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Andrzej Derdzinski

Publications and source records attributed to Andrzej Derdzinski.

At least 37 records · Page 2Linked to original sources
arXiv2023

Rank-one ECS manifolds of dilational type

We study ECS manifolds, that is, pseudo-Riemannian manifolds with parallel Weyl tensor which are neither conformally flat nor locally symmetric. Every ECS manifold has rank 1 or 2, the rank being the dimension of a distinguished null parallel distribution discovered by Olszak, and a rank-one ECS manifold may be called translational or dilational, depending on whether the holonomy group of a natural flat connection in the Olszak distribution is finite or infinite. Some such manifolds are in a natural sense generic, which refers to the algebraic structure of the Weyl tensor. Known examples of compact ECS manifolds, in every dimension greater than 4, are all of rank 1 and translational, some of them generic, none of them locally homogeneous. As we show, generic compact rank-one ECS manifolds must be translational or locally homogeneous, provided that they arise as isometric quotients of a specific class of explicitly constructed "model" manifolds. This result is relevant since the clause starting with "provided that" may be dropped: according to a theorem which we prove in a forthcoming paper, the models just mentioned include the isometry types of the pseudo-Riemannian universal coverings of all generic compact rank-one ECS manifolds.

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arXiv2023

New examples of compact Weyl-parallel manifolds

We prove the existence of compact pseudo-Riemannian manifolds with parallel Weyl tensor which are neither conformally flat nor locally symmetric, and represent all indefinite metric signatures in all dimensions $\,n\ge5$. Until now such manifolds were only known to exist in dimensions $\,n=3j+2$, where $\,j\,$ is any positive integer [11]. As in [11], our examples are diffeomorphic to nontrivial torus bundles over the circle and arise from a quotient-manifold construction applied to suitably chosen discrete isometry groups of diffeomorphically-Euclidean "model" manifolds.

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arXiv2022

Null-projectability of Levi-Civita connections

We study the natural property of projectability of a torsion-free connection along a foliation on the underlying manifold, which leads to a projected torsion-free connection on a local leaf space, focusing on projectability of Levi-Civita connections of pseudo-Riemannian metric along foliations tangent to null parallel distributions. For the neutral metric signature and mid-dimensional distributions, Afifi showed in 1954 that projectability of the Levi-Civita connection characterizes, locally, the case of Patterson and Walker's Riemann extension metrics. We extend this correspondence to null parallel distributions of any dimension, introducing a suitable generalization of Riemann extensions.

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arXiv2022

Harmonic-curvature warped products over surfaces

For warped products with harmonic curvature, nonconstant warping functions $ϕ$, and compact two-dimensional bases $(M,h)$, we establish a dichotomy: either the Gaussian curvature $K$ of the metric $g=ϕ^{-2}h$ is constant and negative, or $ϕ$ equals a specific elementary function of $K$, also depending on the dimension $p$ and Einstein constant $\varepsilon$ of the fibre. In both cases the fibre must be an Einstein manifold with $p>1$ and $\varepsilon>0$, while the function $f=ϕ^{p/2}$ satisfies a Yamabe-type second-order differential equation on $(M,g)$. We prove that both possibilities are realized on every closed orientable surface of genus greater than $1$, and in the latter case -- which also occurs on the $2$-sphere and real projective plane -- the metrics in question constitute uncountably many distinct homothety types.

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arXiv2021

Subspace foliations and collapse of closed flat manifolds

We study relations between certain totally geodesic foliations of a closed flat manifold and its collapsed Gromov-Hausdorff limits. Our main results explicitly identify such collapsed limits as flat orbifolds, and provide algebraic and geometric criteria to determine whether they are singular.

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arXiv2019

Maximally-warped metrics with harmonic curvature

We describe the local structure of Riemannian manifolds with harmonic curvature which admit a maximum number, in a well-defined sense, of local warped-product decompositions, and at the same time their Ricci tensor has, at some point, only simple eigenvalues. We also prove that, in every given dimension greater than two, the local-isometry types of such manifolds form a finite-dimensional moduli space, and a nonempty open subset of this moduli space is realized by locally irreducible complete metrics which are neither Ricci-parallel, nor -- for dimensions greater than three -- conformally flat.

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arXiv2019

Flat manifolds and reducibility

Hiss and Szczepański proved in 1991 that the holonomy group of any compact flat Riemannian manifold, of dimension at least two, acts reducibly on the rational span of the Euclidean lattice associated with the manifold via the first Bieberbach theorem. Geometrically, their result states that such a manifold must admit a nonzero proper parallel distribution with compact leaves. We study algebraic and geometric properties of the sublattice-spanned holonomy-invariant subspaces that exist due to the above theorem, and of the resulting compact-leaf foliations of compact flat manifolds. The class consisting of the former subspaces, in addition to being closed under spans and intersections, also turns out to admit (usually nonorthogonal) complements. As for the latter foliations, we provide descriptions, first -- and foremost -- of the intrinsic geometry of their generic leaves in terms of that of the original flat manifold and, secondly -- as an essentially obvious afterthought -- of the leaf-space orbifold. The general conclusions are then illustrated by examples in the form of generalized Klein bottles.

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arXiv2017

Ricci solitons

English translation of "Solitony Ricciego" (Wiadomości Matematyczne 48, 2012, no. 1, pp. 1-32). Despite the general-sounding title, the text covers just a few narrow topics: Perelman's proof of the fact that compact Ricci solitons are of the gradient type, and a detailed unified description of Page's and Berard Bergery's Einstein manifolds on the one hand, and Koiso's and Cao's Kähler-Ricci solitons on the other, as the Ricci solitons arising via conformal changes from certain special Kähler manifolds.

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arXiv2017

Teichmüller theory and collapse of flat manifolds

We provide an algebraic description of the Teichmüller space and moduli space of flat metrics on a closed manifold or orbifold and study its boundary, which consists of (isometry classes of) flat orbifolds to which the original object may collapse. It is also shown that every closed flat orbifold can be obtained by collapsing closed flat manifolds, and the collapsed limits of closed flat 3-manifolds are classified.

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arXiv2017

Kähler manifolds with geodesic holomorphic gradients

A vector field on a Riemannian manifold is called geodesic if its integral curves are reparametrized geodesics. We classify compact Kähler manifolds admitting nontrivial real-holomorphic geodesic gradient vector fields that satisfy an additional integrability condition. They are all biholomorphic to bundles of complex projective spaces.

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arXiv2013

Curvature spectra of simple Lie groups

The Killing form β of a real (or complex) semisimple Lie group G is a left-invariant pseudo-Riemannian (or, respectively, holomorphic) Einstein metric. Let Ω denote the multiple of its curvature operator, acting on symmetric 2-tensors, with the factor chosen so that Ωβ=2β. The result of Meyberg [8], describing the spectrum of Ω in complex simple Lie groups G, easily implies that 1 is not an eigenvalue of Ω in any real or complex simple Lie group G except those locally isomorphic to SU(p,q), or SL(n,R), or SL(n,C) or, for even n only, SL(n/2,H), where p\ge q\ge0 and p+q=n>2. Due to the last conclusion, on simple Lie groups G other the ones just listed, nonzero multiples of the Killing form β are isolated among left-invariant Einstein metrics. Meyberg's theorem also allows us to understand the kernel of Λ, which is another natural operator. This in turn leads to a proof of a known, yet unpublished, fact: namely, that a semisimple real or complex Lie algebra with no simple ideals of dimension 3 is essentially determined by its Cartan three-form.

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arXiv2013

Indefinite Einstein metrics on simple Lie groups

The set E of Levi-Civita connections of left-invariant pseudo-Riemannian Einstein metrics on a given semisimple Lie group always includes D, the Levi-Civita connection of the Killing form. For the groups SU(l,j) (or SL(n,R), or SL(n,C) or, if n is even, SL(n/2,IH)), with 0<=j<=l and j+l>2 (or, n>2), we explicitly describe the connected component C of E, containing D. It turns out that C, a relatively-open subset of E, is also an algebraic variety of real dimension 2lj (or, real/complex dimension [n^2/2] or, respectively, real dimension 4[n^2/8]), forming a union of (j + 1)(j + 2)/2 (or, [n^2]+1 or, respectively, [n/4] + 1) orbits of the adjoint action. In the case of SU(n) one has 2lj=0, so that a positive-definite multiple of the Killing form is isolated among suitably normalized left-invariant Riemannian Einstein metrics on SU(n).

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arXiv2011

Two-jets of conformal fields along their zero sets

The connected components of the zero set of any conformal vector field $v$, in a pseudo-Riemannian manifold $(M,g)$ of arbitrary signature, are of two types, which may be called `essential' and `nonessential'. The former consist of points at which $v$ is essential, that is, cannot be turned into a Killing field by a local conformal change of the metric. In a component of the latter type, points at which $v$ is nonessential form a relatively-open dense subset that is at the same time a totally umbilical submanifold of $(M,g)$. An essential component is always a null totally geodesic submanifold of $(M,g)$, and so is the set of those points in a nonessential component at which $v$ is essential (unless this set, consisting precisely of all the singular points of the component, is empty). Both kinds of null totally geodesic submanifolds arising here carry a 1-form, defined up to multiplications by functions without zeros, which satisfies a projective version of the Killing equation. The conformal-equivalence type of the 2-jet of $v$ is locally constant along the nonessential submanifold of a nonessential component, and along an essential component on which the distinguished 1-form is nonzero. The characteristic polynomial of the 1-jet of $v$ is always locally constant along the zero set.

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arXiv2011

Special biconformal changes of Kähler surface metrics

The term "special biconformal change" refers, basically, to the situation where a given nontrivial real-holomorphic vector field on a complex manifold is a gradient relative to two Kähler metrics, and, simultaneously, an eigenvector of one of the metrics treated, with the aid of the other, as an endomorphism of the tangent bundle. A special biconformal change is called nontrivial if the two metrics are not each other's constant multiples. For instance, according to a 1995 result of LeBrun, a nontrivial special biconformal change exists for the conformally-Einstein Kähler metric on the two-point blow-up of the complex projective plane, recently discovered by Chen, LeBrun and Weber; the real-holomorphic vector field involved is the gradient of its scalar curvature. The present paper establishes the existence of nontrivial special biconformal changes for some canonical metrics on Del Pezzo surfaces, viz. Kähler-Einstein metrics (when a nontrivial holomorphic vector field exists), non-Einstein Kähler-Ricci solitons, and Kähler metrics admitting nonconstant Killing potentials with geodesic gradients.

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arXiv2010

Zeros of conformal fields in any metric signature

The connected components of the zero set of any conformal vector field, in a pseudo-Riemannian manifold of arbitrary signature, are shown to be totally umbilical conifold varieties, that is, smooth submanifolds except possibly for some quadric singularities. The singularities occur only when the metric is indefinite, including the Lorentzian case. This generalizes an analogous result in the Riemannian case, due to Belgun, Moroianu and Ornea (2010).

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arXiv2009

Noncompactness and maximum mobility of type III Ricci-flat self-dual neutral Walker four-manifolds

It is shown that a self-dual neutral Einstein four-manifold of Petrov type III, admitting a two-dimensional null parallel distribution compatible with the orientation, cannot be compact or locally homogeneous, and its maximum possible degree of mobility is 3. Diaz-Ramos, Garcia-Rio and Vazquez-Lorenzo found a general coordinate form of such manifolds. The present paper also provides a modified version of that coordinate form, valid in a suitably defined generic case and, in a sense, "more canonical" than the usual formulation. Moreover, the local-isometry types of manifolds as above having the degree of mobility equal to 3 are classified. Further results consist in explicit descriptions, first, of the kernel and image of the Killing operator for any torsionfree surface connection with everywhere-nonzero, skew-symmetric Ricci tensor, and, secondly, of a moduli curve for surface connections with the properties just mentioned that are, in addition, locally homogeneous. Finally, hyperbolic plane geometry is used to exhibit examples of codimension-two foliations on compact manifolds of dimensions greater than 2 admitting a transversal torsionfree connection, the Ricci tensor of which is skew-symmetric and nonzero everywhere. No such connection exists on any closed surface, so that there are no analogous examples in dimension 2.

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