Calf Seminar
About the Calf
Calf is the junior COW, an algebraic geometry seminar group primarily aimed at PhD students.
The organisers for academic year 2025/26 are Inés Chung-Halpern (LSGNT), Marc Truter (Warwick), and Charlotte Satchwell (Essex), as well as an extensive network of local organisers at different universities. Calf announcements are made using the COW mailing list. If you would like to get involved in the organisation, or suggest your institution as the next venue, please contact any of the people named above.
The COW seminar has some funding for travel expenses, and information on reimbursement can be found on the main COW webpage.
About the Calf
Calf is the junior COW, an algebraic geometry seminar group primarily aimed at PhD students.
The organisers for academic year 2025/26 are Inés Chung-Halpern (LSGNT), Marc Truter (Warwick), and Charlotte Satchwell (Essex), as well as an extensive network of local organisers at different universities. Calf announcements are made using the COW mailing list. If you would like to get involved in the organisation, or suggest your institution as the next venue, please contact any of the people named above.
The COW seminar has some funding for travel expenses, and information on reimbursement can be found on the main COW webpage.
Upcoming Meeting:
University of Nottingham, Tuesday 9th June 2026
Room c29, Physics building, 13:00-17:00
13:00 Fraser Sparks (University of Nottingham)
Title: Motivic Tensor-Triangular Geometry: The Universal Space of Invariants of Spaces
Abstract: To any given variety one can associate various well-behaved invariants in the form of cohomology groups, and the motive of a space contains all this information. These motives form what is known as a tensor-triangulated category—Voevodsky’s DM(k;R)—and one can ask how to dissect it. Given such a category, one can assign a space to it, called its Balmer spectrum, which classifies its objects up to a natural notion of equivalence. The study of these spaces is the field of tensor-triangular geometry, and it is an open question as to what the Balmer spectrum of Voevodsky’s category of motives is. In this talk, I plan to give an introduction to the subject of motives as well as tensor-triangular geometry. I will focus on motivation and examples, and time permitting I will discuss some of my own research in the intersection of these areas.
14:30 Roktim Mascharak (King's College London)
Title: Introduction to Foliated MMP for log canonical Foliations.
Abstract: In a very introductory terms Foliations can be described as objects capturing differential equation on algebraic varieties or complex manifolds. These objects naturally come up in Birational Geometry on numerous contexts such as Abundance for three-folds, Bogomolov-Beauville decomposition of klt varieties etc. In recent times there have been advancements in understanding the Birational Geometry of Foliations on Projective varieties, and how the geometry of Foliation interacts with that of the variety. One of the issues in the existing literature was that- it was not sufficient to study the birational geometry of Fano foliations from a Mori Theoretic point of view, as we did not have the MMP for lc foliations. In this talk we will discuss about the foliated MMP comparing it with the classical MMP and the construction of the MMP for log canonical co-rank one foliations on klt three folds. This a joint work with Priyankur Chaudhuri.
16:00 Emanuel Roth (University of Edinburgh)
Title: Complementary polyhedra in the moduli of bundles
Abstract: To find the existence and uniqueness of canonical reductions of principal bundles, generalizing Harder-Narasimhan filtrations, Behrend introduced the notion of complementary polyhedra in his 1995 paper on the semistability of reductive group schemes. For every principal bundle, Behrend associates to it a complementary polyhedron via its automorphism group scheme that stores its (semi)-stability profile, to which canonical reductions correspond to "special facets". I will discuss how his work has been applied since then to more general moduli problems, such as that of parabolic bundles (Heinloth-Schmitt), Higgs bundles (Wißdorf), parahoric torsors (Heinloth) and parahoric Higgs torsors (Réga). These canonical reductions correspond to Theta-stratifications of the moduli stack through the Rees construction. Time permitting, in order to get similar results for Jordan-Hölder filtrations, I will discuss my work in progress to extend Behrend's approach to construct Jordan-Hölder filtrations through "Jordan-Hölder facets" of complementary polyhedra, which have an important connection to S-equivalence on the level of good moduli spaces for algebraic stacks.
University of Nottingham, Tuesday 9th June 2026
Room c29, Physics building, 13:00-17:00
13:00 Fraser Sparks (University of Nottingham)
Title: Motivic Tensor-Triangular Geometry: The Universal Space of Invariants of Spaces
Abstract: To any given variety one can associate various well-behaved invariants in the form of cohomology groups, and the motive of a space contains all this information. These motives form what is known as a tensor-triangulated category—Voevodsky’s DM(k;R)—and one can ask how to dissect it. Given such a category, one can assign a space to it, called its Balmer spectrum, which classifies its objects up to a natural notion of equivalence. The study of these spaces is the field of tensor-triangular geometry, and it is an open question as to what the Balmer spectrum of Voevodsky’s category of motives is. In this talk, I plan to give an introduction to the subject of motives as well as tensor-triangular geometry. I will focus on motivation and examples, and time permitting I will discuss some of my own research in the intersection of these areas.
14:30 Roktim Mascharak (King's College London)
Title: Introduction to Foliated MMP for log canonical Foliations.
Abstract: In a very introductory terms Foliations can be described as objects capturing differential equation on algebraic varieties or complex manifolds. These objects naturally come up in Birational Geometry on numerous contexts such as Abundance for three-folds, Bogomolov-Beauville decomposition of klt varieties etc. In recent times there have been advancements in understanding the Birational Geometry of Foliations on Projective varieties, and how the geometry of Foliation interacts with that of the variety. One of the issues in the existing literature was that- it was not sufficient to study the birational geometry of Fano foliations from a Mori Theoretic point of view, as we did not have the MMP for lc foliations. In this talk we will discuss about the foliated MMP comparing it with the classical MMP and the construction of the MMP for log canonical co-rank one foliations on klt three folds. This a joint work with Priyankur Chaudhuri.
16:00 Emanuel Roth (University of Edinburgh)
Title: Complementary polyhedra in the moduli of bundles
Abstract: To find the existence and uniqueness of canonical reductions of principal bundles, generalizing Harder-Narasimhan filtrations, Behrend introduced the notion of complementary polyhedra in his 1995 paper on the semistability of reductive group schemes. For every principal bundle, Behrend associates to it a complementary polyhedron via its automorphism group scheme that stores its (semi)-stability profile, to which canonical reductions correspond to "special facets". I will discuss how his work has been applied since then to more general moduli problems, such as that of parabolic bundles (Heinloth-Schmitt), Higgs bundles (Wißdorf), parahoric torsors (Heinloth) and parahoric Higgs torsors (Réga). These canonical reductions correspond to Theta-stratifications of the moduli stack through the Rees construction. Time permitting, in order to get similar results for Jordan-Hölder filtrations, I will discuss my work in progress to extend Behrend's approach to construct Jordan-Hölder filtrations through "Jordan-Hölder facets" of complementary polyhedra, which have an important connection to S-equivalence on the level of good moduli spaces for algebraic stacks.
Recent Meetings:
Imperial College London, Friday 20th March 2026
ICBS 100 - LT1, Imperial College Business School, 13:00-17:00
14:30 Cat Rust (Queen Mary University London)
Title: Topological Invariants of Stable Maps to Toric Pairs via Tropicalisation
Abstract: We begin with an overview of moduli spaces of stable maps to pairs and their role in enumerative geometry. Specialising to stable maps to toric pairs with prescribed tangency conditions to the toric boundary, we will cover the stratification of the moduli space by combinatorial types of associated tropical maps. Finally, we will use this stratification to study the class of the moduli space in the Grothendieck ring of varieties, describing a wall-and-chamber decomposition of the tangency space such that on open chambers, the class of the moduli space is constant.
16:00 Calum Crossley (University College London)
Title: Gluing curves and quivers
Abstract: Normalization is the most well-behaved step in resolving singularities, but it has one major defect: it completely forgets which singularity you came from. This is fixed by remembering some gluing data, which we can treat as a "non-commutative space" whose category of sheaves relates to both the singularity and the resolution. We will explore some 1-dimensional examples where this boils down to gluing quivers together, and look at special quiver representations which perfectly match the properties of special sheaves on a 3-fold resolution of singularities.
University of Warwick, Friday 28th November 2025
IAS Seminar Room, Zeeman Building, 13:00-17:00
13:00 Yaoqi Yang (University of Warwick)
Title: Bridging K-Stability and GIT Stability
Abstract: K-stability plays a central role in the study of Fano varieties and their moduli, linking algebraic geometry with differential geometry through the existence of Kähler–Einstein metrics. On the other hand, Geometric Invariant Theory (GIT) provides a classical notion of stability used to construct moduli spaces via group actions. In this talk, I’ll explain how K-stability can be viewed as an extension of GIT stability, comparing how both measure the behaviour of degenerations of polarized varieties. If time allows, I will also describe how similar ideas apply to log pairs.
14:30 Harry Shaw (University of Bath)
Title: The existence of quartic del Pezzo surfaces over global function fields which do not admit a quadratic point.
Abstract: Creutz and Viray have recently proven the existence of quartic del Pezzo surfaces over Q which do not admit a point defined over any quadratic field extension, despite every such surface admitting a quadratic point over any local field. We prove the analogous result over global function fields. This is joint work with Giorgio Navone, Katerina Santicola and Haowen Zhang.
16:00 Heath Pearson (University of Nottingham)
Title: Decombinatorialisation
Abstract: This is a case study in approaching algebraic-geometric questions by first interpreting them in a combinatorially tractable class of varieties, then systematically generalising the findings through a sequence of increasingly general classes. The end goal is a general statement. The prototypical starting point is the class of toric varieties. In this talk, we explore the additional geometric insight gained by passing to the more general—yet still combinatorial—spherical varieties. They are a natural generalisation of toric varieties, where the algebraic torus is replaced by an arbitrary reductive group.
Imperial College London, Friday 20th March 2026
ICBS 100 - LT1, Imperial College Business School, 13:00-17:00
14:30 Cat Rust (Queen Mary University London)
Title: Topological Invariants of Stable Maps to Toric Pairs via Tropicalisation
Abstract: We begin with an overview of moduli spaces of stable maps to pairs and their role in enumerative geometry. Specialising to stable maps to toric pairs with prescribed tangency conditions to the toric boundary, we will cover the stratification of the moduli space by combinatorial types of associated tropical maps. Finally, we will use this stratification to study the class of the moduli space in the Grothendieck ring of varieties, describing a wall-and-chamber decomposition of the tangency space such that on open chambers, the class of the moduli space is constant.
16:00 Calum Crossley (University College London)
Title: Gluing curves and quivers
Abstract: Normalization is the most well-behaved step in resolving singularities, but it has one major defect: it completely forgets which singularity you came from. This is fixed by remembering some gluing data, which we can treat as a "non-commutative space" whose category of sheaves relates to both the singularity and the resolution. We will explore some 1-dimensional examples where this boils down to gluing quivers together, and look at special quiver representations which perfectly match the properties of special sheaves on a 3-fold resolution of singularities.
University of Warwick, Friday 28th November 2025
IAS Seminar Room, Zeeman Building, 13:00-17:00
13:00 Yaoqi Yang (University of Warwick)
Title: Bridging K-Stability and GIT Stability
Abstract: K-stability plays a central role in the study of Fano varieties and their moduli, linking algebraic geometry with differential geometry through the existence of Kähler–Einstein metrics. On the other hand, Geometric Invariant Theory (GIT) provides a classical notion of stability used to construct moduli spaces via group actions. In this talk, I’ll explain how K-stability can be viewed as an extension of GIT stability, comparing how both measure the behaviour of degenerations of polarized varieties. If time allows, I will also describe how similar ideas apply to log pairs.
14:30 Harry Shaw (University of Bath)
Title: The existence of quartic del Pezzo surfaces over global function fields which do not admit a quadratic point.
Abstract: Creutz and Viray have recently proven the existence of quartic del Pezzo surfaces over Q which do not admit a point defined over any quadratic field extension, despite every such surface admitting a quadratic point over any local field. We prove the analogous result over global function fields. This is joint work with Giorgio Navone, Katerina Santicola and Haowen Zhang.
16:00 Heath Pearson (University of Nottingham)
Title: Decombinatorialisation
Abstract: This is a case study in approaching algebraic-geometric questions by first interpreting them in a combinatorially tractable class of varieties, then systematically generalising the findings through a sequence of increasingly general classes. The end goal is a general statement. The prototypical starting point is the class of toric varieties. In this talk, we explore the additional geometric insight gained by passing to the more general—yet still combinatorial—spherical varieties. They are a natural generalisation of toric varieties, where the algebraic torus is replaced by an arbitrary reductive group.
