arXiv:2606.22229v1

The ratio spectrum of Lagrange constants under linear fractional transformations

Harold Erazo, Carlos Gustavo Moreira

math.NT11J0611J70

Abstract

In this note we solve a problem posed by Lagarias and Shallit concerning Lagrange constants under linear fractional transformations Mx=ax+bcx+dMx=\frac{ax+b}{cx+d}. For an integer matrix MM with nonzero determinant and relatively prime entries, define the ratio spectrum V(M)={k(Mx)k(x):xBad},\mathcal{V}(M)=\left\{\frac{k(Mx)}{k(x)}:x\in\mathrm{Bad}\right\}, where k(x)k(x) denotes the Lagrange constant of the irrational number xx and Bad\mathrm{Bad} is the set of badly approximable numbers. Lagarias and Shallit proved that V(M)[1detM,detM],\mathcal{V}(M)\subseteq\left[\frac{1}{|\det M|},|\det M|\right], and asked for the determination of V(M)\mathcal{V}(M). We prove that V(M)=[1detM,detM].\mathcal{V}(M)=\left[\frac{1}{|\det M|},|\det M|\right].

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Audited against arXiv v1

Not a correctness certificate. A “Correct” result may include yellow typos or minor formal corrections that do not affect substantive soundness. It means this audit found no unresolved substantive error under the stated criteria; it does not replace expert scrutiny or formal verification.

Current report

Detailed mathematical audit

Generated August 15, 2026
01Statements2 reported findingsCorrect

The interval formula for the ratio spectrum and the full-Hausdorff-dimension refinement are supported by the continued-fraction construction.

Theorem 1.3Correct

The ratio spectrum is the full allowed interval

Pages 3–8 · Theorem 1.3 and Proposition 2.5 · arXiv:2606.22229v1

Smith normal form reduces a primitive integer matrix to multiplication by m=detMm=|\det M|. Proposition 2.5 then realizes every ratio in [1/m,m][1/m,m]: after choosing C/D=γ/mC/D=\gamma/m, Hall's identity C(4)+C(4)C(4)+C(4) prescribes the two limiting continued-fraction values, while the uniformly bounded bridge words enforce the required denominator congruences modulo mm. The calculation gives k(y)=Dk(y)=D and k(my)=mC=γDk(my)=mC=\gamma D, exactly as claimed.

Level setsCorrect

Each ratio level set has Hausdorff dimension one

Pages 3 and 7–8 · paragraph following Proposition 2.5 · arXiv:2606.22229v1

The prescribed blocks and congruence bridges have zero asymptotic density when the free block lengths grow rapidly, while all free digits may range over 1,,A21,\ldots,A-2. The standard bounded-digit Cantor-set estimate therefore gives dimensions approaching dimHC(A2)\dim_H C(A-2); letting AA grow makes their supremum equal to 11. The forced blocks preserve the calculation of both Lagrange constants.

02Proofs1 reported findingCorrect

The proof is correct and complete. The modular bridge construction, reduction of rational approximants, and limiting continued-fraction calculations retain all required coprimality and congruence conditions.

Proposition 2.5Correct and complete

Congruence-controlled continued-fraction construction

Pages 5–7 · Lemma 2.3, Corollary 2.4, and Proposition 2.5 · arXiv:2606.22229v1

Modulo mm, the digit matrices for mm and 11 give the standard generators of GL2(Z/mZ)\operatorname{GL}_2(\mathbb Z/m\mathbb Z), so uniformly bounded bridge words exist. If a sufficiently good reduced approximant p/qp/q to mymy is reduced after viewing p/(mq)p/(mq) as an approximant to yy, the designed congruence qn0(modm)q_n\equiv0\pmod m forces gcd(p,m)=1\gcd(p,m)=1 in the AA-branch, whereas qn1(modm)q_n\equiv1\pmod m forces gcd(p,m)=m\gcd(p,m)=m in the BB-branch. Thus the two branches contributing to k(my)k(my) are exactly the two evaluated in the proof, and the hypothesis C/D1/m2C/D\geq1/m^2 selects mCmC as the maximum.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.22229v1
Authors listed
Harold Erazo, Carlos Gustavo Moreira
Audit date
August 15, 2026
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