2,405 research outputs found
Small covers and the equivariant bordism classification of 2-torus manifolds
Associated with the Davis-Januszkiewicz theory of small covers, this paper
deals with the theory of 2-torus manifolds from the viewpoint of equivariant
bordism. We define a differential operator on the "dual" algebra of the
unoriented -representation algebra introduced by Conner and Floyd, where
. With the help of -colored graphs (or mod 2 GKM graphs), we
may use this differential operator to give a very simple description of tom
Dieck-Kosniowski-Stong localization theorem in the setting of 2-torus
manifolds. We then apply this to study the -equivariant unoriented bordism
classification of -dimensional 2-torus manifolds. We show that the
-equivariant unoriented bordism class of each -dimensional 2-torus
manifold contains an -dimensional small cover as its representative, solving
the conjecture posed in [19]. In addition, we also obtain that the graded
noncommutative ring formed by the equivariant unoriented bordism classes of
2-torus manifolds of all possible dimensions is generated by the classes of all
generalized real Bott manifolds (as special small covers over the products of
simplices). This gives a strong connection between the computation of
-equivariant bordism groups or ring and the Davis-Januszkiewicz theory of
small covers. As a computational application, with the help of computer, we
completely determine the structure of the group formed by equivariant bordism
classes of all 4-dimensional 2-torus manifolds. Finally, we give some essential
relationships among 2-torus manifolds, coloring polynomials, colored simple
convex polytopes, colored graphs.Comment: 32 pages, updated version with the title of paper changed and a large
expansio
The Cosmology of String Theoretic Axions
String theory posesses numerous axion candidates. The recent realization that
the compactification radius in string theory might be large means that these
states can solve the strong CP problem. This still leaves the question of the
cosmological bound on the axion mass. Here we explore two schemes for
accommodating such light axions in cosmology. In the first, we note that in
string theory the universe is likely to be dominated early on by the coherent
oscillations of some moduli. The usual moduli problem assumes that these fields
have masses comparable to the gravitino. We argue that string moduli are likely
to be substantially more massive, eliminating this problem. In such cosmologies
the axion bound is significantly weakened. Plausible mechanisms for generating
the baryon number density are described. In the second, we point out that in
string theory, the axion potentials might be much larger at early times than at
present. In string theory, if CP violation is described by a small parameter,
the axion may sit sufficiently close to its true minimum to invalidate the
bounds.Comment: 24 pages, uses harvmac. Refs corrected plus spellin
It is Not All Fun and Games: Breaking News Consumption on Snapchat
Snapchat is a camera and ephemeral messaging application popular among young adults. Due to its self-destructing content and playful features, Snapchat is often associated with more trivial uses. However, the platform has added functionality to support consumption of news. To understand how users perceive and interact with news content on Snapchat, we conducted semi-structured interviews with 19 users of the platform, focusing on their use of Snapchat during breaking news events, including the 2016/2017 US presidential election and inauguration. Through the lens of Network Gatekeeping, our research explains how users consume breaking news content on Snapchat. We unpack usersâ ambiguous perceptions of news reliability on Snapchat, and demonstrate how this contrasts with traditional news consumption. Our research also describes how usersâ mental models of how Snapchat worksâspecifically their theories about how the platform curates news contentâshape their judgments of reliability, media bias and authenticity
Flux Dendrites of Opposite Polarity in Superconducting MgB rings observed with magneto-optical imaging
Magneto-optical imaging was used to observe flux dendrites with opposite
polarities simultaneously penetrate superconducting, ring-shaped MgB films.
By applying a perpendicular magnetic field, branching dendritic structures
nucleate at the outer edge and abruptly propagate deep into the rings. When
these structures reach close to the inner edge, where flux with opposite
polarity has penetrated the superconductor, they occasionally trigger anti-flux
dendrites. These anti-dendrites do not branch, but instead trace the triggering
dendrite in the backward direction. Two trigger mechanisms, a non-local
magnetic and a local thermal, are considered as possible explanations for this
unexpected behaviour. Increasing the applied field further, the rings are
perforated by dendrites which carry flux to the center hole. Repeated
perforations lead to a reversed field profile and new features of dendrite
activity when the applied field is subsequently reduced.Comment: 6 pages, 6 figures, accepted to Phys. Rev.
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