slug: oeis-a396795-iterate-product-twenty-five-mod-five bibkey: hanna2026a396795 doi: null url: https://oeis.org/A396795 triage: theorem motivation_gids:

• D5/S1/Recurrence/Residue/IterateProductTwentyFiveModFive

Divisibility of the A396795 coefficients

Problem

OEIS A396795, Paul D. Hanna, Jun 06 2026, gives the following NAME and COMMENT, quoted verbatim from Library/Recurrence/hanna2026a396795.md:

G.f. A(x) satisfies A^2(x) * A^3(x) = x^2 + 25*x^3, where A^n(x) is the n-th iteration of A(x).

Conjecture: a(n) == 0 (mod 5) for n > 1.

Motivation

This is a first-tier OEIS conjecture from the 2026 entry. The KPI is open problems resolved. The target proves divisibility for every index greater than one of the uniquely determined normalized integer formal series.

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 access and did not repeat those searches. This is a bounded search report, not a claim of exhaustive literature coverage or first-publication priority.

Route

The escape content is iterate_product_lift, general in the natural modulus m and in both natural iterate indices i, j. For an integer formal series f with constantCoeff f = 0, if m divides every coefficient of f - X and m ∣ i + j, then m² divides every coefficient of iterate f i * iterate f j - X².

Write f = X + C m * b by coefficientwise division and work in ZMod (m²). There (m : ZMod (m²)) squares to zero, and nilpotent_iterate gives iterate f k = X + C (k·m)·b after mapping. The quadratic correction vanishes; the cross-term scalar C ((i+j)·m) vanishes whenever m ∣ i + j, which is the direction the theorem uses and proves. The converse holds for m > 0 but fails at the degenerate modulus zero, where every scalar vanishes: m = 0, i = 1, j = 0 gives a vanishing scalar while 0 ∤ 1. The theorem is stated with m ∣ i + j as a hypothesis, so it covers m = 0 without asserting that equivalence. The public subst_annihilate supplies the substitution step of nilpotent_iterate.

At (m,i,j) = (5,2,3), the lift makes successive degree-by-degree corrections exact and divisible by 5. generatingSeries selects the proved existential witness and a n denotes its degree-n coefficient. generating_equation asserts iterate A 2 * iterate A 3 = X² + 25X³, A(0) = 0, and a(1) = 1. Thus it realizes the NAME: A^n is the frozen compositional iterate from D5/S1/Recurrence/Invariants/CompositionalIterateCongruence, not an ordinary power. generating_unique covers every normalized integer solution. hanna_conjecture concludes n > 1 → 5 ∣ a n through solution_five, error_divisible, and the lift. Target generality is I because the module imports the frozen Invariants/CompositionalIterateCongruence, itself G.

Falsifier

An index n > 1 for which the normalized solution has 5 ∤ a(n) would contradict the conjecture. The orchestrator’s exact coefficient check is supporting evidence only; the theorem has no finite index bound.

Evidence

• Lean module: D5/S1/Recurrence/Residue/IterateProductTwentyFiveModFive.lean.
• Main theorem: hanna_conjecture.
• Companions: subst_annihilate, nilpotent_iterate, iterate_product_lift, generating_equation, generating_unique.
• Public definitions: generatingSeries, a.
• Axioms: std3, exactly propext, Classical.choice, Quot.sound, as reported by the implementation seat’s #print axioms checks.
• Frozen direct import: D5/S1/Recurrence/Invariants/CompositionalIterateCongruence, statement_id sha256:4063c4732963765b6b16b5dda51775b4d179cfc3a203c91ed15e3ebffb0467e7.

iterate_product_lift is NOT an instantiation of any frozen public theorem. The frozen D5/S1/Recurrence/Invariants/TripleIterateProductModFour fixes (m,i,j) = (4,1,3) and the frozen D5/S1/Recurrence/Residue/IterateProductNineModThree fixes (3,1,2). IterateProductNineModThree.lift_mod_nine is public on dev, but its modulus and both iterate indices are fixed; the scalar helpers subst_annihilate and nilpotent_iterate are private in both siblings. That is why those two are published here for the first time, together with the variable-modulus, variable-index lifting theorem, which neither sibling states. Neither sibling is imported by this module.

Triage

theorem. The formal proof closes the universal coefficient divisibility assertion for the unique normalized integer solution of the NAME equation.

ASSUMED-UNVERIFIED

The quotations and attribution were supplied by the orchestrator and copied from the Library note. The search seat, not this seat, read the OEIS revision history on 2026-09-09. The reported literature scope comprised the OEIS entry and history plus identifier searches on arXiv, MathOverflow, and GitHub; private indexes and exhaustive literature coverage were not established. This seat had no network. Source-to-Lean identification, source dates, search completeness, and first-publication priority are not kernel-checked facts. Compilation, std3 axiom output, and the exact numerical check were reported by the implementation seat or orchestrator, not rerun by Stage B.