arXiv:2607.00825v2

The Minimal Absolute Value of Sums of Fifth Roots of Unity

Akihiro Munemasa, Guillermo Núñez Ponasso

math.NTmath.CO11J2511Y8505B20

Abstract

We determine the minimal absolute value of a non-vanishing sum of nn fifth roots of unity chosen with repetition, and characterize the corresponding sums. As a function of nn, the minimal absolute value is monotone non-increasing over congruence classes of nn modulo 55 and its only jumps occur when n=5Fmn=5F_m, n=Lmn=L_m, or n=2Lmn=2L_m, where FmF_m and LmL_m denote the mm-th Fibonacci and Lucas numbers respectively. To prove our results we reduce the problem to a series of inequalities involving rational approximations of the golden ratio φ=(1+5)/2\varphi=(1+\sqrt{5})/2, the solutions of which can be characterized using the theory of continued fractions.

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Audited against arXiv v2

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.

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Detailed mathematical audit

Generated August 15, 2026
01Statements2 reported findingsCorrect

The formulas for the minimum nonzero absolute value of a weight-nn sum of fifth roots of unity are correct, both for 5n5\nmid n and for 5n5\mid n. The stated characterization of minimizers is also supported after the harmless duplicated-set typo recorded under Proofs is corrected.

Theorem 1Correct

Minimum for weights not divisible by five

Pages 3 and 9–15 · Theorem 1 and Section 3 · arXiv:2607.00825v2

The explicit Fibonacci–Lucas sums attain φκ(n)\varphi^{-\kappa(n)}. For the reverse inequality, the real part reduces a putative smaller sum to a restricted Ostrowski representation for b=[n/(φ+2)]b=[n/(\varphi+2)]; Lemmas 10 and 13 leave only the displayed extremal representations. The imaginary-part argument then forces the coefficient patterns in Propositions 11 and 14. The remaining finite range agrees with an independent exhaustive enumeration of all weight-nn coefficient tuples.

Theorem 2Correct

Minimum for weights divisible by five

Pages 3 and 15–19 · Theorem 2 and Section 4 · arXiv:2607.00825v2

For n=5kn=5k, the constructed sum has squared modulus 5/φ2ν(k)1\sqrt5/\varphi^{2\nu(k)-1}. Lemmas 17 and 18 force a competing sufficiently small sum to have a2+a3=Lm1a_2+a_3=L_{m-1} and n=5Fmn=5F_m; Proposition 19 then determines the remaining coefficient differences and proves the matching lower bound. The finite range through n=60n=60 was independently reproduced.

02Proofs2 reported findingsCorrect

The continued-fraction and Ostrowski arguments are correct and complete, including the abbreviated analogous cases. One duplicated set symbol in the final characterization is a typo and does not affect either minimum formula or the overall proof status.

Sections 3–4Correct and complete

Ostrowski reductions and finite base ranges

Pages 10–19 · Lemmas 9–18 and Propositions 11, 14, 19 · arXiv:2607.00825v2

The lower bounds for b(φ+2)\|b(\varphi+2)\| and xφ\|x\varphi\| correctly constrain the least Fibonacci index in each Zeckendorf expansion. Enumerating the resulting short lists gives the asserted congruence-unique candidates; the abbreviated Lemmas 12, 13, 17, and 18 were independently checked over their full parameter ranges. Exhaustive enumeration also confirms every finite case used before the uniform bounds apply.

TypoTypo

The final union repeats the same minimizer set

Page 19 · paragraph following the proof of Theorem 2 · arXiv:2607.00825v2

The text says that the minimizers belong to MM\mathcal M''\cup\mathcal M''. Proposition 19 characterizes a sum only up to complex conjugation, and M\mathcal M'' is not itself closed under conjugation. The second set should therefore be M\overline{\mathcal M''}. This uniquely determined notation correction has no effect on the value of σ5(n)\sigma_5(n) or on the correctness status.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2607.00825v2
Authors listed
Akihiro Munemasa, Guillermo Núñez Ponasso
Audit date
August 15, 2026
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