arXiv subjects

Žan Grad

Publications and source records attributed to Žan Grad.

4 recordsLinked to original sources

arXiv2026

Yang-Mills theory for multiplicative Ehresmann connections

We develop a twofold generalization of classical Yang-Mills theory, extending it from principal bundles to the setting of possibly non-transitive and non-integrable Lie algebroids. The classical theory is recovered when one considers the Atiyah algebroid of a principal bundle. In our framework, principal bundle connections are replaced by the more general notion of (infinitesimal) multiplicative Ehresmann connections. An action functional for such connections is constructed, now including a curvature 3-form contribution, alongside the usual curvature 2-form term, and the resulting variational problem is naturally constrained by a cohomological condition. We derive the associated Euler-Lagrange equations, and define a class of self-dual solutions (instantons) in both 4 and 5 dimensions. We also show that the solution space is invariant under gauge transformations, and compute its tangent space at a solution. As an important example, we show that our framework produces a Yang-Mills theory for connections on bundle gerbes.

math.DG↗

arXiv2025

Fundamentals of Lie categories and Yang-Mills theory for multiplicative Ehresmann connections

The first and shorter part of this thesis deals with the structural assumption of invertibility in a Lie groupoid. When this assumption is dropped, we obtain the notion of a Lie category: a small category, endowed with a compatible differentiable structure. We introduce various examples of Lie categories, examine their differences and similarities with Lie groupoids, and research the notions emerging naturally from the lack of invertibility of arrows. The aim of the second and principal part of this thesis is to provide a far-reaching generalization of Yang-Mills theory, extending it from the classical setting of principal bundles to general Lie groupoids and algebroids. The notion of a principal bundle connection is now replaced with that of a more general multiplicative Ehresmann connection. In obtaining this generalization, we make various advances to the theory of such connections, as well as invariant linear connections on representations. We develop the obstruction classes for their existence, generalize the (horizontal) exterior covariant derivative to the representation-valued Bott-Shulman-Stasheff and Weil complexes, and inspect their relationship with the van Est map. We research the class of multiplicative connections with cohomologically trivial curvature, which are central to obtaining the desired generalization. Applying the variational principle to this framework rests upon our developed formulae for affine deformations of multiplicative connections. Ultimately, we develop the extension of Yang-Mills theory to a non-integrable and non-transitive setting: the classical Yang-Mills equation is upgraded to a gauge-invariant pair of equations, which now describe the dynamics of gauge fields in both the longitudinal and transversal directions with respect to the (singular) orbit foliation. As an example, we obtain a Yang-Mills theory for $S^1$-bundle gerbes.

math.DG↗

arXiv2025

Covariant derivatives in the representation-valued Bott-Shulman-Stasheff and Weil complex

For a Lie groupoid $G$, the differential forms on its nerve comprise a double complex. A natural question is if this statement extends to forms with values in a representation $V$ of $G$. In this paper, we research two types of covariant derivatives which commute with the simplicial differential, yielding two types of "curved" double complexes of forms with coefficients in $V$. The naive approach is to consider a linear connection $\nabla$ on $V$, in which case $d^\nabla$ commutes with the simplicial differential if and only if $\nabla$ satisfies a certain (restrictive) invariance condition. The heart of this paper focuses on another, more compelling approach: using a multiplicative Ehresmann connection for a bundle of ideals. In this case, we obtain a geometrically richer curved double complex, where the cochain map is given by the horizontal exterior covariant derivative $D$, which generalizes the well-known operator from the theory of principal bundles. Moreover, both differential operators $d^\nabla$ and $D$ are researched in the infinitesimal setting of Lie algebroids, as well as their relationship with the van Est map. We conclude by using the operator $D$ to study the curvature of an (infinitesimal) multiplicative Ehresmann connection.

math.DG↗

arXiv2023

Fundamentals of Lie categories

We introduce the basic notions and present examples and results on Lie categories -- categories internal to the category of smooth manifolds. Demonstrating how the units of a Lie category $\mathcal C$ dictate the behavior of its invertible morphisms $\mathcal G(\mathcal C)$, we develop sufficient conditions for $\mathcal G(\mathcal C)$ to form a Lie groupoid. We show that the construction of Lie algebroids from the theory of Lie groupoids carries through, and ask when the Lie algebroid of $\mathcal G(\mathcal C)$ is recovered. We reveal that the lack of invertibility assumption on morphisms leads to a natural generalization of rank from linear algebra, develop its general properties, and show how the existence of an extension $\mathcal C\hookrightarrow \mathcal G$ of a Lie category to a Lie groupoid affects the ranks of morphisms and the algebroids of $\mathcal C$. Furthermore, certain completeness results for invariant vector fields on Lie monoids and Lie categories with well-behaved boundaries are obtained. Interpreting the developed framework in the context of physical processes, we yield a rigorous approach to the theory of statistical thermodynamics by observing that entropy change, associated to a physical process, is a functor.

math.DG↗