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Claus Hertling

Publications and source records attributed to Claus Hertling.

At least 19 recordsLinked to original sources
arXiv2026

Conjugacy classes of regular integer matrices

This paper is devoted to the theory of $GL_n({\mathbb Z})$-conjugacy classes of regular integer $n\times n$ matrices. Such a matrix is $GL_n({\mathbb Q})$-conjugate to the companion matrix of its characteristic polynomial. But the set of $GL_n({\mathbb Z})$-conjugacy classes of regular integer matrices with a fixed characteristic polynomial $f$ is usually nontrivial (finite if $f$ has simple roots, infinite if $f$ has multiple roots). It is in 1:1-correspondence to a subsemigroup of a certain quotient semigroup of the commutative semigroup of full lattices in the algebra ${\mathbb Q}[t]/(f)$. In its first part, the paper gives a survey on old and new results on full lattices and orders in a finite dimensional commutative ${\mathbb Q}$-algebra with unit element and on induced semigroups. In its longer second part, the paper applies this theory to many examples, essentially all cases with $n=2$, many cases with $n=3$ and two cases with arbitrary $n$, the case with $n$ different integer eigenvalues and the case of a single $n\times n$ Jordan block.

math.RA↗
arXiv2026

Semigroups from full lattices in commutative ${\mathbb Q}$-algebras

The full lattices in a finite dimensional commutative ${\mathbb Q}$-algebra form a commutative semigroup. In the case of an algebraic number field the top part of a certain quotient semigroup is the class group. For a separable algebra some basic results, especially the Jordan-Zassenhaus theorem, are known for this quotient semigroup. This paper considers also algebras which are not separable. It studies the commutative semigroup of full lattices in such an algebra and also the quotient semigroup. This leads in this commutative, but not separable situation to a certain extension of the Jordan-Zassenhaus theorem. One application concerns $GL_n({\mathbb Z})$-conjugacy classes of regular integer $n\times n$ matrices.

math.RA↗
arXiv2025

Characterization of distinguished matrices of isolated hypersurface singularities through their spectral numbers

Isolated hypersurface singularities come equipped with distinguished bases of their Milnor lattices and with upper triangular integral matrices, which are called here distinguished matrices. These matrices form an orbit of a braid group and a sign change group. This paper proposes to characterize the distinguished matrices of singularities within all upper triangular integral matrices in terms of the variance of certain spectral numbers. It succeeds in the positive definite and the positive semidefinite cases. The ADE root lattices are crucial. In the semidefinite cases, results on non-reduced presentations of Weyl group elements are used.

math.AG↗
arXiv2025

Odd vanishing cycles in cyclotomic fields

A cusp of a Hecke group $G_q$ has two natural lifts to the ring of integers of a cyclotomic field. These lifts are called here odd vanishing cycles. All lifts of all cusps together form a discrete subset of ${\mathbb C}$ of some exquisite beauty. They form one or two or four orbits of a certain subgroup of the matrix Hecke group. The subgroup can be considered as a monodromy group and is an analog of a rank 2 Coxeter group, so of a dihedral group. The paper has a research part and a larger survey part.

math.NT↗
arXiv2024

Mixed extensions of generic finite games embedded into products of real projective spaces

Finite games in normal form and their mixed extensions are a corner stone of noncooperative game theory. Often generic finite games and their mixed extensions are considered. But the properties which one expects in generic games and the existence of games with these properties are often treated only in passing. The paper considers strong properties and proves that generic games have these properties. The space of mixed strategy combinations is embedded in a natural way into a product of real projective spaces. All relevant hypersurfaces extend to this bigger space. The paper shows that for all games in the complement of a semialgebraic subset of codimension at least one all relevant hypersurfaces in the bigger space are smooth and maximally transversal. The proof uses the theorem of Sard and follows an argument of Khovanskii.

math.OC↗
arXiv2024

Maximal number of mixed Nash equilibria in generic games where each player has two pure strategies

The number of Nash equilibria of the mixed extension of a generic finite game in normal form is finite and odd. This raises the question how large the number can be, depending on the number of players and the numbers of their pure strategies. Here we present a lower bound for the maximal possible number in the case of m-player games where each player has two pure strategies. It is surprisingly close to a known upper bound.

math.CO↗
arXiv2024

Unimodular bilinear lattices, automorphism groups, vanishing cycles, monodromy groups, distinguished bases, braid group actions and moduli spaces from upper triangular matrices

This monograph starts with an upper triangular matrix with integer entries and 1's on the diagonal. It develops from this a spectrum of structures, which appear in different contexts, in algebraic geometry, representation theory and the theory of irregular meromorphic connections. It provides general tools to study these structures, and it studies sytematically the cases of rank 2 and 3. The rank 3 cases lead already to a rich variety of phenomena and give an idea of the general landscape. The first structure associated to the matrix is a Z-lattice with unimodular bilinear form (called Seifert form) and a triangular basis. It leads immediately to an even and an odd intersection form, reflections and transvections, an even and an odd monodromy group, even and odd vanishing cycles. Braid group actions lead to braid group orbits of distinguished bases and of upper triangular matrices. Finally, complex manifolds, which consist of correctly glued Stokes regions, are associated to these braid group orbits. The last chapter is a report on the case of isolated hypersurface singularities.

math.AG↗
arXiv2021

Rank 2 Bundles with Meromorphic Connections with Poles of Poincaré Rank 1

Holomorphic vector bundles on $\mathbb C\times M$, $M$ a complex manifold, with meromorphic connections with poles of Poincaré rank 1 along $\{0\}\times M$ arise naturally in algebraic geometry. They are called $(TE)$-structures here. This paper takes an abstract point of view. It gives a complete classification of all $(TE)$-structures of rank 2 over germs $\big(M,t^0\big)$ of manifolds. In the case of $M$ a point, they separate into four types. Those of three types have universal unfoldings, those of the fourth type (the logarithmic type) not. The classification of unfoldings of $(TE)$-structures of the fourth type is rich and interesting. The paper finds and lists also all $(TE)$-structures which are basic in the following sense: Together they induce all rank $2$ $(TE)$-structures, and each of them is not induced by any other $(TE)$-structure in the list. Their base spaces $M$ turn out to be 2-dimensional $F$-manifolds with Euler fields. The paper gives also for each such $F$-manifold a classification of all rank 2 $(TE)$-structures over it. Also this classification is surprisingly rich. The backbone of the paper are normal forms. Though also the monodromy and the geometry of the induced Higgs fields and of the bases spaces are important and are considered.

math.AG↗
arXiv2021

The combinatorics of weight systems and characteristic polynomials of isolated quasihomogeneous singularities

A paper of the first author and Zilke proposed seven combinatorial problems around formulas for the characteristic polynomial and the exponents of an isolated quasihomogeneous singularity. The most important of them was a conjecture on the characteristic polynomial. Here the conjecture is proved, and some of the other problems are solved, too. In the cases where also an old conjecture of Orlik on the integral monodromy holds, this has implications on the automorphism group of the Milnor lattice. The combinatorics used in the proof of the conjecture consists of tuples of orders on sets $\{0,1,...,n\}$ with special properties and may be of independent interest.

math.CO↗
arXiv2020

3-dimensional F-manifolds

F-manifolds are complex manifolds with a multiplication with unit on the holomorphic tangent bundle with a certain integrability condition. Here the local classification of 3-dimensional F-manifolds with or without Euler fields is pursued.

math.DG↗
arXiv2020

The integral monodromy of isolated quasihomogeneous singularities

The integral monodromy on the Milnor lattice of an isolated quasihomogeneous singularity is subject of an almost untouched conjecture of Orlik from 1972. We prove this conjecture for all iterated Thom-Sebastiani sums of chain type singularities and cycle type singularities. The main part of the paper is purely algebraic. It provides tools for dealing with sums and tensor products of ${\mathbb Z}$-lattices with automorphisms of finite order and with cyclic generators. The calculations are involved. They use fine properties of unit roots, cyclotomic polynomials, their resultants and discriminants.

math.AG↗
arXiv2020

The integral monodromy of the cycle type singularities

The middle homology of the Milnor fiber of a quasihomogeneous polynomial with an isolated singularity is a ${\mathbb Z}$-lattice and comes equipped with an automorphism of finite order, the integral monodromy. Orlik (1972) made a precise conjecture, which would determine this monodromy in terms of the weights of the polynomial. Here we prove this conjecture for the cycle type singularities. A paper of Cooper (1982) with the same aim contained two mistakes. Still it is very useful. We build on it and correct the mistakes. We give additional algebraic and combinatorial results.

math.AT↗
arXiv2020

(TE)-structures over the irreducible 2-dimensional globally nilpotent F-manifold germ

We find formal and holomorphic normal forms for a class of meromorphic connections (the so-called $(TE)$-structures) over the irreducible $2$-dimensional globally nilpotent $F$-manifold germ $\mathcal N_{2}$. We find normal forms for Euler fields on $\mathcal N_{2}$ and we characterize the Euler fields on $\mathcal N_{2}$ which are induced by a $(TE)$-structure.

math.DG↗
arXiv2019

Meromorphic connections over F-manifolds

This paper review one construction of Frobenius manifolds (and slightly weaker structures). It splits it into several steps and discusses the freedom and the constraints in these steps. The steps pass through holomorphic bundles with meromorphic connections. A conjecture on existence and uniqueness of certain such bundles, a proof of the conjecture in the 2-dimensional cases, and some other new results form a research part of this paper.

math.DG↗
arXiv2019

(T)-structures over 2-dimensional F-manifolds: formal classification

A $(TE)$-structure $\nabla$ over a complex manifold $M$ is a meromorphic connection defined on a holomorphic vector bundle over $\mathbb{C}\times M$, with poles of Poincaré rank one along $\{ 0 \} \times M.$ Under a mild additional condition (the so called unfolding condition), $\nabla$ induces a multiplication on $TM$ and a vector field on $M$ (the Euler field), which make $M$ into an $F$-manifold with Euler field. By taking the pull-backs of $\nabla$ under the inclusions $\{ z\} \times M \rightarrow \mathbb{C}\times M$ we obtain a family of flat connections on vector bundles over $M$, parameterized by $z\in \mathbb{C}^{*}$. The properties of such a family of connections give rise to the notion of $(T)$-structure. Therefore, any $(TE)$-structure underlies a $(T)$-structure but the converse is not true. The unfolding condition can be defined also for $(T)$-structures. A $(T)$-structure with the unfolding condition induces on its parameter space the structure of an $F$-manifold (without Euler field). After a brief review on the theory of $(T)$ and $(TE)$-structures, we determine normal forms for the equivalence classes, under formal isomorphisms, of $(T)$-structures which induce a given irreducible germ of $2$-dimensional $F$-manifolds.

math.DG↗
arXiv2018

Distinguished bases and Stokes regions for the simple and the simple elliptic singularities

Isolated hypersurface singularities come equipped with a Milnor lattice, a ${\mathbb Z}$-lattice of finite rank, and a set of $distinguished$ ${\mathbb Z}$-bases of this lattice. Usually these bases are constructed from $one$ morsification and $all\ possible$ choices of distinguished systems of paths. But what does one obtain if one considers $all\ possible$ morsifications and $one$ fixed distinguished system of paths? Looijenga asked this question 1974 for the simple singularities. He and Deligne found that one obtains a bijection between Stokes regions in a universal unfolding and the set of distinguished bases modulo signs. This allows to see the base space of the universal unfolding as an atlas of Stokes data. Here we reprove their result and extend it to the simple elliptic singularities. We use more conceptual arguments, moduli spaces of marked singularities (i.e. Teichmüller spaces for singularities), extensions of them to F-manifolds, and the actions of symmetries of singularities on the Milnor lattices and these moduli spaces. We use and extend results of Jaworski on the Lyashko-Looijenga maps for the simple elliptic singularities. The sections 2 and 3 give a survey on singularities and the associated objects which allows to read the paper independently of other sources.

math.AG↗
arXiv2018

Seven combinatorial problems around quasihomogeneous singularities

This paper proposes seven combinatorial problems around formulas for the characteristic polynomial and the spectral numbers of a quasihomogeneous singularity. One of them is a new conjecture on the characteristic polynomial. It is an amendment to an old conjecture of Orlik on the integral monodromy of a quasihomogeneous singularity. The search for a combinatorial proof of the new conjecture led us to the seven purely combinatorial problems.

math.CO↗