arXiv subjects

Tamás László

Publications and source records attributed to Tamás László.

At least 19 recordsLinked to original sources

arXiv2026

Splice formulae for Poincaré series of integral homology spheres

Let M be a plumbed integral homology sphere 3-manifold associated with a connected negative definite plumbing graph $Γ$. One of its most important invariants is its multivariable Poincaré series (or zeta function) $f_Γ(\mathbf {t})$. Several numerical invariants can be read from $f_Γ(\mathbf{t})$, or even from its `polynomial part' $ \operatorname{Pol}_Γ(\mathbf{t})$. For example, the `normalized' Casson's invariant equals $ \operatorname{Pol}_Γ(1)$. In this note we provide several splice (surgery) formulae for $f_Γ$ and $\operatorname{Pol}_Γ$ (reduced to the node variables, or to the node variables of connected sub-graphs). In this way, these global invariants can be recovered from a collection of certain smaller graphs. Recall that some (integral homology sphere) 3-manifold invariants are additive with respect to the splice decomposition (like the Casson's invariant). However, some invariants need some `splice correction terms'. In our formulae the correction terms are easily computable one variable Alexander polynomials.

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arXiv2026

Representability of systems of proportionally modular numerical semigroups

In this short note we prove that every system of proportionally modular numerical semigroups is representable by a canonical equivariant resolution of a weighted homogeneous surface singularity with rational homology sphere link. The construction starts from the quotient descriptions of proportionally modular numerical semigroups by two-generator numerical semigroups, realizes each quotient by a two-legged canonical equivariant resolution graph, and then glues these graphs with suitable multiplicities.

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arXiv2025

Delta invariant of $\mathbb{Q}$-Cartier curve germs and the genus of representable numerical semigroups

In this article, first we give two formulae for the delta invariant of a complex curve singularity that can be embedded as a ${\mathbb Q}$-Cartier divisor in a normal surface singularity with rational homology sphere link. Next, we consider representable numerical semigroups, they are semigroups associated with normal weighted homogeneous surface singularities with rational homology sphere links (via the degrees of the homogeneous functions). We then prove that such a semigroup can be interpreted as the value semigroup of a generic orbit (as a curve singularity) given by the $\mathbb{C}^*$-action on the weighted homogeneous germ. Furthermore, we use the delta invariant formula to derive a combinatorially computable formula for the genus of representable semigroups. Finally, we characterize topologically those representable semigroups which are symmetric.

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arXiv2024

Multiplier ideals of normal surface singularities

We study the multiplier ideals and the corresponding jumping numbers and multiplicities $\{m(c)\}_{c\in \mathbb{R}}$ in the following context: $(X,o)$ is a complex analytic normal surface singularity, ${\mathfrak a}\subset \mathcal{O}_{X,o}$ is an ${\mathfrak m}_{X,o}$--primary ideal, $ϕ:\widetilde{X}\to X$ is a log resolution of $\mathfrak{a}$ such that $\mathfrak{a}\mathcal{O}_{\widetilde{X}}=\mathcal{O}_{\widetilde{X}}(-F)$, for some nonzero effective divisor $F$ supported on $ϕ^{-1}(0)$. We show that $\{m(c)\}_{c>0}$ is combinatorially computable from $F$ and the resolution graph $Γ$ of $ϕ$, and we provide several formulae. We also extend Budur's result (valid for $(X,o)=(\mathbb{C}^2,0)$), which makes an identification of $\sum_{c\in[0,1]}m(c)t^c$ with a certain Hodge spectrum. In our general case we use Hodge spectrum with coefficients in a mixed Hodge module. We show that $\{m(c)\}_{c\leq 0}$ usually depends on the analytic type of $(X,o)$. However, for some distinguished analytic types we determine it concretely. E.g., when $(X,o)$ is weighted homogeneous (and $F$ is associated with the central vertex), we recover $\sum_cm(c)t^c$ from the Poincaré series of $(X,o)$ and when $(X,o)$ is a splice quotient then we recover $\sum_cm(c)t^c$ from the multivariable topological Poincaré (zeta) function of $Γ$.

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arXiv2024

Flat semigroups and weighted homogeneous surface singularities

We consider numerical semigroups associated with normal weighted homogeneous surface singularities with rational homology sphere links. We say that a semigroup is representable if it can be realized in this way. In this article, we study the representability of flat semigroups and prove that a numerical semigroup is representable if and only if it can be written as a quotient of a flat semigroup.

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arXiv2024

On a canonical polynomial for links of elliptic singularities

The canonical polynomial is an important output of the multivariable topological Poincaré series associated with a normal surface singularity. It can be considered as a multivariable polynomial generalization of the Seiberg--Witten invariant of the link. In the case of elliptic germs, another key topological invariant was considered, the elliptic sequence, which mirrors the specific structure of the elliptic germs and guides several properties of them. In this note we study the relationship of these two objects. First of all, we describe the structure of the exponents of the canonical polynomial and prove that they determine the elliptic sequence. For the converse problem, we consider an inductive setup of elliptic germs via natural extension of their graphs and compare the corresponding sets of exponents. This leads to the definition of a good extension which can be characterized by an inclusion type formula for the corresponding canonical polynomials. This reflects in a compatible way the `flag structure' of the elliptic sequence.

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arXiv2021

Brill-Noether problem on splice quotient singularities and duality of topological Poincaré series

In this manuscript we investigate the analouge of the Brill-Noether problem for smooth curves in the case of normal surface singularities. We determine the maximal possible value of $h^1$ of line bundles without fixed components in the Picard group $\pic^{l'}(\tX)$ in the following cases: for some special Chern classes $l'$ if $\tX$ is a resolution of a splice quotient singularity $(X, 0)$ and for arbitrary Chern classes in the case of weighted homogenous singularities. Motivated by this problem, we define the \emph{virtual cohomology numbers} $h^1_{virt}(l')$ for all Chern classes $l'$ such that $h^1_{virt}(0)$ is the canonical normalized Seiberg-Witten invariant and we generalize the duality formulae of Seiberg-Witten invariants obtained by the authors and A. Némethi in \cite{LNNdual}, for the virtual cohomology numbers.

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arXiv2020

Local invariants of minimal generic curves on rational surfaces

Let (C,0) be a reduced curve germ in a normal surface singularity (X,0). The main goal is to recover the delta invariant of the abstract curve (C,0) from the topology of the embedding. We give explicit formulae whenever (C,0) is minimal generic and (X,0) is rational (as a continuation of previous works of the authors). Additionally we prove that if (X,0) is a quotient singularity, then the delta invariant of (C,0) only admits the values r-1 or r, where r is the number or irreducible components of (C,0). (r-1 realizes the extremal lower bound, valid only for `ordinary r-tuples'.)

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arXiv2020

The delta invariant of curves on rational surfaces II: Poincaré series and topological aspects

In this article we study abstract and embedded invariants of reduced curve germs via topological techniques. One of the most important numerical analytic invariants of an abstract curve is its delta invariant. Our primary goal is to develop delta invariant formulae for curves embedded in rational singularities in terms of embedded data. The topological machinery not only produces formulae, but it also creates deep connections with the theory of (analytical and topological) multivariable Poincaré series.

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arXiv2019

Delta invariant of curves on rational surfaces I. The analytic approach

We prove that if (C,0) is a reduced curve germ on a rational surface singularity (X,0) then its delta invariant can be recovered by a concrete expression associated with the embedded topological type of the pair (X,C). Furthermore, we also identify it with another (a priori) embedded analytic invariant, which is motivated by the theory of adjoint ideals. Finally, we connect our formulae with the local correction term at singular points of the global Riemann--Roch formula, valid for projective normal surfaces, introduced by Blache.

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arXiv2018

On the geometry of strongly flat semigroups and their generalizations

Our goal is to convince the readers that the theory of complex normal surface singularities can be a powerful tool in the study of numerical semigroups, and, in the same time, a very rich source of interesting affine and numerical semigroups. More precisely, we prove that the strongly flat semigroups, which satisfy the maximality property with respect to the Diophantine Frobenius problem, are exactly the numerical semigroups associated with negative definite Seifert homology spheres via the possible 'weights' of the generic $S^1$-orbit. Furthermore, we consider their generalization to the Seifert rational homology sphere case and prove an explicit (up to a Laufer computation sequence) formula for their Frobenius number. The singularities behind are the weighted homogeneous ones, whose several topological and analytical properties are exploited.

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arXiv2018

Combinatorial duality for Poincaré series, polytopes and invariants of plumbed 3-manifolds

Assume that the link of a complex normal surface singularity is a rational homology sphere. Then its Seiberg-Witten invariant can be computed as the `periodic constant' of the topological multivariable Poincaré series (zeta function). This involves a complicated regularization procedure (via quasipolynomials measuring the asymptotic behaviour of the coefficients). We show that the (a Gorenstein type) symmetry of the zeta function combined with Ehrhart-Macdonald-Stanley reciprocity (of Ehrhart theory of polytopes) provide a simple expression for the periodic constant. Using these dualities we also find a multivariable polynomial generalization of the Seiberg-Witten invariant, and we compute it in terms of lattice points of certain polytopes. All these invariants are also determined via lattice point counting, in this way we establish a completely general topological analogue of formulae of Khovanskii and Morales valid for singularities with non-degenerate Newton principal part.

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arXiv2017

Némethi's division algorithm for zeta-functions of plumbed 3-manifolds

A polynomial counterpart of the Seiberg-Witten invariant associated with a negative definite plumbed 3-manifold has been proposed by earlier work of the authors. It is provided by a special decomposition of the zeta-function defined by the combinatorics of the manifold. In this article we give an algorithm, based on multivariable Euclidean division of the zeta-function, for the explicit calculation of the polynomial, in particular for the Seiberg--Witten invariant.

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arXiv2017

Surgery formulae for the Seiberg-Witten invariant of plumbed 3-manifolds

Assume that $M(\mathcal{T})$ is a rational homology sphere plumbed 3-manifold associated with a connected negative definite graph $\mathcal{T}$. We consider the combinatorial multivariable Poincaré series associated with $\mathcal{T}$ and its counting functions, which encode rich topological information. Using the `periodic constant' of the series (with reduced variables) we prove surgery formulae for the normalized Seiberg-Witten invariants: the periodic constant appears as the difference of the Seiberg-Witten invariants associated with $M(\mathcal{T})$ and $M(\mathcal{T}\setminus\mathcal{I})$, where $\mathcal{I}$ is an arbitrary subset of the set of vertices of $\mathcal{T}$.

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arXiv2016

Non-normal affine monoids, modules and Poincaré series of plumbed 3-manifolds

We construct a non-normal affine monoid together with its modules associated with a negative definite plumbed $3$-manifold $M$. In terms of their structure, we describe the $H_1(M,\mathbb{Z})$-equivariant parts of the topological Poincaré series. In particular, we give combinatorial formulas for the Seiberg--Witten invariants of $M$ and for polynomial generalizations defined in a previous paper of the authors.

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arXiv2015

On Poincaré series associated with links of normal surface singularities

We study the counting function of topological Poincaré series associated with rational homology sphere plumbed 3-manifold with connected negative definite tree, interpreting as an alternating sum of coefficient functions associated with some Taylor expansions. It is motivated by a theorem of Szenes and Vergne which expresses these coefficient functions in terms of Jeffrey--Kirwan residues. This is used to prove the uniqueness of the quasipolynomiality inside a special cone, the structure of the counting function in terms of the graph and construction for a polynomial generalization of the Seiberg--Witten invariant given by the Poincaré series. We also reprove and discuss surgery formulas of Némethi for the counting function, and of Braun and Némethi for the Seiberg--Witten invariant.

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arXiv2013

Lattice cohomology and Seiberg-Witten invariants of normal surface singularities

One of the main questions in the theory of normal surface singularities is to understand the relations between their geometry and topology. The lattice cohomology is an important tool in the study of topological properties of a plumbed 3-manifold M associated with a connected negative definite plumbing graph G. It connects the topological properties with analytic ones when M is realized as a singularity link, i.e. when G is a good resolution graph of the singularity. Its computation is based on the (Riemann-Roch) weights of the lattice points of \Z^s, where s is the number of vertices of G. The first part of the thesis reduces the rank of this lattice to the number of `bad' vertices of the graph. Usually, the geometry/topology of M is encoded exactly by these `bad' vertices and their number measures how far the plumbing graph stays from a rational one. In the second part, we identify the following three objects: the Seiberg-Witten invariant of a plumbed 3-manifold, the periodic constant of its topological Poincare series, and a coefficient of an equivariant multivariable Ehrhart polynomial. For this, we construct the corresponding polytope from the plumbing graph, together with an action of H_1(M,\Z), and we develop Ehrhart theory for them. The effect of the reduction appears also at the level of the multivariable topological Poincare series, simplifying the corresponding polytope and the Ehrhart theory as well. We end the thesis with detailed calculations and examples.

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arXiv2013

Reduction theorem for lattice cohomology

The lattice cohomology of a plumbed 3--manifold $M$ associated with a connected negative definite plumbing graph is an important tool in the study of topological properties of $M$, and in the comparison of the topological properties with analytic ones when $M$ is realized as complex analytic singularity link. By definition, its computation is based on the (Riemann--Roch) weights of the lattice points of $\Z^s$, where $s$ is the number of vertices of the plumbing graph. The present article reduces the rank of this lattice to the number of `bad' vertices of the graph. (Usually the geometry/topology of $M$ is codified exactly by these `bad' vertices via surgery or other constructions. Their number measures how far is the plumbing graph from a rational one.) The effect of the reduction appears also at the level of certain multivariable (topological Poincaré) series as well. Since from these series one can also read the Seiberg--Witten invariants, the reduction theorem provides new formulae for these invariants too. The reduction also implies the vanishing $\bH^q=0$ of the lattice cohomology for $q\geq ν$, where $ν$ is the number of `bad' vertices. (This bound is sharp.)

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