slug: oeis-a383377-absolute-reciprocal-geometric-parity bibkey: hanna2025a383377 doi: null url: https://oeis.org/A383377 triage: theorem motivation_gids:

• D5/S1/Recurrence/Invariants/AbsoluteReciprocalGeometricParity

Parity of the A383377 coefficients

Problem

Paul D. Hanna, OEIS A383377, May 15 2025. The following quotations are verbatim from Library/Arith/hanna2025a383377.md.

NAME:

G.f. satisfies A(x) = Sum_{n>=0} x^n * abs(1/A(x)^n), where abs(F(x)) equals the series expansion formed by the unsigned coefficients in F(x).

COMMENT:

Conjecture: a(n) is even for n > 1.

Motivation

This is a first-tier recent OEIS conjecture. KPI = open problems resolved. The target module proves the assertion for every natural index greater than one.

Gap

The search seat reported no proof found after reading the OEIS entry and revision history on 2026-09-09 and performing identifier searches on arXiv, MathOverflow, and GitHub. This Stage-B seat has no network and did not independently repeat those searches. This bounded search does not establish publication priority.

Route

absSeries F is mk (fun n => |coeff n F|). The integer series A is built by stabilised iteration (approximation) of the coefficientwise map Phi: at each degree N its coefficient is the sum, for n from zero through N, of coeff N (X ^ n * absSeries (invOfUnit F 1 ^ n)). The map preserves constant coefficient one and improves agreement below degree d to agreement below degree d+1, so each coefficient stabilises.

generating_equation states A(0) = 1 and, for every N, coeff N A = sum (n in range (N+1)) (coeff N (X^n * absSeries (invOfUnit A 1 ^ n))). This is exactly the coefficientwise form of the OEIS infinite sum: summands with n > N vanish at degree N. Mathlib has no infinite sums of formal series; the honesty boundary is the finite coefficientwise statement, with no analytic convergence or separate infinite-sum operator asserted. generating_unique covers every integer series B with constant coefficient one satisfying this equation.

Over ZMod 2, absolute values disappear. geometric_fixed establishes coefficient agreement with arbitrary finite geometric sums, transports that agreement through multiplication, and obtains F*(1 - XU) = 1 with UF = 1. Consequently F = 1 + X (mod_two_identity). hanna_conjecture reads off Even (a n) for n > 1. The module has no D5 import and declares generality G.

Falsifier

A counterexample index n > 1 with odd a(n) in the uniquely defined integer series would contradict the assertion. The orchestrator’s exact coefficient check is supporting evidence only, not the universal proof.

Evidence

• Lean module: D5/S1/Recurrence/Invariants/AbsoluteReciprocalGeometricParity.lean.
• Main theorem: hanna_conjecture.
• Companions: generating_equation, generating_unique, mod_two_identity.
• Axioms: std3, namely propext, Classical.choice, Quot.sound, for each public theorem, as recorded in the implementation seat’s envelope.

Triage

theorem. The formal proof closes the universal parity assertion recorded by OEIS.

ASSUMED-UNVERIFIED

The OEIS quotations, attribution, and date were supplied by the orchestrator. The OEIS entry and revision history were read by the search seat on 2026-09-09, not by this seat. The reported literature scope was identifier searches on arXiv, MathOverflow, and GitHub; this seat had no network access. No exhaustive literature or first-publication claim follows. Source-to-Lean identification and the reported source search are not kernel-checked facts.