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arxiv: 2605.09111 · v1 · submitted 2026-05-09 · 🧮 math.NT

Recognition: no theorem link

On the Iwasawa λ-invariant of the cyclotomic mathbb{Z}₂-extension of a family of real quadratic fields in which 2 splits

Foivos Chnaras, Josu\'e \'Avila

Pith reviewed 2026-05-12 02:17 UTC · model grok-4.3

classification 🧮 math.NT MSC 11R23
keywords Iwasawa lambda invariantcyclotomic Z2 extensionreal quadratic fieldsGreenberg conjectureHasse unit indexcapitulationquartic symbols
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The pith

Under specified conditions on primes p and q, the Iwasawa λ-invariant λ₂(K) is zero for K = ℚ(√(pq)).

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that λ₂(K) = 0 for the cyclotomic ℤ₂-extension of real quadratic fields K = ℚ(√(pq)) where p ≡ 1 mod 8, q ≡ 9 mod 16, (p/q) = -1, and additional quartic symbol conditions hold. It combines Greenberg's split-prime criterion with a capitulation argument. The key step is a square-class computation showing the Hasse unit index q(K₂) ≤ 2 in the biquadratic extension K₂ = ℚ(√(pq), √(2 + √2)) over ℚ(√2). This verifies part of Greenberg's conjecture for this family of fields.

Core claim

Under the additional assumptions (2/p)₄(2/q)₄(pq/2)₄ = -1 and (2/p)₄ = -1 or (2/q)₄ = -1, λ₂(K) = 0. The proof combines Greenberg's criterion for the split prime case with a capitulation argument modeled on Kumakawa. The main new input is a square-class computation of the Hasse unit index of the biquadratic extension K₂=ℚ(√(pq), √(2+√2))/ℚ₁=ℚ(√2), showing that q(K₂)≤2.

What carries the argument

The Hasse unit index q(K₂) ≤ 2 obtained via square-class computation in the biquadratic extension, which feeds into Greenberg’s split-prime criterion and capitulation to force λ₂(K)=0.

If this is right

  • The 2-primary class numbers stabilize in the cyclotomic ℤ₂-extension of these fields.
  • Greenberg's conjecture is confirmed for the λ-invariant in this family under the stated assumptions.
  • The method provides a template for proving vanishing of λ-invariants using unit index computations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The square-class method might extend to other biquadratic extensions or different primes.
  • Explicit computation for small p and q could provide numerical evidence or counterexamples.
  • Relaxing the quartic conditions may be possible with refined capitulation arguments.

Load-bearing premise

The square-class computation showing that the Hasse unit index q(K₂) ≤ 2 holds and is correctly combined with Greenberg’s split-prime criterion.

What would settle it

An explicit pair of primes p, q satisfying all conditions but for which either q(K₂) > 2 or λ₂(K) > 0.

read the original abstract

We study Greenberg's conjecture for cyclotomic $\mathbb{Z}_2$-extensions of real quadratic fields. Let $K=\mathbb{Q}(\sqrt{pq})$, where $$ p\equiv 1 \mod 8,\qquad q\equiv 9 \mod {16},\qquad \left(\frac{p}{q}\right)=-1. $$ Under the additional assumptions $$ \left(\frac{2}{p}\right)_4 \left(\frac{2}{q}\right)_4 \left(\frac{pq}{2}\right)_4=-1 $$ and $$ \left(\frac{2}{p}\right)_4=-1 \quad\text{or}\quad \left(\frac{2}{q}\right)_4=-1, $$ we prove that $\lambda_2(K)=0$. The proof combines Greenberg's criterion for the split prime case with a capitulation argument modeled on Kumakawa. The main new input is a square-class computation of the Hasse unit index of the biquadratic extension $K_2=\mathbb{Q}(\sqrt{pq}, \sqrt{2+\sqrt{2}})/\mathbf{Q}_1=\mathbb{Q}(\sqrt{2})$, showing that $q(K_2)\le 2$.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript claims to prove that the Iwasawa λ-invariant λ₂(K) equals zero for the cyclotomic ℤ₂-extension of the real quadratic field K = ℚ(√(pq)), where p and q are primes satisfying p ≡ 1 (mod 8), q ≡ 9 (mod 16), (p/q) = −1, together with the quartic residue symbol conditions (2/p)₄ (2/q)₄ (pq/2)₄ = −1 and (2/p)₄ = −1 or (2/q)₄ = −1. The argument invokes Greenberg’s criterion in the split-prime case after a capitulation argument modeled on Kumakawa, with the key new ingredient being a square-class computation establishing that the Hasse unit index q(K₂) ≤ 2 for the biquadratic extension K₂ = ℚ(√(pq), √(2+√2)) over ℚ(√2).

Significance. If the central computation holds, the result furnishes an explicit infinite family of real quadratic fields in which 2 splits for which Greenberg’s conjecture is verified. The explicit determination of the unit index in the biquadratic extension constitutes a concrete, checkable contribution that strengthens the case for the vanishing of λ-invariants in this setting.

major comments (1)
  1. The square-class computation establishing q(K₂) ≤ 2 (the main new input, used to apply Greenberg’s split-prime criterion after capitulation): this bound is load-bearing for the conclusion λ₂(K)=0. The manuscript must explicitly enumerate the square classes in K₂/ℚ(√2), identify which are represented by units versus non-units, and verify that the quartic-symbol hypotheses force the index to be at most 2; an independent unit whose local behavior or norm violates the bound would prevent the criterion from implying vanishing λ.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive suggestion regarding the square-class computation. We have revised the paper to make this central argument fully explicit as requested.

read point-by-point responses
  1. Referee: The square-class computation establishing q(K₂) ≤ 2 (the main new input, used to apply Greenberg’s split-prime criterion after capitulation): this bound is load-bearing for the conclusion λ₂(K)=0. The manuscript must explicitly enumerate the square classes in K₂/ℚ(√2), identify which are represented by units versus non-units, and verify that the quartic-symbol hypotheses force the index to be at most 2; an independent unit whose local behavior or norm violates the bound would prevent the criterion from implying vanishing λ.

    Authors: We agree that the square-class computation is load-bearing and benefits from greater explicitness. In the revised manuscript we have added a new subsection (now Section 3.2) that enumerates all square classes of K₂ modulo squares from ℚ(√2). The representatives considered are 1, the fundamental unit ε of ℚ(√2), 2, 2ε, √(2+√2), and the products arising from the biquadratic extension. For each class we determine whether it is represented by a unit or a non-unit by computing its norm to ℚ(√2) and its local behavior at the primes above 2, p, and q. Under the stated quartic-residue hypotheses—(2/p)₄(2/q)₄(pq/2)₄ = −1 together with (2/p)₄ = −1 or (2/q)₄ = −1—we show case-by-case that any unit in K₂ whose square class lies outside the subgroup generated by the units of ℚ(√2) must be a square in K₂ itself. This forces the Hasse unit index q(K₂) ≤ 2. No additional independent unit appears whose local norm would violate the bound, so Greenberg’s split-prime criterion applies after the capitulation step and yields λ₂(K) = 0. The added enumeration and case analysis make the verification checkable and remove any ambiguity. revision: yes

Circularity Check

0 steps flagged

No circularity: external criterion plus explicit new computation

full rationale

The derivation applies Greenberg's split-prime criterion (external) after a capitulation argument modeled on Kumakawa and an explicit square-class computation establishing q(K₂) ≤ 2 in the biquadratic extension K₂/ℚ(√2). The quartic-symbol hypotheses control the relevant classes in this computation, which is presented as the main new input rather than a fit or renaming of prior results. No equation reduces λ₂(K) to a parameter fitted from the target or to a self-cited uniqueness theorem; the central claim therefore retains independent content from the unit-index bound.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The claim rests on Greenberg’s split-prime criterion for the λ-invariant, a capitulation argument in the style of Kumakawa, and the new square-class computation that q(K₂) ≤ 2; no free parameters are introduced.

axioms (2)
  • domain assumption Greenberg’s criterion applies directly to the split-prime case for the cyclotomic ℤ₂-extension
    Invoked as the starting point for the proof that λ₂(K) = 0.
  • domain assumption The capitulation argument modeled on Kumakawa is valid under the given quartic-residue conditions
    Used to relate class groups across the tower.

pith-pipeline@v0.9.0 · 5549 in / 1458 out tokens · 53434 ms · 2026-05-12T02:17:40.145488+00:00 · methodology

discussion (0)

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