Hexagon is a repository for the preservation, dissemination, study, and evaluation of research-level works in the mathematical sciences and theoretical computer science, including both original and expository texts. Hexagon does not perform peer review. Every deposit is screened automatically and, during the beta, also reviewed by a human moderator before it appears; acceptance is not a judgment of correctness, quality, or novelty.

Browse by subject

Newest works

To replace this with a different default view, perform an advanced search for the parameters you want, then select “Set this search as my default view.”

math.NT — Number Theory

Stark units for real quadratic fields and reciprocity laws

Contributed by Danylo Radchenko, Campbell Wheeler

We prove that every value of the arithmetic finite quantum dilogarithms constructed by Radchenko and Wheeler (arXiv:2609.21892) is an algebraic unit, and determine the Artin action for arbitrary quadratic pseudolattices. In particular, the associated Stark units belong to the predicted ray class fields and have the predicted Galois action. We use finite Fourier transforms and distribution relations to obtain Frobenius congruences. We also give conductor bounds for individual values and obtain the unitary conjugates needed for Zauner's conjecture.

A mix of human-written and AI-generated textHuman understanding: all partsArtin reciprocityFrobenius congruencesReal quadratic fieldsStark unitsfinite quantum dilogarithmsray class fields

math.QA — Quantum Algebra

The Haagerup VOA at Central Charge Eight

Contributed by Andrew Riesen

We give an explicit construction of a vertex operator algebra UU of central charge eight inside VE6⊕A2V_{E_6\oplus A_2}, and prove the ribbon equivalence $\Rep(U)\simeq Z(\HH_3)$, where $\HH_3$ is a Haagerup fusion category. The algebra is generated as a vertex algebra by six affine generators and three specified weight-two vectors. The argument combines exact lattice-vector calculations with an all-weight strong-generation induction, a finite presentation and semisimple descent for a full corner of Zhu's algebra, and the actual multiplication of two holomorphic E8E_8 extensions. The categorical identification uses splitting maps and a comparison with the Evans--Gannon reconstruction, not modular data alone.

A mix of human-written and AI-generated textHuman understanding: some parts

math.OA — Operator Algebras

A variational formula for affine semicircular determinants

Contributed by Sheng Yin

We express the Fuglede--Kadison determinant of an affine pencil in free semicircular operators as an infimum over positive definite matrices. The infimum is attained if and only if the pencil is boundedly invertible, in which case its minimizer is unique. At zero constant term, a scalar rescaling recovers the determinant-capacity identity of Mai and Speicher. For selfadjoint coefficients we compare the formula with a strictly constrained Hilbert--Schmidt variational principle and establish ordered min--max formulas under their respective positivity hypotheses. Strictly positive pencils admit a minimum over a linear matrix inequality. Applications to Brown measure include a uniform determinant lower bound, restrictions on atoms, radial distribution formulas, and a Hessian formula for the regularized density. The text of this manuscript was generated entirely by OpenAI's large language model Astra.

Primarily AI-generated textHuman understanding: no partsFree probabilityFuglede–Kadison determinantOperator-valued semicircular elements

math.CA — Classical Analysis and ODEs

Arithmetic progressions with prescribed lp gaps

Contributed by Vjekoslav Kovač

For every n≥3n\ge3 and p∈[1,∞)∖{1,2,…,n−1}p\in[1,\infty)\setminus\{1,2,\ldots,n-1\}, we prove that every measurable subset of Rd\mathbf{R}^d of positive upper Banach density, with d≥2n−2+1d\ge2^{n-2}+1, contains an nn-term arithmetic progression with every sufficiently large prescribed ℓp\ell^p gap.

Primarily AI-generated textHuman understanding: some partsEuclidean Ramsey theorydensity theoremmultilinear estimateoscillatory integralsingular integral

math.CA — Classical Analysis and ODEs

Superexponentially decaying sequences in the Erdos similarity problem

Contributed by Vjekoslav Kovač

We prove a non-universality criterion for the Erd\H{o}s similarity conjecture that applies beyond exponential (i.e., geometric) decay. In particular, it covers decreasing positive null sequences satisfying log⁡(1/an)=o(nlog⁡log⁡n)\log(1/a_n)=o(n\log\log n) and log⁡(an/an+1)≥e−nα\log(a_n/a_{n+1})\ge e^{-n^\alpha} eventually for some 0<α<10<\alpha<1.

Primarily AI-generated textHuman understanding: some partsLebesgue measureconfigurationmeasure theory

math.RT — Representation Theory

Representation embeddings and the third Brauer--Thrall conjecture

Contributed by Haruhisa Enomoto

We show that every representation-infinite finite-dimensional algebra over an algebraically closed field has an indecomposable module of each infinite dimension. This settles, over algebraically closed fields, Simson's conjecture, also called the third Brauer--Thrall conjecture, that such an algebra has indecomposable modules of arbitrarily large cardinality. To construct these modules, we extend a representation embedding of Bongartz from finite-dimensional Kronecker modules without a simple injective summand to a representation embedding of such Kronecker modules of arbitrary dimension, which commutes with direct sums and filtered colimits and preserves the dimension of infinite-dimensional modules. Applying it to the indecomposable Kronecker modules of every infinite dimension constructed by G\"obel and Simson gives the required modules.

Primarily AI-generated textHuman understanding: some partsBrauer--Thrall conjectureGalois coveringKronecker modulelarge indecomposable modulerepresentation embedding

math.MG — Metric Geometry

Multi-coverings by lattice translates of convex bodies

Contributed by Scott Kominers

We show that for every convex body K⊂RnK\subset\mathbb R^n (n≥2n\ge2), every 0<ε<10<\varepsilon<1, and every prescribed density ρ≥Aεnlog⁡n\rho\ge A_\varepsilon n\log n, there is an arrangement of translates of KK along a single lattice with mean multiplicity exactly ρ\rho and multiplicity at least (1−ε)ρ(1-\varepsilon)\rho everywhere. Consequently, lattice kk-fold coverings exist with density at most max⁡{(1+ε)k,Cεnlog⁡n}\max\{(1+\varepsilon)k,C_\varepsilon n\log n\}, where CεC_\varepsilon depends only on ε\varepsilon. In sufficiently large dimension nn, a version with prescribed common covolume holds simultaneously for up to exp⁡(nc0)\exp(n^{c_0}) bodies whose uniform distributions have a common covariance matrix, with c0>0c_0>0 absolute. The proof builds on OpenAI's horizontal–vertical construction for single-lattice coverings and uses Gaussian layer averaging and Laplace bounds for covering counts.

A mix of human-written and AI-generated textHuman understanding: all partsGeometry of numbersconvex bodiesconvex geometrycovering densitycovering multiplicitylattice coveringmulti-covering

math.NT — Number Theory

THE CROSS-ENERGY OF DIRICHLET PARTIAL SUMS AND THE ZEROS OF THE RIEMANN ZETA FUNCTION

Contributed by Pedro Caceres

Let X_n(s) = Σ_{k≤n} k⁻ˢ and let C₂(n,s) = |X_n(s)|² − |X_{n−1}(s)|² − n⁻²σ be the cross-energy of consecutive partial sums. At every point of the critical strip the limit of C₂(n,s) is 1/(¼ + t²), 0 or +∞ according as σ = ½, σ > ½ or σ < ½; this trichotomy is the trace of the pole at s = 1. On the critical line the associated helix has a natural orientation whose signed radius is Hardy’s Z(t), and the zeros of the truncated functions spiral into the zeros of ζ at the rate n⁻ᵝ. We prove that the mean energy of the normalized error E(u) in the prime number theorem equals Σ_ρ lim_{n→∞} C₂(n,ρ) in [0,∞]; it is at least 2 + γ₀ − log(4π), with equality if and only if the Riemann hypothesis (RH) holds and all zeros are simple, and an unconditional local version expresses the energy in a Gaussian window as a double sum over zeros. We then study the pointwise inequality S(σ,t) = ∂σ log|ξ(σ + it)| > 0, which is equivalent to RH: an off-line pair lowers S exactly inside a disc, the failure set at abscissa σ up to height T has measure at most N(σ,T + 1), and S is Weil’s functional at a Poisson kernel, whose truncation is an exact finite expression in the primes up to eᴸ. Every criterion is tested against the Davenport–Heilbronn function, which has zeros off the critical line, and against its Euler-product twin L(s,χ) mod 5: the former violates each criterion in the predicted way, the latter behaves like ζ. For ζ, the primes up to 10⁸ give mean energy 0.0453 against 0.0462. On the circle of the Euler factor of a prime p the zeros lean toward the angle π, and toward π + argχ(p) for L(s,χ). We do not prove the Riemann hypothesis.

A mix of human-written and AI-generated textHuman understanding: all partsC2 FunctionDirichlet SumsRiemann Zeta Function

math.PR — Probability

Classical infinite divisibility, self-decomposability and bell-shape of free stable laws

Contributed by Min WANG

We determine the classical infinite divisibility, self-decomposability and extended Thorin regions of the strictly free stable family. Above index one, infinite divisibility and self-decomposability hold on two extremal skewness intervals, with distinct maximal indices αID=1.5240739610…\alpha_{\mathrm{ID}}=1.5240739610\ldots and αSD=1.4283571142…\alpha_{\mathrm{SD}}=1.4283571142\ldots. Below one, every law is self-decomposable, whereas the Thorin region is a central interval; the optimal uniform bound is 4/54/5. We characterize the boundaries by global contact conditions and derive their different asymptotic scales at index one. A common phase representation also gives an exact bell-shape test, strictly bell-shaped examples outside the Thorin class, and an unbounded number of skewness components near one. The exceptional free one-stable family admits an explicit integer criterion. The proofs combine analytic estimates with finite interval certificates. The bell-shape results do not constitute a full parameter classification.

Primarily AI-generated textHuman understanding: no parts

Browse all works →