arXiv subjects

Jonathan Rosenberg

Publications and source records attributed to Jonathan Rosenberg.

At least 19 recordsLinked to original sources

arXiv2025

Classification of spin$^c$ manifolds with generalized positive scalar curvature

Suppose $M$ is a closed $n$-dimensional spin$^c$ manifold with spin$^c$ structure $\sigma$ and associated spin$^c$ line bundle $L$. If one fixes a Riemannian metric $g$ on $M$ and a connection $\nabla_L$ on $L$, the generalized scalar curvature $R^{\text{gen}}$ of $(M,L)$ is $R_g - 2|\Omega_L|_{\text{op}}$, where $|\Omega_L|_{\text{op}}$ is the pointwise operator norm of the curvature $2$-form $\Omega_L$ of $\nabla_L$, acting on spinors. In a previous paper, we showed that positivity of $R^{\text{gen}}$ is obstructed by the non-vanishing of the index of the spin$^c$ Dirac operator on $(M,g,L,\nabla_L)$, and that in some cases, the vanishing of this index guarantees the existence of a pair $(g,\nabla_L)$ with positive generalized scalar curvature. Building on this and on surgery techniques inspired by those that have been developed in the theory of positive scalar curvature on spin manifolds, we show that if $\dim M = n \ge 5$, if the fundamental group $\pi$ of $M$ is in a large class including surface groups and finite groups with periodic cohomology, and if $M$ is totally non-spin (meaning that the universal cover is not spin), then $(M,L)$ admits positive generalized scalar curvature if and only if the generalized $\alpha$-invariant of $(M,L)$ vanishes in the $K$-homology group $K_n(B\pi)$. We also develop an analogue of Stolz's sequence for computing the group of concordance classes of positive generalized scalar curvature metrics, and connect this to the analytic surgery sequence of Roe and Higson. Finally, we give a number of applications to moduli spaces of positive generalized scalar curvature metrics.

math.DG↗

arXiv2025

Orientifolds for F-theory on K3 Surfaces

We study F-theory orientifolds, starting with products of two elliptic curves, but focusing mostly on a family of K3 surfaces, lattice polarized by the rank-17 lattice $\langle 8 \rangle \oplus 2D_8(-1)$, generalizing the family (to which it degenerates) of Kummer surfaces of products of two non-isogenous elliptic curves. After a thorough study of the complex geometry of this family and its elliptic fibrations, we proceed to study real structures on the K3 surfaces in the family which are equivariant with respect to an elliptic fibration. We also study the physics of the associated F-theory orientifolds with a particular focus on the impact of the real structure on the charge spectrum. We also study how these orientifolds degenerate to the case of isotrivial Kummer surface fibrations.

hep-th↗

arXiv2025

Complex K-theory of 4-complexes

This short note summarizes a number of facts about the ring $K^0(X)$ for $X$ a $4$-dimensional CW-complex. Unusual features of this dimension are that every complex vector bundle is determined up to stable isomorphism by its Chern classes, that every even cohomology class arises as a Chern class of a vector bundle, and that $K^0(X)$ is completely determined as a ring by knowledge of the even-dimensional cohomology ring $H^{\text{even}}(X; \mathbb Z)$. (All of these fail in high dimensions.)

math.KT↗

arXiv2024

Generalized positive scalar curvature on spin$^c$ manifolds

Let $(M,L)$ be a (compact) non-spin spin$^c$ manifold. Fix a Riemannian metric $g$ on $M$ and a connection $A$ on $L$, and let $D_L$ be the associated spin$^c$ Dirac operator. Let $R^{tw}_{(g,A)}:=R_g + 2ic(Ω)$ be the twisted scalar curvature (which takes values in the endomorphisms of the spinor bundle), where $R_g$ is the scalar curvature of $g$ and $2ic(Ω)$ comes from the curvature $2$-form $Ω$ of the connection $A$. Then the Lichnerowicz-Schrödinger formula for the square of the Dirac operator takes the form $D_L^2 =\nabla^*\nabla+\frac{1}{4}R^{tw}_{(g,A)}$. In a previous work we proved that a closed non-spin simply-connected spin$^c$-manifold $(M,L)$ of dimension $n\geq 5$ admits a pair $(g,A)$ such that $R^{tw}_{(g,A)}>0$ if and only if the index $α^c(M,L):=\text{ind}\, D_L$ vanishes in $K_n$. In this paper we introduce a scalar-valued generalized scalar curvature $R^{gen}_{(g,A)}:=R_g - 2|Ω|_{op}$, where $|Ω|_{op}$ is the pointwise operator norm of Clifford multiplication $c(Ω)$, acting on spinors. We show that the positivity condition on the operator $R^{tw}_{(g,A)}$ is equivalent to the positivity of the scalar function $R^{gen}_{g,A}$. We prove a corresponding trichotomy theorem concerning the curvature $R^{gen}_{(g,A)}$, and study its implications. We also show that the space $\mathcal{R}^{gen+}(M,L)$ of pairs $(g,A)$ with $R^{gen}_{(g,A)}>0$ has non-trivial topology, and address a conjecture about non-triviality of the ``index difference'' map.

math.DG↗

arXiv2024

New results on tilings via cup products and Chern characters on tiling spaces

We study the cohomology rings of tiling spaces $\Omega$ given by cubical substitutions. While there have been many calculations before of cohomology groups of such tiling spaces, the innovation here is that we use computer-assisted methods to compute the cup-product structure. This leads to examples of substitution tilings with isomorphic cohomology groups but different cohomology rings. Part of the interest in studying the cup product comes from Bellissard's gap-labeling conjecture, which is known to hold in dimensions $\le 3$, but where a proof is known in dimensions $\ge 4$ only when the Chern character from $K^0(\Omega)$ to $H^*(\Omega,\mathbb{Q})$ lands in $H^*(\Omega,\mathbb{Z})$. Computation of the cup product on cohomology often makes it possible to compute the Chern character. We introduce a natural generalization of the gap-labeling conjecture, called the equivariant gap-labeling conjecture, which applies to tilings with a finite symmetry group. Again this holds in dimensions $\le 3$, but we are able to show that it fails in general in dimensions $\ge 4$. This, plus some of our cup product calculations, makes it plausible that the gap-labeling conjecture might fail in high dimensions.

math.DS↗

arXiv2024

Twisted cohomology

We discuss twisted cohomology, not just for ordinary cohomology but also for $K$-theory and other exceptional cohomology theories, and discuss several of the applications of these in mathematical physics. Our list of applications is by no means exhaustive, but we are hoping that it is extensive enough to give the reader a feel for the possible applications of twisted theories in many different contexts. We also give many suggestions for further reading, but this subject has now expanded to the point where the bibliography is necessarily very incomplete.

math.AT↗

arXiv2023

Positive scalar curvature on manifolds with fibered singularities

A (compact) manifold with fibered $P$-singularities is a (possibly) singular pseudomanifold $M_Σ$ with two strata: an open nonsingular stratum $\mathring M$ (a smooth open manifold) and a closed stratum $βM$ (a closed manifold of positive codimension), such that a tubular neighborhood of $βM$ is a fiber bundle with fibers each looking like the cone on a fixed closed manifold $P$. We discuss what it means for such an $M_Σ$ with fibered $P$-singularities to admit an appropriate Riemannian metric of positive scalar curvature, and we give necessary and sufficient conditions (the necessary conditions based on suitable versions of index theory, the sufficient conditions based on surgery methods and homotopy theory) for this to happen when the singularity type $P$ is either $\mathbb Z/k$ or $S^1$, and $M$ and the boundary of the tubular neighborhood of the singular stratum are simply connected and carry spin structures. Along the way, we prove some results of perhaps independent interest, concerning metrics on spin$^c$ manifolds with positive "twisted scalar curvature," where the twisting comes from the curvature of the spin$^c$ line bundle.

math.DG↗

arXiv2022

Derived categories of curves of genus one and torsors over abelian varieties

Suppose $C$ is a smooth projective curve of genus 1 over a perfect field $F$, and $E$ is its Jacobian. In the case that $C$ has no $F$-rational points, so that $C$ and $E$ are not isomorphic, $C$ is an $E$-torsor with a class $\delta(C)\in H^1(\text{Gal}(\bar F/F), E(\bar F))$. Then $\delta(C)$ determines a class $\beta \in \text{Br}(E)/\text{Br}(F)$ and there is a Fourier-Mukai equivalence of derived categories of (twisted) coherent sheaves $\mathcal D(C) \xrightarrow{\cong} \mathcal D(E, \beta^{-1})$. We generalize this result to higher dimensions; namely, we prove it also for torsors over abelian varieties.

math.AG↗

arXiv2022

Positive scalar curvature on manifolds with boundary and their doubles

This paper is about positive scalar curvature on a compact manifold $X$ with non-empty boundary $\partial X$. In some cases, we completely answer the question of when $X$ has a positive scalar curvature metric which is a product metric near $\partial X$, or when $X$ has a positive scalar curvature metric with positive mean curvature on the boundary, and more generally, we study the relationship between boundary conditions on $\partial X$ for positive scalar curvature metrics on $X$ and the positive scalar curvature problem for the double $M=\operatorname{Dbl}(X,\partial X)$.

math.DG↗

arXiv2021

Positive Scalar Curvature on Spin Pseudomanifolds: the Fundamental Group and Secondary Invariants

In this paper we continue the study of positive scalar curvature (psc) metrics on a depth-1 Thom-Mather stratified space $M_Σ$ with singular stratum $βM$ (a closed manifold of positive codimension) and associated link equal to $L$, a smooth compact manifold. We briefly call such spaces manifolds with $L$-fibered singularities. Under suitable spin assumptions we give necessary index-theoretic conditions for the existence of wedge metrics of positive scalar curvature. Assuming in addition that $L$ is a simply connected homogeneous space of positive scalar curvature, $L=G/H$, with the semisimple compact Lie group $G$ acting transitively on $L$ by isometries, we investigate when these necessary conditions are also sufficient. Our main result is that our conditions are indeed sufficient for large classes of examples, even when $M_Σ$ and $βM$ are not simply connected. We also investigate the space of such psc metrics and show that it often splits into many cobordism classes.

math.DG↗

arXiv2021

Positive scalar curvature on $\mathbf{Pin}^\pm$- and $\mathbf{Spin}^c$-manifolds

It is well-known that spin structures and Dirac operators play a crucial role in the study of positive scalar curvature metrics (psc-metrics) on compact manifolds. Here we consider a class of non-spin manifolds with "almost spin" structure, namely those with spin$^c$ or pin$^\pm$-structures. It turns out that in those cases (under natural assumptions on such a manifold $M$), the index of a relevant Dirac operator completely controls existence of a psc-metric which is $S^1$- or $C_2$-invariant near a "special submanifold" $B$ of $M$. This submanifold $B\subset M$ is dual to the complex (respectively, real) line bundle $L$ which determines the spin$^c$ or pin$^\pm$ structure on $M$. We also show that these manifold pairs $(M,B)$ can be interpreted as "manifolds with fibered singularities" equipped with "well-adapted psc-metrics". This survey is based on our recent work as well as on our joint work with Paolo Piazza.

math.DG↗

arXiv2021

Positive scalar curvature on simply connected spin pseudomanifolds

Let $M_Σ$ be an $n$-dimensional Thom-Mather stratified space of depth $1$. We denote by $βM$ the singular locus and by $L$ the associated link. In this paper we study the problem of when such a space can be endowed with a wedge metric of positive scalar curvature. We relate this problem to recent work on index theory on stratified spaces, giving first an obstruction to the existence of such a metric in terms of a wedge $α$-class $α_w (M_Σ)\in KO_n$. In order to establish a sufficient condition we need to assume additional structure: we assume that the link of $M_Σ$ is a homogeneous space of positive scalar curvature, $L=G/K$, where the semisimple compact Lie group $G$ acts transitively on $L$ by isometries. Examples of such manifolds include compact semisimple Lie groups and Riemannian symmetric spaces of compact type. Under these assumptions, when $M_Σ$ and $βM$ are spin, we reinterpret our obstruction in terms of two $α$-classes associated to the resolution of $M_Σ$, $M$, and to the singular locus $βM$. Finally, when $M_Σ$, $βM$, $L$, and $G$ are simply connected and $\dim M$ is big enough, and when some other conditions on $L$ (satisfied in a large number of cases) hold, we establish the main result of this article, showing that the vanishing of these two $α$-classes is also sufficient for the existence of a well-adapted wedge metric of positive scalar curvature.

math.DG↗

arXiv2020

Relating Diffraction and Spectral Data of Aperiodic Tilings: Towards a Bloch theorem

The purpose of this paper is to show the relationship in all dimensions between the structural (diffraction pattern) aspect of tilings (described by Čech cohomology of the tiling space) and the spectral properties (of Hamiltonians defined on such tilings) defined by $K$-theory, and to show their equivalence in dimensions $\leq 3$. A theorem makes precise the conditions for this relationship to hold. It can be viewed as an extension of the "Bloch Theorem" to a large class of aperiodic tilings. The idea underlying this result is based on the relationship between cohomology and $K$-theory traces and their equivalence in low dimensions.

math-ph↗

arXiv2019

The Riemann-Roch Theorem on higher dimensional complex noncommutative tori

We prove analogues of the Riemann-Roch Theorem and the Hodge Theorem for noncommutative tori (of any dimension) equipped with complex structures, and discuss implications for the question of how to distinguish "noncommutative abelian varieties" from "non-algebraic" noncommutative complex tori.

math.OA↗

arXiv2018

A new approach to twisted K-theory of compact Lie groups

This paper explores further the computation of the twisted K-theory and K-homology of compact simple Lie groups, previously studied by Hopkins, Moore, Maldacena-Moore-Seiberg, Braun, and Douglas, with a focus on groups of rank 2. We give a new method of computation based on the Segal spectral sequence which seems to us appreciably simpler than the methods used previously, at least in many key cases. The exposition has been clarified and one mistake in the previous version has been fixed. Also the references have been updated.

math.KT↗

arXiv2017

Different definitions of conic sections in hyperbolic geometry

In classical Euclidean geometry, there are several equivalent definitions of conic sections. We show that in the hyperbolic plane, the analogues of these same definitions still make sense, but are no longer equivalent, and we discuss the relationships among them.

math.MG↗

arXiv2017

Group dualities, T-dualities, and twisted K-theory

This paper explores further the connection between Langlands duality and T-duality for compact simple Lie groups, which appeared in work of Daenzer-Van Erp and Bunke-Nikolaus. We show that Langlands duality gives rise to isomorphisms of twisted K-groups, but that these K-groups are trivial except in the simplest case of SU(2) and SO(3). Along the way we compute explicitly the map on $H^3$ induced by a covering of compact simple Lie groups, which is either 1 or 2 depending in a complicated way on the type of the groups involved. We also give a new method for computing twisted K-theory using the Segal spectral sequence, giving simpler computations of certain twisted K-theory groups of compact Lie groups relevant for D-brane charges in WZW theories and rank-level dualities. Finally we study a duality for orientifolds based on complex Lie groups with an involution.

hep-th↗

arXiv2016

Algebraic K-theory and derived equivalences suggested by T-duality for torus orientifolds

We show that certain isomorphisms of (twisted) KR-groups that underlie T-dualities of torus orientifold string theories have purely algebraic analogues in terms of algebraic K-theory of real varieties and equivalences of derived categories of (twisted) coherent sheaves. The most interesting conclusion is a kind of Mukai duality in which the "dual abelian variety" to a smooth projective genus-1 curve over R with no real points is (mildly) noncommutative.

math.AG↗