slug: oeis-a378495-terminal-difference
bibkey: kagey2024a378495
doi: null
url: https://oeis.org/A378495
triage: theorem
motivation_gids:
  - D5/S3/Zeros/Convolution/PerfectMatchingCount.card_fixedPointFreeInvolution_mul

The terminal difference of OEIS A378495

Problem

The FORMULA field of OEIS A378495, revision 70, states verbatim:

Conjecture: T(n,n-2) - T(n,n) = (n-3)!(n-4)(n-1)/2 for n >= 5.

On the source-agreeing columns $2\le k\le n$, $T(n,k)$ counts derangements in $S_n$ with no $k$-cycle; the source explicitly sets $T(n,1)=0$. The triangle is read by rows, with offset 1,5. Both cardinalities are cast to $\mathbb{Q}$ before ordinary subtraction; the division by two is rational division.

Statement echo

Motivation

This is the tier-one externally published question preregistered in #23899. The source is Peter Kagey’s 2024 triangle, with the terminal formula introduced in revision 60. The frozen motivation theorem above is a related application of permutation cycle-type counting to perfect matchings. It is an analogous mathematical anchor, not a dependency or an asserted substitute for the present settlement.

Gap

The exact mathematical gap is closed by D5.S3.Combinatorics.Derangements.TerminalDifference.result. The source note records the fixed-pin repository and bounded external searches. No exact prior settlement was found in that successful scope. Kallat v1, section 9.2 and supplementary sec:open, does not settle this stronger identity. OpenAlex search and Semantic Scholar HTTP 429 responses leave literature coverage incomplete; absence of a result in those endpoints is not claimed.

Route

Partition the actual finite set of derangements into permutations with and without a $k$-cycle. The retained count plus the excluded count is $|D(n)|$. Among derangements an $n$-cycle forces cycle type $\{n\}$, and an $(n-2)$-cycle forces $\{n-2,2\}$ when $n\ge 5$. The latter follows because the remaining lengths sum to two and every nontrivial cycle length is at least two.

Pinned Mathlib counts these cycle types: the excluded terminal count is $(n-1)!$, while the other excluded count multiplied by $2(n-2)$ is $n!$. Subtract the two partition equalities over $\mathbb{Q}$, expand factorials and cancel the nonzero factor $n-2$. This gives $T(n,n-2)-T(n,n)=(n-3)!(n-4)(n-1)/2$. The lower bound justifies every natural subtraction and rational cast. No finite positive table is used to infer the universal conclusion.

Falsifier

A natural $n\ge 5$ whose actual permutation cardinalities violate the displayed rational equality would contradict the stated formal result. A different triangle, a truncated natural subtraction between counts, or an extra assumption on cycle multiplicities would change the question.

Evidence

The independent scientific terminal is 6509681032. It accepts the original universal identity. Its compiled result statement identity is sha256:c63f5c740b67d90f69fd542bf3d6e5bd57e53b6f35910ac068920bcddf197a0a. The proof source is D5/S3/Combinatorics/Derangements/TerminalDifference.lean; its typed descriptions and generated mirror are under the corresponding Blueprint path. The accepted axiom closure is contained in Classical.choice, Quot.sound, propext; no new axiom is introduced. The displayed formulas use a hand-authored typed Scribe AST of the exact Lean statements; FromLean had no unfrozen fixture entries (missing). They are not claimed to be kernel-derived formula projections. Their quantifiers, casts, natural subtractions, factorials and proof references agree with the Lean declarations and source-agreeing column domain.

Declaration form and actual consumers

Names below are relative to TerminalDifference; Adjacent means AdjacentDifferenceRefutation. No declaration supplies an escape witness.

sum_two’s induction hypothesis is unused in the compiled proof; it is not an escape argument. The actual suppliers are the pinned Mathlib cycle-type cardinalities, support identities and finite-set partition, combined by logical decomposition and factorial/rational normalization. The terminal module imports only pinned Mathlib suppliers; it has no dependency on another project module. The admission basis is open-problem-resolution, with the exact preregistration above. The result Describe carries the typed Proved resolution claim for this dossier and names the exact canonical frozen declaration. Remote required CI and final independent integration review remain separate obligations; this dossier asserts no delivery completion.

Utility and unfinished audit

The module’s utility classification is none: every definition describes the full unbounded counting family, and every theorem is a general structural or symbolic identity. No bounded enumeration, checker, numeric reduction or certified finite instance is a delivered result. The admission exception concerns the external named problem, not a fabricated computational consumer or escape witness.

The nine public theorems in the table have an unfinished four-slot audit under the concrete section 3.9 issue exception. The faithful design attempt inspected Reg.Support.DependentFamily and its typed family contract. Missing are complete source reconstruction and selection, an enrolled readout, and full-family realization, variation, sensitivity and observational dependence for each target. No compiling Reg source or declared_validated registration is claimed. The linked issue permits the mathematical unit to proceed to review; it does not discharge those audit obligations.

The first-event research readings remain unproduced: the existing execution-start fence is rejected by the readout parser, as recorded in 6509713581. Missing counts are not zeros. The original pre-mathematics full native closure ordering is unestablished. Neither limitation changes the separate kernel-checked mathematical conclusion.

Triage

What the settlement shows

• Proved: the exact original terminal identity for every $n\ge 5$, from actual cardinalities, with only the original lower bound.
• Mechanism: the two complete cycle types $\{n\}$ and $\{n-2,2\}$ give a symbolic count and rational cancellation, rather than a table.
• Computed: no independent finite experiment is needed or credited for this universal identity.
• Open: extensions below five or a corrected adjacent shift are not settled here. The separate literal adjacent question has its own refutation dossier.
• Boundary: remote required CI and final independent integration review remain pending; no delivery or discovery KPI follows from this dossier.

ASSUMED-UNVERIFIED

Global literature completeness, novelty and priority are not established. The source qualification is bounded, including the named HTTP 429 gaps. Historical execution times are recorded workflow assertions supported by unchanged server evidence, not recovered process telemetry. The original pre-mathematics closure ordering remains unestablished. The unfinished escape audit and unproduced research readings remain explicit limitations; neither is claimed repaired or validated.