Pablo Portilla Cuadrado

Research

(Preprint) Vanishing arcs for isolated plane curve singularities, 2025, Joint work with Hanwool Bae, Cheol-Hyun Cho, Dongwook Choa and Wonbo Jeong.
Abstract arXiv

The variation operator associated with an isolated hypersurface singularity is a classical topological invariant that relates relative and absolute homologies of the Milnor fiber via a non trivial isomorphism. Here we work with a topological version of this operator that deals with proper arcs and closed curves instead of homology cycles. Building on the classical framework of geometric vanishing cycles, we introduce the concept of vanishing arcsets as their counterpart using this geometric variation operator. We characterize which properly embedded arcs are sent to geometric vanishing cycles by the geometric variation operator in terms of intersections numbers of the arcs and their images by the geometric monodromy. Furthermore, we prove that for any distinguished collection of vanishing cycles arising from an A'Campo's divide, there exists a topological exceptional collection of arcsets whose variation images match this collection.
(Survey) Plane curve singularities via divides, 2025, Joint work with Norbert A'Campo. To appear in the Handbook of Geometry and Topology of Singularities VIII.
Abstract arXiv

Generic relative immersions of compact one-manifolds in the closed unit disk, i.e. divides, provide a powerful combinatorial framework, and allow a topological construction of fibered classical links, for which the monodromy diffeomorphism is explicitly given as a product of Dehn twists. Complex isolated plane curve singularities provide a classical fibered link, the Milnor fibration, with its Milnor monodromy, monodromy group, and vanishing cycles. This surveys puts together much of the work done on divides and their role in the topology of isolated plane curve singularities. We review two complementary approaches for constructing divides: one via embedded resolution techniques and controlled real deformations, and another via Chebyshev polynomials, which yield explicit real morsifications. A combinatorial description of the Milnor fiber is developed, leading to an explicit factorization of the geometric monodromy as a product of right-handed Dehn twists. We further explore the structure of reduction curves that arise from the Nielsen description of quasi-finite mapping classes and from iterated cabling operations on divides. The interplay between the geometric and integral homological monodromies is analyzed, with special attention to symmetries induced by complex conjugation and strong invertibility phenomena. In particular, the integral homological monodromy for isolated plane curve singularities can be computed effectively. In contrast, for complex hypersurface singularities in higher dimensions no method of computation of the integral homology monodromy is known. Connections with mapping class groups, contact and symplectic geometry, and Lefschetz fibrations are also discussed. We conclude by outlining several open problems and conjectures related to the characterization of divides among fibered links, the presentation of geometric monodromy groups, and the existence of symplectic fillings compatible with the natural fibration structures.
(Preprint) The total spine of the Milnor fibration of a plane curve singularity, 2023, Joint work with Baldur Sigurðsson.
Abstract arXiv

We study the separatrices at the origin of the vector field -\nabla \log |f| where f is any plane curve singularity. Under some genericity conditions on the metric we produce a natural partition of the set of separatrices S into segments and disks. As a byproduct of this theory we construct a smooth fibration equivalent to the Milnor fibration that lives on a quotient of the Milnor fibration at radius 0, and, furthermore we see how the strict transform of S in this space induces a spine for each Milnor fiber of this fibration..
On a quadratic form associated with a surface automorphism and its applications to Singularity Theory, 2022, Indag. Math. Joint work with Lilia Alanís-López, Enrique Artal Bartolo, Christian Bonatti, Xavier Gómez-Mont and Manuel González Villa.
Abstract arXiv Journal

We study the nilpotent part of certain pseudoperiodic automorphisms of surfaces appearing in singularity theory. We associate a quadratic form \tilde{Q} defined on the first (relative to the boundary) homology group of the Milnor fiber F of any germ analytic curve on a normal surface. Using the twist formula and techniques from mapping class group theory, we prove that the form \tilde{Q} obtained after killing \ker N is definite positive, and that its restriction to the absolute homology group of F is even whenever the Nielsen-Thurston graph of the monodromy automorphism is a tree. The form \tilde{Q} is computable in terms of the Nielsen-Thurston or the dual graph of the semistable reduction, as illustrated with several examples. Numerical invariants associated to \tilde{Q} are able to distinguish plane curve singularities with different topological types but same spectral pairs or Seifert form. Finally, we discuss a generic linear germ defined on a superisolated surface with not smooth ambient space..
Vanishing cycles, plane curve singularities, and framed mapping class groups, 2021, Geom. Topol. Joint work with Nick Salter.
Abstract arXiv Journal

Let f be an isolated plane curve singularity with Milnor fiber of genus at least 5. For all such f, we give (a) an intrinsic description of the geometric monodromy group that does not invoke the notion of the versal deformation space, and (b) an easy criterion to decide if a given simple closed curve in the Milnor fiber is a vanishing cycle or not. With the lone exception of singularities of type A_n and D_n, we find that both are determined completely by a canonical framing of the Milnor fiber induced by the Hamiltonian vector field associated to f. As a corollary we answer a question of Sullivan concerning the injectivity of monodromy groups for all singularities having Milnor fiber of genus at least 7.
Positive factorizations of pseudoperiodic homeomorphisms, 2020, Math. Ann.
Abstract arXiv Journal

We generalize a classical result concerning smooth germs of surfaces, by proving that monodromies on links of isolated complex surface singularities associated with reduced holomorphic map germs admit a positive factorization. As a consequence of this and a topological characterization of these monodromies by Anne Pichon, we conclude that a pseudoperiodic homeomorphism on an oriented surface with boundary with positive fractional Dehn twist coefficients and screw numbers, admits a positive factorization. We use the main theorem to give a sufficiency criterion for certain pseudoperiodic homeomorphisms with negative screw numbers to admit a positive factorization.
Mixed tête-à-tête twists as monodromies associated with holomorphic function germs, 2019, Geom. Dedicata. Joint work with Baldur Sigurðsson.
Abstract arXiv Journal

Tête-à-tête graphs were introduced by N. A'Campo in 2010 with the goal of modeling the monodromy of isolated plane curves. Mixed tête-à-tête graphs provide a generalization which define mixed tête-à-tête twists, which are pseudo-periodic automorphisms on surfaces. We characterize the mixed tête-à-tête twists as those pseudo-periodic automorphisms that have a power which is a product of right-handed Dehn twists around disjoint simple closed curves, including all boundary components. It follows that the class of tête-à-tête twists coincides with that of monodromies associated with reduced function germs on isolated complex surface singularities.
Tête-à-tête twists, monodromies and representation of elements of Mapping Class Group, 2018, Ann. Inst. Fourier. Joint work with Norbert A'Campo, Javier Fernández de Bobadilla and María Pe Pereira.
Abstract arXiv Journal

We study monodromies of plane curve singularities and pseudo-periodic homeomorphisms of oriented surfaces with boundary, following an original idea of the first author: tête-à-tête graphs and twists. We completely characterize mapping classes that can be represented by tête-à-tête twists, and generalize the notion to be able to represent any class of the mapping class group relative to the boundary which is boundary-free periodic. This improves previous work on the subject by C. Graf. Furthermore, we introduce the class of mixed tête-à-tête graphs and twists, and prove that mixed tête-à-tête twists contain monodromies of irreducible plane curve singularities. In a sequel paper, the fourth author and B. Sigurdsson have extended this to the reducible case.
(PhD Thesis) Monodromies as tête-à-tête graphs, 2018, Under the supervision of Javier Fernández de Bobadilla and María Pe Pereira.
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