Metadata

slug: mckean-2025-toda-primes-conjecture
bibkey: mckean2025todaprimes
doi: null
triage: theorem
motivation_gids:
  - D5/S3/Arith/McKeanTodaPrimes/Result.result
url: https://arxiv.org/abs/2511.19744v1

Refutation of both Toda-prime denominator assertions

Problem

Stephen McKean, Toda primes, arXiv:2511.19744v1, §5.1, Conjecture 5.6, printed p. 8:

Conjecture 5.6. Let d be a Bernoulli denominator. (i) If F (d) = 4a for some integer a, then T (am) = T (a) whenever D₄ₐₘ = D₄ₐ. (ii) If D₄ₐₘ ≠ D₄ₐ = d, then t(am) ≥ t(a).

Definition 1.1, §1, printed p. 1:

Definition 1.1. Let n be an integer. A Toda prime of n is an odd prime p such that p − 1 | 4n and gcd(p, 4n/(p−1)) = 1. Denote the set of Toda primes of n by T (n) (see Table 1), and let t(n) := |T (n)| (see Figure 1).

Notation 5.5, §5.1, printed p. 8:

Notation 5.5. Given an even integer 2m, let P (2m) denote the set of primes such that p − 1 | 2m, so that D₂ₘ = ∏ₚ∈P(2m) p. Let F (d) = min{2m > 0 : D₂ₘ = d} [PW23].

Formal reading

Lean names have prefix D5.S3.Arith.McKeanTodaPrimes.Result.

Source object	Lean name	Correspondence
$T (n)$	`todaPrimes`	Odd primes with predecessor dividing $4 n$ and coprime quotient.
$t (n)$	`t`	Cardinality of the finite Toda-prime set.
$D_{k}$	`bernDen`	Denominator of Mathlib’s rational `bernoulli k`.
$P (k)$	`denominatorPrimes`	Primes whose predecessor divides $k$ .
Conjecture 5.6(i)	`clauseI`	Positive $a, m$ , first even index $4 a$ , denominator unchanged.
Conjecture 5.6(ii)	`clauseII`	Positive $a, m$ , denominator changed.
The full conjecture	`claim`	Conjunction of the two universal assertions.
Both negations	`refutedClauses`	Each component assertion is false.
Refutation	`result`	Exactly `¬ claim`.

The condition $F (D_{4 a}) = 4 a$ is expressed equivalently as the absence of a smaller positive even $k$ with $D_{k} = D_{4 a}$ . The equality at $4 a$ is automatic. For positive $n$ , the range $p < 4 n + 2$ is exhaustive because $p - 1 ∣ 4 n$ implies $p - 1 \leq 4 n$ . At $n = 0$ , the quotient is zero and no prime is coprime to zero, so $T (0)$ is empty. All counterexamples have positive indices. The natural-number restriction suffices to refute the integer conjecture.

Motivation

The settling declaration is D5/S3/Arith/McKeanTodaPrimes/Result.result. The source note is D5/L/Factorization/mckean2025todaprimes. A Bernoulli denominator records predecessor divisibility, whereas Toda primality additionally depends on a prime not dividing the input. These two pieces of information behave differently under multiplication.

Gap

Both clauses are false. For clause (i), take $a = 23$ , $m = 47$ : $D_{92} = D_{4324} = 1410$ , with first positive even index $92$ , but $T (23) = {3, 5, 47}$ and $T (1081) = {3, 5}$ . For clause (ii), take $a = 11$ , $m = 5$ : $D_{44} = 690 \neq = 7590 = D_{220}$ , but $T (11) = {3, 5, 23}$ and $T (55) = {3, 23}$ .

Route

The von Staudt–Clausen identity gives

$D_{2 k} = p \in P (2 k) \prod p (k > 0) .$

The denominator of the prime reciprocal sum divides this product. Each prime in the product divides $D_{2 k}$ , and distinct primes are coprime, which gives the opposite divisibility. Toda-prime membership reduces to

$T (n) = {p \in P (4 n) : p odd, p ∤ n} (n > 0) .$

For every prime $a \equiv 23 (mod 30)$ such that $p = 2 a + 1$ is prime, the divisors of $4 a p$ have twelve forms obtained from divisors of $4, a, p$ . The only prime successors are $2, 3, 5, p$ ; other successors are even or are divisible by $3$ or $5$ . Hence

$D_{4 a} = D_{4 a p} = 30 p, T (a) = {3, 5, p}, T (a p) = {3, 5} .$

If $D_{k} = 30 p$ for a positive even $k$ , the prime divisors $5$ and $p$ force $4 ∣ k$ and $2 a ∣ k$ . Since $a$ is odd, $4 a ∣ k$ . Thus $F (30 p) = 4 a$ . This is a conditional family theorem; no infinitude of prime pairs satisfying its hypotheses is asserted.

The second counterexample uses the denominator prime sets $P (44) = {2, 3, 5, 23}$ and $P (220) = {2, 3, 5, 11, 23}$ . It uses finite prime-set evaluation, without evaluating $B_{220}$ directly.

Falsifier

An error in the Bernoulli-denominator formula, the exhaustiveness of the prime ranges, the coprimality reduction or the first-index argument would invalidate the corresponding counterexample. The separate negations in refutedClauses establish failure of both clauses, rather than only failure of their conjunction.

Evidence

PROVED. denominatorFormula is the exact Bernoulli-denominator product; todaReduction is the Toda-prime membership reduction. family states the conditional family including minimality of the first index and both Toda sets. refutedClauses refutes both universal assertions, and result refutes their conjunction.

The immutable experiment directory contains mckean-c56.py, SHA-256 162bd80015ea2c9615399d49ddc6868b15bd7665326e2ad92fec71b8d092ec4e. COMPUTED. python3 mckean-c56.py, exit 0: direct Bernoulli denominators agree with the product formula at even indices through $400$ ; the two counterexamples have the exact denominators and Toda sets above. The script also computes $F (690) = 44$ and $F (1410) = 92$ by a finite first-index search.

The supplied literature search found no settlement in arXiv, Semantic Scholar, web searches, Formal Conjectures or the repository scope recorded with the source. The MathOverflow question concerns existence of Toda primes, a different assertion. These are bounded supplied findings, not a claim of exhaustive literature coverage or priority.

Triage

What the settlement shows

PROVED (Lean). family exhibits the failure mechanism for clause (i): the Toda prime $p = 2 a + 1$ divides the multiplier, disappears from the Toda set, and creates no new denominator prime. The minimal-first-index hypothesis does not prevent this loss.
PROVED (prose, using the reduction). An old Toda prime of $a$ remains in $T (am)$ exactly when it does not divide $m$ : predecessor divisibility persists, and $p ∤ am$ is equivalent to $p ∤ m$ for $p \in T (a)$ . Thus coprimality of $m$ with every old Toda prime prevents a decrease in cardinality. If the denominator prime set is also unchanged, it ensures equality of the Toda sets. This describes a sufficient repair and is not a proof of the source’s unqualified assertions.
PROVED (Lean). The private witnessII, used in refuteII and refutedClauses, gives $D_{44} = 690$ , $D_{220} = 7590$ , $T (11) = {3, 5, 23}$ and $T (55) = {3, 23}$ . It reverses the roles of $a$ and $m$ in the paper’s own tightness example. The new denominator prime $11$ divides the input $55$ , so it does not compensate for losing the old Toda prime $5$ .
PROVED (prose). Lemma 5.7’s sufficient route fails on the family. For $p = 2 a + 1$ , the divisors of $2 a$ are $1, 2, a, 2 a$ . Their corresponding successors $2 p + 1, 4 p + 1, 2 p a + 1, 4 p a + 1$ are divisible respectively by $5, 3, 3, 5$ . Each is strictly larger than that divisor because $a \geq 23$ . Hence none is prime. This explanation is not an additional Lean theorem.
COMPUTED. The cited script lists all prime $a < 2000$ with $a \equiv 23 (mod 30)$ and $2 a + 1$ prime: $23, 53, 83, 113, 173, 233, 293, 443, 593, 653, 683, 743, 953, 1013, 1103, 1223, 1583, 1733, 1973$ . It checks the denominator, minimal-index and Toda-set identities separately for the first eight listed members. The general conditional identities are the statement of family; this finite computation asserts no infinitude.
OPEN. Conjecture 1.3 and Question 1.2 about existence of Toda primes are untouched. Refuting Conjecture 5.6 removes its unrestricted proposed route to them; it refutes neither existence assertion nor the source’s propositions under their original hypotheses. The restricted cases $a = 3$ and $a = 5$ mentioned in the paper are not settled here.

ASSUMED-UNVERIFIED

Exhaustive literature coverage, citation-index completeness and priority are unverified. Google Scholar was not read in the supplied search. No new network search is asserted. Infinitude of the prime pairs in the conditional family is not established. The explanatory repair and the Lemma 5.7 obstruction are proved in prose, not separate Lean declarations.

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