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Amir Babak Aazami

Publications and source records attributed to Amir Babak Aazami.

At least 19 recordsLinked to original sources
arXiv2026

Higher Self-Duality

We use the totally trace-free component of Thorpe's middle curvature operator $R_{2n}\colonΛ^{2n}\to Λ^{2n}$ to define higher self-duality on oriented Riemannian $4n$-manifolds. We show that higher self-dual metrics are absolute minimizers of a natural conformal energy, whose Bach tensor is symmetric, trace-free, and divergence-free, resolving a question posed by Kastor. We further show that the self- and anti-self-dual blocks give a signed-square formula for the top Pontryagin density. Compact self-dual examples include the Fubini-Study metric, though Einstein metrics need not be higher Bach-flat in general.

math.DG↗
arXiv2026

Hodge Splittings and Einstein 4-manifolds

On an oriented $4$-manifold, we study pairs of Riemannian metrics $(g,h)$ for which the curvature tensor of $g$ preserves the Hodge splitting determined by $h$. This extends the Einstein condition in dimension four, which is recovered when $h$ is conformal to $g$. We prove that such pairs satisfy a generalized Hitchin-Thorpe inequality, which reduces to the classical one when $h$ is conformal to $g$. We then exhibit a pair $(g,h)$ on $\#_5\mathbb{CP}^2$, which violates Hitchin-Thorpe and hence admits no Einstein metric, thus showing that our condition is indeed broader than the Einstein condition.

math.DG↗
arXiv2026

$p$-orderings: From Slater to Kemeny-Young to Ranked Pairs

We introduce a family of ranking rules for preferential elections, called $p$-orderings, obtained by minimizing the $p$-norm of the pairwise majority margins that disagree with a given ranking. This family is defined on the margin-of-victory matrix of the election and has the Slater orderings as its limit as $p \to 0^+$, includes the Kemeny-Young rule as the case $p=1$, and coincides with Ranked Pairs for all sufficiently large $p$. We show that, under natural assumptions of scale invariance, dependence only on margin magnitude, and monotonicity with respect to margin size, the score function underlying this construction is uniquely of the form $c|x|^p$. Thus Ranked Pairs arises as the eventual large-$p$ member of a canonical family of margin-based ranking rules.

econ.TH↗
arXiv2026

On normal forms of gradient Ricci 4-solitons

In this note we analyze the normal form of the operator $\hat{R} + \frac{1}{2}\hat{H}$ of a gradient Ricci 4-soliton in Cao & Tran. In particular, we show that the curvature operator $\hat{R}$ of the Koiso-Cao soliton inherits this normal form. By work of D. Johnson, this yields a normal form for the curvature operator of the Koiso-Cao soliton relative to the space of algebraic Kähler curvature operators.

math.DG↗
arXiv2025

Geometry via Plane wave limits

Utilizing the covariant formulation of Penrose's plane wave limit by Blau et~al., we construct for any semi-Riemannian metric $g$ a family of "plane wave limits." These limits are taken along any geodesic of $g$, yield simpler metrics of Lorentzian signature, and are isometric invariants. We show that they generalize Penrose's limit to the semi-Riemannian regime and, in certain cases, encode $g$'s tensorial geometry and its geodesic deviation. As an application of the latter, we partially extend a well known result by Hawking & Penrose to the semi-Riemannian regime: On any semi-Riemannian manifold, if the Ricci curvature is nonnegative along any complete geodesic without conjugate points that is "causally independent" (in a sense we make precise), then the curvature tensor along that geodesic must vanish in all normal directions. A Morse Index Theorem is also proved for such geodesics.

math.DG↗
arXiv2024

Petrov Types for the Weyl Tensor via the Riemannian-to-Lorentzian Bridge

We analyze oriented Riemannian 4-manifolds whose Weyl tensors $W$ satisfy the conformally invariant condition $W(T,\cdot,\cdot,T) = 0$ for some nonzero vector $T$. While this can be algebraically classified via $W$'s normal form, we find a further geometric classification by deforming the metric into a Lorentzian one via $T$. We show that such a $W$ will have the analogue of Petrov Types from general relativity, that only Types I and D can occur, and that each is completely determined by the number of critical points of $W$'s associated Lorentzian quadratic form. A similar result holds for the Lorentzian version of this question, with $T$ timelike.

math.DG↗
arXiv2024

The Eisenhart Lift and Hamiltonian Systems

It is well known in general relativity that trajectories of Hamiltonian systems lift to geodesics of pp-wave spacetimes, an example of a more general phenomenon known as the "Eisenhart lift." We review and expand upon the benefits of this correspondence for dynamical systems theory. One benefit is the use of curvature and conjugate points to study the stability of Hamiltonian systems. Another benefit is that this lift unfolds a Hamiltonian system into a family of ODEs akin to a moduli space. One such family arises from the conformal invariance of lightlike geodesics, by which any Hamiltonian system unfolds into a "conformal class" of non-diffeomorphic ODEs with solutions in common. By utilizing higher-index versions of pp-waves, a similar lift and conformal class are shown to exist for certain second-order complex ODEs. Another such family occurs by lifting to a Riemannian metric that is dual to a pp-wave, a process that in certain cases yields a "square root" for the Hamiltonian. We prove a two-point boundary result for the family of ODEs arising from this lift, as well as the existence of a constant of the motion generalizing conservation of energy.

math.DG↗
arXiv2024

Obstructions to distinguished Riemannian metrics via Lorentzian geometry

We approach the problem of finding obstructions to curvature distinguished Riemannian metrics by considering Lorentzian metrics to which they are dual in a suitable sense. Obstructions to the latter then yield obstructions to the former. This framework applies both locally and globally, including to compact manifolds, and is sensitive to various aspects of curvature. Here we apply it in two different ways. First, by embedding a Riemannian manifold into a Lorentzian one and utilizing Penrose's "plane wave limit," we find necessary local conditions, in terms of the Hessian of just one function, for large classes of Riemannian metrics to contain within them those that have parallel Ricci tensor, or are Ricci-flat, or are locally symmetric. Second, by considering Riemannian metrics dual to constant curvature Lorentzian metrics via a type of Wick rotation, we are able to rule out the existence of a family of compact Riemannian manifolds (in all dimensions) that deviate from constant curvature in a precise sense.

math.DG↗
arXiv2024

On the Petrov Type of a 4-manifold

On an oriented 4-manifold, we examine the geometry that arises when the curvature operator of a Riemannian or Lorentzian metric $g$ commutes, not with its own Hodge star operator, but rather with that of another semi-Riemannian metric $h$ that is a suitable deformation of $g$. We classify the case when one of these metrics is Riemannian and the other Lorentzian by generalizing the concept of Petrov Type from general relativity; the case when $h$ is split-signature is also examined. The "generalized Petrov Types" so obtained are shown to relate to the critical points of $g$'s sectional curvature, and sometimes yield unique normal forms. They also carry topological information independent of the Hitchin-Thorpe inequality, and yield a direct geometric formulation of "almost-Einsten" metric via the Ricci or sectional curvature of $g$.

math.DG↗
arXiv2023

Killing vector fields on Riemannian and Lorentzian 3-manifolds

We give a complete local classification of all Riemannian 3-manifolds $(M,g)$ admitting a nonvanishing Killing vector field $T$. We then extend this classification to timelike Killing vector fields on Lorentzian 3-manifolds, which are automatically nonvanishing. The two key ingredients needed in our classification are the scalar curvature $S$ of $g$ and the function $\text{Ric}(T,T)$, where $\text{Ric}$ is the Ricci tensor; in fact their sum appears as the Gaussian curvature of the quotient metric obtained from the action of $T$. Our classification generalizes that of Sasakian structures, which is the special case when $\text{Ric}(T,T) = 2$. We also give necessary, and separately, sufficient conditions, both expressed in terms of $\text{Ric}(T,T)$, for $g$ to be locally conformally flat. We then move from the local to the global setting, and prove two results: in the event that $T$ has unit length and the coordinates derived in our classification are globally defined on $\mathbb{R}^3$, we give conditions under which $S$ completely determines when the metric will be geodesically complete. In the event that the 3-manifold $M$ is compact, we give a condition stating when it admits a metric of constant positive sectional curvature.

math.DG↗
arXiv2022

Exact Parallel Waves in General Relativity

We conduct a review of the basic definitions and the principal results in the study of wavelike spacetimes, that is spacetimes whose metric models massless radiation moving at the speed of light, focusing in particular on those geometries with parallel rays. In particular, we motivate and connect their various definitions, outline their coordinate descriptions and present some classical results in their study in a language more accessible to modern readers, including the existence of "null coordinates" and the construction of Penrose limits. We also present a thorough summary of recent work on causality in pp-waves, and describe progress in addressing an open question in the field - the Ehlers-Kundt conjecture.

gr-qc↗
arXiv2022

Riemannian counterparts to Lorentzian space forms

On a smooth $n$-manifold $M$ with $n \geq 3$, we study pairs $(g,T)$ consisting of a Riemannian metric $g$ and a unit length closed vector field $T$. Motivated by how Ricci solitons generalize Einstein metrics via a distinguished vector field, we propose to generalize space forms by considering those pairs $(g,T)$ whose corresponding Lorentzian metric $g_{\scriptscriptstyle L} = g - 2T^{\flat} \otimes T^{\flat}$ has constant curvature. We show by examples that such pairs exist when $M$ is noncompact, and that complete metrics exist among them. When $M$ is compact, however, the situation is more rigid. In the compact setting, we prove that the only pairs $(g,T)$ whose corresponding Lorentzian metric $g_{\scriptscriptstyle L}$ is a space form are those where $(M,g)$ is flat and its universal covering splits isometrically as a product $\mathbb{R} \times N$. The nonexistence of compact Lorentzian spherical space forms plays a key role in our proof.

math.DG↗
arXiv2022

Almost Kähler metrics and pp-wave spacetimes

We establish a one-to-one correspondence between a class of strictly almost Kähler metrics on the one hand, and Lorentzian pp-wave spacetimes on the other; the latter metrics are well known in general relativity, where they model radiation propagating at the speed of light. Specifically, we construct families of complete almost Kähler metrics by deforming pp-waves via their propagation wave vector. The almost Kähler metrics we obtain exist in all dimensions $2n \geq 4$, and are defined on both $\mathbb{R}^{2n}$ and $\mathbb{S}^1\times\mathbb{S}^1 \times M$, where $M$ is any closed almost Kähler manifold; they are not warped products, they include noncompact examples with constant negative scalar curvature, and all of them have the property that their fundamental 2-forms are also co-closed with respect to the Lorentzian pp-wave metric. Finally, we further deepen this relationship between almost Kähler and Lorentzian geometry by utilizing Penrose's "plane wave limit," by which every spacetime has, locally, a pp-wave metric as a limit: using Penrose's construction, we show that in all dimensions $2n \geq 4$, every Lorentzian metric admits, locally, an almost Kähler metric of this form as a limit.

math.DG↗
arXiv2022

Finsler pp-waves and the Penrose Limit

The Penrose plane wave limit is a remarkable property of Lorentzian spacetimes. Here, we discuss its extension to Finsler spacetimes by introducing suitable lightlike coordinates and adapting the Lorentzian definition of pp-waves. New examples of such Finsler pp-waves are also presented.

math.DG↗
arXiv2021

On the Einstein condition for Lorentzian 3-manifolds

It is well known that in Lorentzian geometry there are no compact spherical space forms; in dimension 3, this means there are no closed Einstein 3-manifolds with positive Einstein constant. We generalize this fact here, by proving that there are also no closed Lorentzian 3-manifolds $(M,g)$ whose Ricci tensor satisfies $$ \text{Ric} = fg+(f-λ)T^{\flat}\otimes T^{\flat}, $$ for any unit timelike vector field $T$, any positive constant $λ$, and any smooth function $f$ that never takes the values $0,λ$. (Observe that this reduces to the positive Einstein case when $f = λ$.) We show that there is no such obstruction if $λ$ is negative. Finally, the "borderline" case $λ= 0$ is also examined: we show that if $λ= 0$ and $f > 0$, then $(M,g)$ must be isometric to $(\mathbb{S}^1\!\times \!N,-dt^2\oplus h)$ with $(N,h)$ a Riemannian manifold.

math.DG↗
arXiv2020

Canonical Kähler metrics on classes of Lorentzian $4$-manifolds

Conditions for the existence of Kähler-Einstein metrics and central Kähler metrics [MS] along with examples, both old and new, are given on classes of Lorentzian $4$-manifolds with two distinguished vector fields. The results utilize the general construction [AM] of Kähler metrics on such manifolds. The examples include both complete and incomplete metrics, and some reside on Lie groups associated to four types of Lie algebras. An appendix includes a similar construction for scalar-flat Kähler metrics.

math.DG↗
arXiv2020

Kähler metrics via Lorentzian Geometry in dimension four

Given a semi-Riemannian $4$-manifold $(M,g)$ with two distinguished vector fields satisfying properties determined by their shear, twist and various Lie bracket relations, a family of Kähler metrics $g_K$ is constructed, defined on an open set in $M$, which coincides with $M$ in many typical examples. Under certain conditions $g$ and $g_K$ share various properties, such as a Killing vector field or a vector field with a geodesic flow. In some cases the Kähler metrics are complete. The Ricci and scalar curvatures of $g_K$ are computed under certain assumptions in terms of data associated to $g$. Many examples are described, including classical spacetimes in warped products, for instance de Sitter spacetime, as well as gravitational plane waves, metrics of Petrov type $D$ such as Kerr and NUT metrics, and metrics for which $g_K$ is an SKR metric. For the latter an inverse ansatz is described, constructing $g$ from the SKR metric.

math.DG↗