1,579 research outputs found
Nonsingular black holes in nonlinear gravity coupled to Euler-Heisenberg electrodynamics
We study static, spherically symmetric black holes supported by
Euler-Heisenberg theory of electrodynamics and coupled to two different
modified theories of gravity. Such theories are the quadratic model and
Eddington-inspired Born-Infeld gravity, both formulated in metric-affine
spaces, where metric and affine connection are independent fields. We find
exact solutions of the corresponding field equations in both cases,
characterized by mass, charge, the Euler-Heisenberg coupling parameter and the
modified gravity one. For each such family of solutions, we characterize its
horizon structure and the modifications in the innermost region, finding that
some subclasses are geodesically complete. The singularity regularization is
achieved under two different mechanisms: either the boundary of the manifold is
pushed to an infinite affine distance, not being able to be reached in finite
time by any geodesic, or the presence of a wormhole structure allows for the
smooth extension of all geodesics overcoming the maximum of the potential
barrier.Comment: 14 pages, 8 figures, revtex4-1 style. v2: some new discussion and
minor corrections. Version to appear in Phys. Rev.
An introduction to learning technology in tertiary education in the UK.
Contents: 1. The Learning Technology Arena
2. The Learning Technology Community
3. Learning Technology Tools
4. Key issues and developments in the Learning Technology Field
5. Implementing Learning Technologies
6. Further Resource
On the Ancestral Compatibility of Two Phylogenetic Trees with Nested Taxa
Compatibility of phylogenetic trees is the most important concept underlying
widely-used methods for assessing the agreement of different phylogenetic trees
with overlapping taxa and combining them into common supertrees to reveal the
tree of life. The notion of ancestral compatibility of phylogenetic trees with
nested taxa was introduced by Semple et al in 2004. In this paper we analyze in
detail the meaning of this compatibility from the points of view of the local
structure of the trees, of the existence of embeddings into a common supertree,
and of the joint properties of their cluster representations. Our analysis
leads to a very simple polynomial-time algorithm for testing this
compatibility, which we have implemented and is freely available for download
from the BioPerl collection of Perl modules for computational biology.Comment: Submitte
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