Almost all primes are partially regular

Abstract: For odd primes $p$, we let $K_p:=\mathbb{Q}(ζ<em>p)$ be the $p$th cyclotomic field and let $ω$ denote its Teichmuller character. For $α&gt;1/2$, we say that an odd prime $p$ is partially regular if the eigenspaces of the $p$-Sylow subgroup of $\operatorname{Cl}(K_p)$ under the Galois action vanish for all characters $ω<sup>{p-2k}$ with [ 2\le 2k \le \frac{\sqrt{p}}{(\log p)^α}. ] Equivalently, $p\nmid \operatorname{num}(B</em>{2k})$ throughout this range. We prove that a density-one subset of primes is partially regular in this sense. By Leopoldt reflection, this yields a partial Vandiver Theorem: for a density-one set of primes $p$, the even eigenspaces $A_p(ω<sup>{2k})$ vanish for all even $2k$ satisfying the inequality above. This result has consequences for Kubota-Leopoldt $p$-adic $L$-functions, congruences between cusp forms and Eisenstein series, and $p$-torsion in algebraic $K$-groups. The theorem proving partial regularity for almost all $p$ is fully formalized in Lean/Mathlib and was produced automatically by AxiomProver from a natural-language statement of the conjecture.

• The paper proves that, for every α > 1/2, primes failing Bernoulli non-divisibility through √p/(log p)^α number at most 10X/(log X)^{2α} up to X, a density-zero exceptional set.
• The paper derives partial Vandiver-type vanishing of even cyclotomic class-group eigenspaces and corresponding unit, zero-free Kubota–Leopoldt p-adic L-functions throughout the same index range.
• The paper also presents an autonomous Lean formalization of its analytic theorem, while clarifying that the result does not establish full regularity, Vandiver’s conjecture, or Siegel’s irregular-prime density conjecture.

Overview and main result

This paper establishes a density-one statement about the distribution of irregular primes, together with a corresponding "partial Vandiver" theorem. For an odd prime $p$, let $K_p = \mathbb{Q}(\zeta_p)$ be the $p$th cyclotomic field with class group $\mathrm{Cl}(K_p)$ and $p$-Sylow subgroup $A_p$. The classical notion of regularity due to Kummer asserts that $p$ is regular when $p \nmid \mathrm{num}(B_{2k})$ for every even index $2k$ in the full range $2 \le 2k \le p-3$; equivalently (via Herbrand–Ribet) that all odd eigenspaces of $K_p = \mathbb{Q}(\zeta_p)$0 vanish.

The authors weaken this to partial regularity: for $K_p = \mathbb{Q}(\zeta_p)$1, a prime $K_p = \mathbb{Q}(\zeta_p)$2 is $K_p = \mathbb{Q}(\zeta_p)$3-regular if $K_p = \mathbb{Q}(\zeta_p)$4 for all $K_p = \mathbb{Q}(\zeta_p)$5, where

$K_p = \mathbb{Q}(\zeta_p)$6

The main theorem states that the set of primes failing this condition has density zero (2602.05090):

$K_p = \mathbb{Q}(\zeta_p)$7

Since $K_p = \mathbb{Q}(\zeta_p)$8 by the Prime Number Theorem, the exceptional set is negligible relative to all primes: almost every prime is partially regular in this sense. A notable refinement, verified by an additional Lean proof run, is that the constant can be taken to be $K_p = \mathbb{Q}(\zeta_p)$9 independently of $p$0 — a concrete, uniform numerical bound rather than merely existential.

The window $p$1 grows without bound but is far shorter than the full Kummer range $p$2; the theorem says nothing about indices beyond it, so this does not resolve Siegel's conjecture on the density $p$3 of irregular primes, nor prove any individual prime fully regular.

Consequences via Herbrand–Ribet and Leopoldt reflection

The arithmetic payoff comes from two standard structural results. First, the Herbrand–Ribet theorem identifies nonvanishing of odd eigenspaces with Bernoulli divisibility: $p$4 if and only if $p$5. Second, Leopoldt's Spiegelungssatz gives a one-way injection $p$6, linking even ("plus", Vandiver) components to odd components governed by Bernoulli numbers.

Combining these, the paper deduces a partial Vandiver theorem: for almost every prime $p$7,

$p$8

with the same $p$9 bound on exceptions. This is a density-one partial result toward the Kummer–Vandiver conjecture, which predicts vanishing of all even eigenspaces for every prime and remains open in general.

Analytic corollaries

Via cyclotomic Iwasawa theory, the vanishing of eigenspaces translates into statements about the Kubota–Leopoldt $\mathrm{Cl}(K_p)$0-adic $\mathrm{Cl}(K_p)$1-functions. Using the Mazur–Wiles main conjecture, the paper shows that for almost every prime $\mathrm{Cl}(K_p)$2,

$\mathrm{Cl}(K_p)$3

so these functions are units on all of $\mathrm{Cl}(K_p)$4 and have no zeros at all on $\mathrm{Cl}(K_p)$5. The authors emphasize that this is qualitatively different from, and in the $\mathrm{Cl}(K_p)$6-adic sense stronger than, complex zero-free regions: it corresponds to the Iwasawa characteristic polynomial being a unit (degree zero). The argument proceeds through the standard control theorem identifying $\mathrm{Cl}(K_p)$7-coinvariants of the unramified Iwasawa module eigenspace with $\mathrm{Cl}(K_p)$8, Nakayama's lemma forcing triviality of the whole eigenspace, and the main conjecture equating vanishing of the module with invertibility of the $\mathrm{Cl}(K_p)$9-function in $p$0.

Because Bernoulli numerators also control congruences between Eisenstein series and cusp forms (Swinnerton-Dyer theory, exemplified by Ramanujan's congruence $p$1 arising from $p$2) and torsion in algebraic $p$3-groups via the Quillen–Lichtenbaum/Borel picture, the main theorem yields a four-part corollary holding simultaneously for almost every prime throughout the range:

1. The power-sum congruence $p$4 holds;
2. $p$5 is a $p$6-adic unit;
3. No level-one cuspidal Hecke eigenform of weight $p$7 is congruent mod $p$8 to the Eisenstein series $p$9;
4. $A_p$0 has no $A_p$1-torsion.

The paper notes that analogous statements for generalized Bernoulli numbers $A_p$2 attached to Dirichlet characters follow mutatis mutandis.

Method of proof

The proof is elementary analytic number theory and occupies only a few pages. The strategy is:

• Reduction to a fixed range: since $A_p$3 is increasing for $A_p$4, every relevant prime $A_p$5 satisfies $A_p$6, so failures of partial regularity among such primes imply failures of $A_p$7-regularity.
• Counting by divisibility: if $A_p$8 is not $A_p$9-regular then $p$0, so the count of bad primes is at most $p$1, the number of distinct prime divisors.
• Growth bounds: combining Euler's formula for $p$2 with the von Staudt–Clausen theorem (which makes denominators square-free and bounded by $p$3) gives $p$4, hence $p$5. Conversely, Stirling's lower bound gives $p$6 for large $p$7, so $p$8.
• Prime divisor estimate: a Chebyshev/PNT argument bounds $p$9.

Chaining these, $p \nmid \mathrm{num}(B_{2k})$0, which is the theorem after absorbing finite error terms. The threshold $p \nmid \mathrm{num}(B_{2k})$1 appears exactly where the counting function drops below $p \nmid \mathrm{num}(B_{2k})$2.

Autonomous formalization in Lean

A distinctive feature of the paper is that Theorem 2.1 was proved autonomously by AxiomProver, an AI system under development at Axiom Math, from only a natural-language problem statement and a Lean version pin. The system produced both a Lean/Mathlib formalization of the statement (~3500 lines of verified code) and a complete proof, checked by Lean 4.26.0, with no human assistance; the human-readable exposition was written afterward by the authors from the formal artifact.

Two aspects of the case study are worth noting. The machine-found proof does not resemble the narrative exposition — Lean code makes explicit all bookkeeping, coercions, and case splits, so translating it required the human authors to reconstruct the conceptual argument. In particular, AxiomProver re-derived the von Staudt–Clausen theorem within its solution rather than citing it, spending over 1000 lines establishing the denominator consequence used in the numerator growth bound. In a separate experiment with additional resources supplied (including Kontorovich–Tao's PNT formalization, which ultimately went unused), the system discovered the uniform constant $p \nmid \mathrm{num}(B_{2k})$3 and an alternative route avoiding von Staudt–Clausen entirely; both findings were incorporated into the paper. Deep external inputs — Herbrand–Ribet, Leopoldt reflection, Mazur–Wiles — were deliberately excluded from the automated task scope and cited from the literature.

This positions the work simultaneously as a number-theoretic result and as evidence about the current capability boundary of autonomous theorem proving: the successfully automated portion is a self-contained analytic estimate using standard techniques, while the structurally deep machinery connecting it to class groups remained a human-supplied citation layer.

Limitations and open questions

Several qualifications bear directly on the strength of the results. The density-one claim covers only indices up to $p \nmid \mathrm{num}(B_{2k})$4; nothing is asserted about Bernoulli numerators or eigenspaces beyond $p \nmid \mathrm{num}(B_{2k})$5, and the gap between this window and the full Kummer range is where irregular behavior may persist. Consequently the paper does not address Siegel's density conjecture, does not prove any single prime fully regular, and does not establish Vandiver's conjecture for any prime outright. The reflection principle used is one-directional ($p \nmid \mathrm{num}(B_{2k})$6), so the partial Vandiver corollary cannot be reversed into new information about minus parts beyond what Herbrand–Ribet already provides. The constants in the growth lemmas are ineffective in the usual asymptotic sense until specific values are fixed, though Remark on $p \nmid \mathrm{num}(B_{2k})$7 supplies one explicit value. Finally, the autonomous-proof claims are scoped precisely: they cover Theorem 2.1 alone and one guided experiment, leaving open whether systems like AxiomProver can autonomously handle proofs invoking deep cited theorems such as Herbrand–Ribet or the Iwasawa main conjecture, and whether the density-one method can be pushed to larger windows of the form $p \nmid \mathrm{num}(B_{2k})$8 with $p \nmid \mathrm{num}(B_{2k})$9.

Conclusion

The paper proves that a density-one set of primes satisfies Bernoulli non-divisibility up to index $2k$0, with explicit exceptional-set bound $2k$1 and a verified uniform constant of 10. Through Herbrand–Ribet and Leopoldt reflection this yields partial Vandiver-type vanishing of even class group eigenspaces for almost all primes, and through the Iwasawa main conjecture yields zero-free Kubota–Leopoldt $2k$2-adic $2k$3-functions, Eisenstein-cusp non-congruences, and $2k$4-torsion-freeness of $2k$5 across the same range. The mathematics itself uses only classical tools; its secondary significance lies in demonstrating that an AI prover could autonomously produce a fully Lean-verified proof of a new research-level estimate from its natural-language statement alone.