arXiv subjects

Janusz Grabowski

Publications and source records attributed to Janusz Grabowski.

At least 19 recordsLinked to original sources

arXiv2026

Homogeneity actions, N-manifolds, and the Frobenius theorem

This paper is devoted to $\mathbb{N}$-graded supermanifolds $\mathcal{M}$ whose grading is induced by a homogeneity action, i.e., a smooth action $\mathbb{R}\ni t\mapsto h_t$ of the multiplicative monoid of real numbers on $\mathcal{M}$. We show that the map $h_0$ is a smooth retraction onto a submanifold $M=h_0(\mathcal{M})$, and that $h_0:\mathcal{M}\to M$ is a fiber bundle with typical fiber $\mathbb{R}^{m|n}$. Using this homogeneity approach, we obtain a simple proof of a homogeneous version of the Frobenius theorem. If, in addition, $h_{-1}$ acts as the parity operator on $\mathcal{M}$, we provide a geometric characterization equivalent to the recent definition of $\mathbb{N}$-manifolds due to Bursztyn, Cueca, and Mehta in terms of sheaves of graded algebras with prescribed local models. As a consequence, the homogeneous Frobenius theorem for $\mathbb{N}$-manifolds proved by these authors appears as a special case of our more general result.

math.DG↗

arXiv2026

A Darboux classification of homogeneous Pfaffian forms on graded manifolds

We study the local classification problem for differential Pfaffian forms on a supermanifold $M$ that are homogeneous with respect to a given homogeneity structure on $M$. The most familiar examples of homogeneity structures are those associated with vector bundle structures. Our aim is to show that, for a homogeneous form of fixed degree, there exist homogeneous Darboux coordinates. As a consequence, we obtain Darboux-type normal forms for homogeneous Pfaffian forms, recovering as special cases the classical Darboux theorem together with its contact and presymplectic counterparts. To formulate an analogue of Darboux classification in the supergeometric setting, we associate to a differential form $\alpha$ the characteristic distribution $\chi(\alpha)=\ker(\alpha)\cap\ker(\mathrm{d}\alpha)$, and define the class of $\alpha$ as the rank of $\chi(\alpha)$. We prove that, under suitable regularity and constant-rank assumptions, this distribution completely determines the local equivalence problem for homogeneous Pfaffian forms. Our results apply equally well to ordinary (purely even) manifolds.

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arXiv2025

Nijenhuis operators on Banach fibration

In the infinite-dimensional Banach setting, we consider general smooth Banach fibrations $\tau:M\to M_0$ and `$(1,1)$-tensors' $N:TM\to TM$ that are projectable (in the obvious sense) onto Nijenhuis operators $N_0:TM_0\to TM_0$ on $M_0$. We prove that the vanishing of the Nijenhuis torsion of $N_0$ is equivalent to the fact that the Nijenhuis torsion of $N$ takes only vertical values, i.e., values in $ker(T\tau)$. Consequences for almost complex structures on (real) Banach manifolds are also derived. As canonical examples, we define tangent lifts $d_T(N_0):TT M_0\to TT M_0$ of Nijenhuis operators $N_0$ in the Banach category, and prove that they are automatically projectable for the canonical fibrations $\tau_{M_0}:TM_0\to M_0$. Finally, we comment on the projectability in the case of Banach homogeneous manifolds $\tau:G\to G/K$, studied recently by some authors.

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arXiv2025

Equivalence functors in graded supergeometry

It has recently been proved that the category of N-manifolds of degree $n$, that is, $\mathbb N$-graded supermanifolds of degree $n$ for which the parity agrees with the gradation, is equivalent to the category of purely even $n$-tuple vector superbundles equipped with a suitable action of the symmetric group $S_n$ permuting the vector bundle structures. This equivalence may be interpreted as a `desuperization' of N-manifolds. In the present paper, we place this result within a broader framework of graded structures on supermanifolds and explicitly describe several canonical equivalences between the corresponding categories in a purely geometric, constructive manner. The desuperization equivalence functor appears as a composition of some of these canonical equivalences. Our constructions are entirely canonical and rely on standard tools of supergeometry, including iterated tangent functors, parity reversion in vector superbundles, and the interpretation of $n$-tuple vector bundles in terms of commuting Euler vector fields associated with the underlying vector bundle structures.

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arXiv2024

The regularity and products in contact geometry

We study regular contact manifolds $(M,\eta)$ whose Reeb vector field is complete and prove that they are canonically principal bundles with the structure group $S^1$ or $\mathbb{R}$. For compact $M$, our proof is very short and elementary and covers the celebrated Boothby-Wang theorem, but we do not assume compactness from the very beginning. However, to prove our result in full generality we use some topological tools adapted to smooth fibrations. In the second part of the paper, we describe a natural concept of contact products of general contact manifolds as well as a product of principal contact manifolds, which exists if the periods of the Reeb vector fields are commensurate, and corresponds to the construction of products of prequantization bundles of symplectic manifolds.

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arXiv2024

Sasaki structures on general contact manifolds

We extend the notion of a Sasakian structure from the classical setting of a cooriented contact manifold, where it is given by a compatibility between a contact form $\eta$ and a Riemannian metric $g_M$ on $M$, to the case of an arbitrary contact structure understood as a contact distribution. In the cooriented case, this compatibility can be equivalently expressed by the fact that the symplectic form $\omega=\mathrm{d}(s^2\eta)$ and the cone metric $g(x,s)=\mathrm{d} s\otimes\mathrm{d} s+s^2g_M(x)$ define a K\"ahler structure on the cone $\mathcal{M}=M\times\mathbb{R}_+$. Since general contact structures admit canonical realizations as homogeneous symplectic structures $\omega$ on principal $\mathbb{R}^\times$-bundles $P\to M$, it is natural to interpret Sasakian geometry in full generality in terms of suitable homogeneous K\"ahler structures on $P$. We characterize homogeneous K\"ahler structures on symplectizations $(P,\omega)$ associated with arbitrary contact structures on $M$, and show that they canonically determine a two-sheeted covering $\tilde M$ of $M$ equipped with a contact form. This reduces the problem to the cooriented case and leads to a notion of a generalized Sasakian structure on $M$ associated with a homogeneous K\"ahler structure on $(P,\omega)$. Moreover, since products of K\"ahler manifolds are again K\"ahler, our framework naturally yields a concept of a product of Sasakian manifolds. The whole constructions are intrinsic and conceptual, avoiding any ad hoc choices.

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arXiv2024

Principal bundles in the category of $\mathbb{Z}_2^n$-manifolds

We introduce and examine the notion of principal $\mathbb{Z}_2^n$-bundles, i.e., principal bundles in the category of $\mathbb{Z}_2^n$-manifolds. The latter are higher graded extensions of supermanifolds in which a $\mathbb{Z}_2^n$-grading replaces $\mathbb{Z}_2$-grading. These extensions have opened up new areas of research of great interest in both physics and mathematics. In principle, the geometry of $\mathbb{Z}_2^n$-manifolds is essentially different than that of supermanifolds, as for $n>1$ we have formal variables of even parity, so local smooth functions are formal power series. On the other hand, a full version of differential calculus is still valid. We show in this paper that the fundamental properties of classical principal bundles can be generalised to the setting of this `higher graded' geometry, with properly defined frame bundles of $\mathbb{Z}_2^n$-vector bundles as canonical examples. However, formulating these concepts and proving these results relies on many technical upshots established in earlier papers. A comprehensive introduction to $\mathbb{Z}_2^n$-manifolds is therefore included together with basic examples.

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arXiv2024

Graded supermanifolds and homogeneity

We introduce the concept of a homogeneity supermanifold, which is, roughly speaking, a supermanifold equipped with a privileged atlas whose coordinates carry prescribed (real) homogeneity degrees. This structure defines a sheaf of graded algebras on the supermanifold, regarded as an additional geometric structure. The guiding principle of this approach is that grading is ultimately related to homogeneity. Assigning homogeneity degrees to coordinates in a consistent way is equivalent to fixing a global vector field, the weight vector field. This approach is simple and substantially more general than most existing approaches to graded manifolds. In particular, the homogeneity degrees may be arbitrary real numbers, and the resulting category includes compact supermanifolds. We systematically study homogeneity submanifolds, homogeneity Lie supergroups, tangent and cotangent lifts of homogeneity structures, homogeneous distributions and codistributions, as well as related notions such as double homogeneity. The main achievements of this framework include proofs of the homogeneous Poincar\'e Lemma, the homogeneous Frobenius Theorem, and the homogeneous symplectic Darboux Theorem, results that are of independent interest even in the purely even case.

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arXiv2024

Jacobi algebroids and Jacobi sigma models

The definition of an action functional for the Jacobi sigma models, known for Jacobi brackets of functions, is generalized to \emph{Jacobi bundles}, i.e., Lie brackets on sections of (possibly nontrivial) line bundles, with the particular case of contact manifolds. Different approaches are proposed, but all of them share a common feature: the presence of a \emph{homogeneity structure} appearing as a principal action of the Lie group $\mathbb{R}^{\times}=\mathrm{GL}(1;\mathbb{R})$. Consequently, solutions of the equations of motions are morphisms of certain \emph{Jacobi algebroids}, i.e., principal $\mathbb{R}^{\times}$-bundles equipped additionally with a compatible Lie algebroid structure. Despite the different approaches we propose, there is a one-to-one correspondence between the space of solutions of the different models. The definition can be immediately extended to \emph{almost Poisson} and \emph{almost Jacobi brackets}, i.e., to brackets that do not satisfy the Jacobi identity. Our sigma models are geometric and fully covariant.

math-ph↗

arXiv2024

Contactifications: a Lagrangian description of compact Hamiltonian systems

If $\eta$ is a contact form on a manifold $M$ such that the orbits of the Reeb vector field form a simple foliation $\mathcal{F}$ on $M$, then the presymplectic 2-form $d\eta$ on $M$ induces a symplectic structure $\omega$ on the quotient manifold $N=M/\mathcal{F}$. We call $(M,\eta)$ a $\textit contactification$ of the symplectic manifold $(N,\omega)$. First, we present an explicit geometric construction of contactifications of some coadjoint orbits of connected Lie groups. Our construction is a far going generalization of the well-known contactification of the complex projective space $\mathbb{C}P^{n-1}$, being the unit sphere $S^{2n-1}$ in $\mathbb{C}^{n}$, and equipped with the restriction of the Liouville 1-form on $\mathbb{C}^n$. Second, we describe a constructive procedure for obtaining contactification in the process of the Marsden-Weinstein-Meyer symplectic reduction and indicate geometric obstructions for the existence of compact contactifications. Third, we show that contactifications provide a nice geometrical tool for a Lagrangian description of Hamiltonian systems on compact symplectic manifolds $(N,\omega)$, on which symplectic forms never admit a `vector potential'.

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arXiv2023

Regular contact manifolds: a generalization of the Boothby-Wang theorem

A regular contact manifold is a manifold $M$ equipped with a globally defined contact form $\eta$ such that the topological space $M/\mathcal{R}$ of orbits (trajectories) of the Reeb vector field $\mathcal{R}$ of $\eta$ carries a smooth manifold structure, so the canonical projection $p:M\to M/\mathcal{R}$ is a smooth fibration. We show that, under the additional assumption that $\mathcal{R}$ is a complete vector field, this fibration is actually either an $S^1$- or an $\mathbb{R}$-principal bundle. Moreover, there exists a unique symplectic form $\omega$ on $M/\mathcal{R}$ such that $p^*(\omega)=\mathrm{d}\eta$ which is $\rho$-integral in the $S^1$-bundle case, where $\rho$ is the minimal period of the $S^1$-action, so the symplectic manifold $(M/\mathcal{R},\omega)$ admits a prequantization. We do not assume that $M$ is compact.

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arXiv2022

Reductions: precontact versus presymplectic

We show that contact reductions can be described in terms of symplectic reductions in the traditional Marsden-Weinstein-Meyer as well as the constant rank picture. The point is that we view contact structures as particular (homogeneous) symplectic structures. A group action by contactomorphisms is lifted to a Hamiltonian action on the corresponding symplectic manifold, called the symplectic cover of the contact manifold. In contrast to the majority of the literature in the subject, our approach includes general contact structures (not only co-oriented) and changes the traditional view point: contact Hamiltonians and contact moment maps for contactomorphism groups are no longer defined on the contact manifold itself, but on its symplectic cover. Actually, the developed framework for reductions is slightly more general than purely contact, and includes a precontact and presymplectic setting which is based on the observation that there is a one-to-one correspondence between isomorphism classes of precontact manifolds and certain homogeneous presymplectic manifolds.

math.SG↗

arXiv2022

Contact geometric mechanics: the Tulczyjew triples

We propose a generalization of the classical Tulczyjew triple as a geometric tool in Hamiltonian and Lagrangian formalisms which serves for contact manifolds. The r\^ole of the canonical symplectic structures on cotangent bundles in Tulczyjew's case is played by the canonical contact structures on the bundles $J^1L$ of first jets of sections of line bundles $L\to M$. Contact Hamiltonians and contact Lagrangians are understood as sections of certain line bundles, and they determine (generally implicit) dynamics on the contact phase space $J^1L$. We also study a contact analog of the Legendre map and the Legendre transformation of generating objects in both contact formalisms. Several explicit examples are offered.

math.SG↗

arXiv2022

A novel approach to contact Hamiltonians and contact Hamilton-Jacobi theory

We propose a novel approach to contact Hamiltonian mechanics which, in contrast to the one dominating in the literature, serves also for non-trivial contact structures. In this approach Hamiltonians are no longer functions on the contact manifold M itself but sections of a line bundle over M or, equivalently, 1-homogeneous functions on a certain GL(1,R)-principal bundle P\to M which is equipped with a homogeneous symplectic form \omega. In other words, our understanding of contact geometry is that it is not an `odd-dimensional cousin' of symplectic geometry but rather a part of the latter, namely `homogeneous symplectic geometry'. This understanding of contact structures is much simpler than the traditional one and very effective in applications, reducing the contact Hamiltonian formalism to the standard symplectic picture. We develop in this language contact Hamiltonian mechanics in autonomous, as well as time-dependent case, and the corresponding Hamilton-Jacobi theory. Fundamental examples are based on canonical contact structures on the first jet bundles J^1(L) of sections of line bundles L, which play in contact geometry a fundamental role similar to that played by cotangent bundles in symplectic geometry.

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arXiv2022

Lifting statistical structures

We consider some natural (functorial) lifts of geometric objects associated with statistical manifolds (metric tensor, dual connections, skewness tensor, etc.) to higher tangent bundles. It turns out that the lifted objects form again a statistical manifold structure, this time on the higher tangent bundles, with the only difference that the metric tensor is pseudo-Riemannian. What is more, natural lifts of potentials (called also divergence or contrast functions) turn out to be again potentials, this time for the lifted statistical structures. We propose an analogous procedure for lifting statistical structures on Lie algebroids and lifting contrast functions which are defined on Lie groupoids. In particular, we study in detail Lie groupoid structures of higher tangent bundles of Lie groupoids. Our geometric constructions of lifts are illustrated by explicit examples, including some important statistical models and potential functions on Lie groupoids.

math.DG↗

arXiv2021

VB-structures and generalizations

Motivated by properties of higher tangent lifts of geometric structures, we introduce concepts of weighted structures for various geometric objects on a manifold F equipped with a homogeneity structure. The latter is a smooth action on F of the monoid of multiplicative reals. Vector bundles are particular cases of homogeneity structures with the action being the multiplication of vectors by reals, and weighted structures on them we call VB-structures. In the case of Lie algebroids and Lie groupoids, the weighted structures include the concepts of VB-algebroids and VB-groupoids, intensively studied recently in the literature. Investigating various weighted structures, we prove some interesting results about their properties.

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arXiv2021

Symplectic $\mathbb{Z}_2^n$-manifolds

Roughly speaking, $\mathbb{Z}_2^n$-manifolds are `manifolds' equipped with $\mathbb{Z}_2^n$-graded commutative coordinates with the sign rule being determined by the scalar product of their $\mathbb{Z}_2^n$-degrees. We examine the notion of a symplectic $\mathbb{Z}_2^n$-manifold, i.e., a $\mathbb{Z}_2^n$-manifold equipped with a symplectic two-form that may carry non-zero $\mathbb{Z}_2^n$-degree. We show that the basic notions and results of symplectic geometry generalise to the `higher graded' setting, including a generalisation of Darboux's theorem.

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arXiv2021

Nonassociative analogs of Lie groupoids

We introduce nonassociative geometric objects generalising naturally Lie groupoids and called (smooth) quasiloopoids and loopoids. We prove that the tangent bundles of smooth loopoids are canonically smooth loopoids again (it is nontrivial in the case of loopoids). We show also that this is not true if the cotangent bundles are concerned. After providing a few natural constructions, we show how the Lie-like functor associates with loopoids skew-algebroids and almost Lie algebroids and how discrete mechanics on Lie groupoids can be reformulated in the nonassociative case.

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