slug: oeis-a329278-triangular-residue-descent-count bibkey: kagey2019a329278 doi: null url: https://oeis.org/A329278 triage: theorem motivation_gids:

• D5/S3/Arith/Congruence/TriangularResidueDescentCount

The triangular-residue descent count in A329278

Problem

OEIS A329278, NAME (verbatim):

Irregular table read by rows. The n-th row is the permutation of {0, 1, 2, …, 2^n-1} given by T(n,k) = k(k+1)/2 (mod 2^n).

COMMENT (verbatim; Peter Kagey, Nov 11 2019):

Conjecture: for n > 0, the n-th row has 2^(n-1)-1 descents.

A descent is an index k with 0 <= k < 2^n - 1 and T(n,k) > T(n,k+1). The target concerns every positive natural n.

Motivation

This 2019 OEIS conjecture asks for the exact number of adjacent strict descents in every positive row of the triangular-number permutation modulo a power of two. The theorem proves the full unbounded count rather than a finite collection of rows.

Gap

The checked surfaces on 2026-09-13 gave five OpenAlex results, all false positives; zero exact MathOverflow identifier hits, while MathOverflow answer 507816 discusses the permutation but contains no descent count; zero exact GitHub phrase hits; and zero matches in the pinned Mathlib tree and its dependencies. The arXiv endpoint returned Rate exceeded, so arXiv was not verified. No proof or refutation was found in the surfaces that completed. This is a bounded search report, not an exhaustive literature or priority claim.

Route

Write s_k = T(n,k) and M = 2^n. The triangular-number identity

k(k+1)/2 + (k+1) = (k+1)(k+2)/2

gives s_(k+1) = (s_k + (k+1)) mod M; a wrap is equivalent to a descent. Induction on j <= M-1 then proves the prefix invariant

card {k < j : T(n,k+1) < T(n,k)} = floor((j(j+1)/2)/M).

At the endpoint, (2^n-1)2^n/2 = (2^n-1) * 2^(n-1), so the quotient is 2^(n-1)-1.

Falsifier

Any positive natural n whose row has a number of adjacent strict descents different from 2^(n-1)-1 would refute the result. A failure of either the wrap/descent equivalence or the prefix-quotient invariant would invalidate the proof route.

Evidence

• Lean module: D5/S3/Arith/Congruence/TriangularResidueDescentCount.lean.
• Main theorem: kagey_a329278 : forall n, 0 < n -> descents n = 2^(n-1)-1.
• Public definitions: T and descents.
• The theorem’s axiom report is std3: propext, Classical.choice, and Quot.sound.
• The orchestrator checked n = 1..16 with zero descent-count mismatches and also checked that each row is a permutation and has the stated endpoint.
• The probe checked n = 1..20, covering 2,097,150 entries, with zero descent-count mismatches.

Triage

theorem. The Lean theorem proves the stated descent count for every positive natural n, matching the scope of the OEIS conjecture.

ASSUMED-UNVERIFIED

The OEIS quotation, attribution, external search results, and finite numerical checks are external evidence rather than kernel-checked facts. The arXiv surface was not verified because its endpoint was rate-limited. The completed search was bounded to the surfaces listed in Gap, so no exhaustive literature or first-publication claim follows. Source-to-Lean identification is not itself a kernel-checked bibliographic fact.