6,154 research outputs found
The conjugacy problem for automorphism groups of countable homogeneous structures
We consider the conjugacy problem for the automorphism groups of a number of
countable homogeneous structures. In each case we find the precise complexity
of the conjugacy relation in the sense of Borel reducibility
Pion-Nucleon Scattering at Low Energies
We study pion-nucleon scattering at tree level with a chiral lagrangian of
pions, nucleons, and -isobars using a K-matrix unitarization procedure.
Evaluating the scattering amplitude to order , where is a generic
small momentum scale, we obtain a good fit to the experimental phase shifts for
pion center-of-mass kinetic energies up to MeV. The fit can be extended to
150 MeV when we include the order- contributions. Our results are
independent of the off-shell parameter.Comment: 13 pages, RevTex, Two tables and two figures, Minor changes, To
appear in PR
A global conformal extension theorem for perfect fluid Bianchi space-times
A global extension theorem is established for isotropic singularities in
polytropic perfect fluid Bianchi space-times. When an extension is possible,
the limiting behaviour of the physical space-time near the singularity is
analysed.Comment: 13 pages, no figures, revised and shortened, improved proof of main
theorem, some correction
A Faint Star-Forming System Viewed Through the Lensing Cluster Abell 2218: First Light at z~5.6?
We discuss the physical nature of a remarkably faint pair of Lyman
alpha-emitting images discovered close to the giant cD galaxy in the lensing
cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified
star-forming galaxies beyond z=5. A well-constrained mass model suggests the
pair arises via a gravitationally-lensed source viewed at high magnification.
Keck spectroscopy confirms the lensing hypothesis and implies the unlensed
source is a very faint (I~30) compact (<150 pc) and isolated object at z=5.576
whose optical emission is substantially contained within the Lyman alpha
emission line; no stellar continuum is detectable. The available data suggest
the source is a promising candidate for an isolated ~10^6 solar mass system
seen producing its first generation of stars close to the epoch of
reionization.Comment: 11 pages, 3 figures, to appear in Ap J Lett, minor revision following
referee's repor
- …