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Manuel Amann

Publications and source records attributed to Manuel Amann.

28 records · Page 2Linked to original sources
arXiv2014

Positive curvature and rational ellipticity

Simply-connected manifolds of positive sectional curvature $M$ are speculated to have a rigid topological structure. In particular, they are conjectured to be rationally elliptic, i.e., all but finitely many homotopy groups are conjectured to be finite. In this article we combine positive curvature with rational ellipticity to obtain several topological properties of the underlying manifold. These results include a small upper bound on the Euler characteristic and confirmations of famous conjectures by Hopf and Halperin under additional torus symmetry. We prove several cases (including all known even-dimensional examples of positively curved manifolds) of a conjecture by Wilhelm.

math.DG↗
arXiv2014

On a generalized conjecture of Hopf with symmetry

A famous conjecture of Hopf is that the product of the two-dimensional sphere with itself does not admit a Riemannian metric with positive sectional curvature. More generally, one may conjecture that this holds for any nontrivial product. We provide evidence for this generalized conjecture in the presence of symmetry.

math.DG↗
arXiv2012

Non-formal Homogeneous Spaces

Several large classes of homogeneous spaces are known to be formal---in the sense of Rational Homotopy Theory. However, it seems that far fewer examples of non-formal homogeneous spaces are known. In this article we provide several construction principles and characterisations for non-formal homogeneous spaces, which will yield a lot of examples. This will enable us to prove that, from dimension 72 on, such a space can be found in each dimension.

math.AT↗
arXiv2012

On fibrations with formal elliptic fibers

We prove that for a fibration of simply-connected spaces of finite type $F\hookrightarrow E\to B$ with $F$ being positively elliptic and $H^*(F,\qq)$ not possessing non-trivial derivations of negative degree, the base $B$ is formal if and only if the total space $E$ is formal. Moreover, in this case the fibration map is a formal map. As a geometric application we show that positive quaternion Kähler manifolds are formal and so are their associated twistor fibration maps.

math.AT↗
arXiv2012

Cohomological consequences of (almost) free torus actions

The long-standing Halperin--Carlsson conjecture (basically also known as the toral rank conjecture) states that the sum of all Betti numbers of a well-behaved space $X$ (with cohomology taken with coefficients in the cyclic group $\zz_p$ in characteristic $p>0$ respectively with rational coefficients for characteristic 0) is at least $2^n$ where $n$ is the rank of an $n$-torus $\zz_p \times \stackrel{(n)}...\times \zz_p$ (in characteristic $p>0$) respectively $\s^1 \times \stackrel{(n)}...\times \s^1$ (in characteristic 0) acting freely (characteristic $p>0$) respectively almost freely (characteristic 0) on $X$. This conjecture was addressed by a multitude of authors in several distinct contexts and special cases. However, having undergone various reformulations and reproofs over the decades, the best known general lower bound on the Betti numbers remains a very low linear one. This article investigates the conjecture in characteristics 0 and 2, which results in improving the lower bound in characteristic 0.

math.AT↗
arXiv2011

Formality of Positive Quaternion Kaehler Manifolds

Positive Quaternion Kaehler Manifolds are Riemannian manifolds with holonomy contained in Sp(n)Sp(1) and with positive scalar curvature. Conjecturally, they are symmetric spaces. We offer a new approach to this field of study via Rational Homotopy Theory, thereby proving the formality of Positive Quaternion Kaehler Manifolds. This result is established by means of an in-depth investigation on how formality behaves under spherical fibrations.

math.GN↗
arXiv2011

Computational complexity of topological invariants

We answer the following question posed by Lechuga: Given a simply-connected space $X$ with both $H_*(X,\qq)$ and $π_*(X)\otimes \qq$ being finite-dimensional, what is the computational complexity of an algorithm computing the cup-length and the rational Lusternik--Schnirelmann category of $X$? Basically, by a reduction from the decision problem whether a given graph is $k$-colourable (for $k\geq 3$) we show that (even stricter versions of the) problems above are $\mathbf{NP}$-hard.

math.AT↗
arXiv2011

Mapping degrees of self-maps of simply-connected manifolds

An oriented compact closed manifold is called inflexible if the set of mapping degrees ranging over all continuous self-maps is finite. Inflexible manifolds have become of importance in the theory of functorial semi-norms on homology. Although inflexibility should be a generic property in large dimensions, not many simply-connected examples are known. We show that from a certain dimension on there are infinitely many inflexible manifolds in each dimension. Besides, we prove flexibility for large classes of manifolds and, in particular, as a spin-off, for homogeneous spaces. This is an outcome of a lifting result which also permits to generalise a conjecture of Copeland--Shar to the "real world". Moreover, we then provide examples of simply-connected smooth compact closed manifolds in each dimension from dimension 70 on which have the following properties: They do not admit any self-map which reverses orientation. (For this we consider the lack of orientation reversal in the strongest sense possible, i.e. we prove the non-existence of any self-map of arbitrary negative mapping degree.) Moreover, the manifolds neither split as non-trivial Cartesian products nor as non-trivial connected sums.

math.AT↗
arXiv2009

Partial Classification Results for Positive Quaternion Kaehler Manifolds

Positive Quaternion Kaehler Manifolds are Riemannian manifolds with holonomy contained in Sp(n)Sp(1) and with positive scalar curvature. Conjecturally, they are symmetric spaces. We prove this conjecture in dimension 20 under additional assumptions and we provide recognition theorems for quaternionic projective spaces (in low dimensions) as well as the real Grassmanian (which is Positive Quaternion Kaehler).

math.DG↗