive. Source-to-Lean identification, first-publication priority, and the orchestrator’s numerical receipt are not kernel-checked facts.