8,025 research outputs found
The geometry of Grassmannian manifolds and Bernstein type theorems for higher codimension
We identify a region \Bbb{W}_{\f{1}{3}} in a Grassmann manifold
\grs{n}{m}, not covered by a usual matrix coordinate chart, with the
following important property. For a complete submanifold in \ir{n+m} \,
(n\ge 3, m\ge2) with parallel mean curvature whose image under the Gauss map
is contained in a compact subset K\subset\Bbb{W}_{\f{1}{3}}\subset\grs{n}{m},
we can construct strongly subharmonic functions and derive a priori estimates
for the harmonic Gauss map. While we do not know yet how close our region is to
being optimal in this respect, it is substantially larger than what could be
achieved previously with other methods. Consequently, this enables us to obtain
substantially stronger Bernstein type theorems in higher codimension than
previously known.Comment: 36 page
Existence and non-existence of area-minimizing hypersurfaces in manifolds of non-negative Ricci curvature
We study minimal hypersurfaces in manifolds of non-negative Ricci curvature,
Euclidean volume growth and quadratic curvature decay at infinity. By
comparison with capped spherical cones, we identify a precise borderline for
the Ricci curvature decay. Above this value, no complete area-minimizing
hypersurfaces exist. Below this value, in contrast, we construct examples.Comment: 31 pages. Comments are welcome
The Gauss image of entire graphs of higher codimension and Bernstein type theorems
Under suitable conditions on the range of the Gauss map of a complete
submanifold of Euclidean space with parallel mean curvature, we construct a
strongly subharmonic function and derive a-priori estimates for the harmonic
Gauss map. The required conditions here are more general than in previous work
and they therefore enable us to improve substantially previous results for the
Lawson-Osseman problem concerning the regularity of minimal submanifolds in
higher codimension and to derive Bernstein type results.Comment: 28 page
Minimal graphic functions on manifolds of non-negative Ricci curvature
We study minimal graphic functions on complete Riemannian manifolds \Si
with non-negative Ricci curvature, Euclidean volume growth and quadratic
curvature decay. We derive global bounds for the gradients for minimal graphic
functions of linear growth only on one side. Then we can obtain a Liouville
type theorem with such growth via splitting for tangent cones of \Si at
infinity. When, in contrast, we do not impose any growth restrictions for
minimal graphic functions, we also obtain a Liouville type theorem under a
certain non-radial Ricci curvature decay condition on \Si. In particular, the
borderline for the Ricci curvature decay is sharp by our example in the last
section.Comment: 38 page
The regularity of harmonic maps into spheres and applications to Bernstein problems
We show the regularity of, and derive a-priori estimates for (weakly)
harmonic maps from a Riemannian manifold into a Euclidean sphere under the
assumption that the image avoids some neighborhood of a half-equator. The
proofs combine constructions of strictly convex functions and the regularity
theory of quasi-linear elliptic systems.
We apply these results to the spherical and Euclidean Bernstein problems for
minimal hypersurfaces, obtaining new conditions under which compact minimal
hypersurfaces in spheres or complete minimal hypersurfaces in Euclidean spaces
are trivial
KMS, etc
A general form of the ``Wick rotation'', starting from imaginary-time Green
functions of quantum-mechanical systems in thermal equilibrium at positive
temperature, is established. Extending work of H. Araki, the role of the KMS
condition and of an associated anti-unitary symmetry operation, the ``modular
conjugation'', in constructing analytic continuations of Green functions from
real- to imaginary times, and back, is clarified.
The relationship between the KMS condition for the vacuum with respect to
Lorentz boosts, on one hand, and the spin-statistics connection and the PCT
theorem, on the other hand, in local, relativistic quantum field theory is
recalled.
General results on the reconstruction of local quantum theories in various
non-trivial gravitational backgrounds from ``Euclidian amplitudes'' are
presented. In particular, a general form of the KMS condition is proposed and
applied, e.g., to the Unruh- and the Hawking effects.
This paper is dedicated to Huzihiro Araki on the occasion of his seventieth
birthday, with admiration, affection and best wishes.Comment: 56 pages, submitted to J. Math. Phy
Mode spectrum and temporal soliton formation in optical microresonators
The formation of temporal dissipative solitons in optical microresonators
enables compact, high repetition rate sources of ultra-short pulses as well as
low noise, broadband optical frequency combs with smooth spectral envelopes.
Here we study the influence of the resonator mode spectrum on temporal soliton
formation. Using frequency comb assisted diode laser spectroscopy, the measured
mode structure of crystalline MgF2 resonators are correlated with temporal
soliton formation. While an overal general anomalous dispersion is required, it
is found that higher order dispersion can be tolerated as long as it does not
dominate the resonator's mode structure. Mode coupling induced avoided
crossings in the resonator mode spectrum are found to prevent soliton
formation, when affecting resonator modes close to the pump laser. The
experimental observations are in excellent agreement with numerical simulations
based on the nonlinear coupled mode equations, which reveal the rich interplay
of mode crossings and soliton formation
Calculating Optimum Daily Gain for Wintering Replacement Beef Heifers
Research has demonstrated that weight of the yearling heifer is an important factor affecting puberty and initiation of the reproductive cycle. Work at several institutions including South Dakota State has demonstrated that rate of gain from weaning to start of the breeding season influences the proportion of heifers that settle. The objective of this project was to provide the producer with an easy way of calculating this desired rate of growth
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