174 research outputs found
Bounding Helly numbers via Betti numbers
We show that very weak topological assumptions are enough to ensure the
existence of a Helly-type theorem. More precisely, we show that for any
non-negative integers and there exists an integer such that
the following holds. If is a finite family of subsets of such that for any
and every
then has Helly number at most . Here
denotes the reduced -Betti numbers (with singular homology). These
topological conditions are sharp: not controlling any of these first Betti numbers allow for families with unbounded Helly number.
Our proofs combine homological non-embeddability results with a Ramsey-based
approach to build, given an arbitrary simplicial complex , some well-behaved
chain map .Comment: 29 pages, 8 figure
Some remarks on barycentric-sum problems over cyclic groups
We derive some new results on the k-th barycentric Olson constants of abelian
groups (mainly cyclic). This quantity, for a finite abelian (additive) group
(G,+), is defined as the smallest integer l such that each subset A of G with
at least l elements contains a subset with k elements {g_1, ..., g_k}
satisfying g_1 + ... + g_k = k g_j for some 1 <= j <= k.Comment: to appear in European Journal of Combinatoric
Topological methods in zero-sum Ramsey theory
A cornerstone result of Erd\H os, Ginzburg, and Ziv (EGZ) states that any
sequence of elements in contains a zero-sum subsequence
of length . While algebraic techniques have predominated in deriving many
deep generalizations of this theorem over the past sixty years, here we
introduce topological approaches to zero-sum problems which have proven
fruitful in other combinatorial contexts. Our main result (1) is a topological
criterion for determining when any -coloring of an -uniform
hypergraph contains a zero-sum hyperedge. In addition to applications for
Kneser hypergraphs, for complete hypergraphs our methods recover Olson's
generalization of the EGZ theorem for arbitrary finite groups. Furthermore, we
(2) give a fractional generalization of the EGZ theorem with applications to
balanced set families and (3) provide a constrained EGZ theorem which imposes
combinatorial restrictions on zero-sum sequences in the original result.Comment: 18 page
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The SDSS-III APOGEE Radial Velocity Survey Of M Dwarfs. I. Description Of The Survey And Science Goals
We are carrying out a large ancillary program with the Sloan Digital Sky Survey, SDSS-III, using the fiber-fed multi-object near-infrared APOGEE spectrograph, to obtain high-resolution H-band spectra of more than 1200 M dwarfs. These observations will be used to measure spectroscopic rotational velocities, radial velocities, physical stellar parameters, and variability of the target stars. Here, we describe the target selection for this survey, as well as results from the first year of scientific observations based on spectra that will be publicly available in the SDSS-III DR 10 data release. As part of this paper we present radial velocities and rotational velocities of over 200 M dwarfs, with a v sin i precision of similar to 2 km s(-1) a measurement floor at v sin i = 4 km s(-1). This survey significantly increases the number of M dwarfs studied for rotational velocities and radial velocity variability (at similar to 100-200 m s(-1)), and will inform and advance the target selection for planned radial velocity and photometric searches for low-mass exoplanets around M dwarfs, such as the Habitable Zone Planet Finder, CARMENES, and TESS. Multiple epochs of radial velocity observations enable us to identify short period binaries, and adaptive optics imaging of a subset of stars enables the detection of possible stellar companions at larger separations. The high-resolution APOGEE spectra, covering the entire H band, provide the opportunity to measure physical stellar parameters such as effective temperatures and metallicities for many of these stars. At the culmination of this survey, we will have obtained multi-epoch spectra and radial velocities for over 1400 stars spanning the spectral range M0-L0, providing the largest set of near-infrared M dwarf spectra at high resolution, and more than doubling the number of known spectroscopic a sin i values for M dwarfs. Furthermore, by modeling telluric lines to correct for small instrumental radial velocity shifts, we hope to achieve a relative velocity precision floor of 50 m s(-1) for bright M dwarfs. With three or more epochs, this precision is adequate to detect substellar companions, including giant planets with short orbital periods, and flag them for higher-cadence followup. We present preliminary, and promising, results of this telluric modeling technique in this paper.Center for Exoplanets and Habitable WorldsPennsylvania State UniversityEberly College of SciencePennsylvania Space Grant ConsortiumNSF AST 1006676, AST 1126413National Science FoundationNational Aeronautics and Space Administration NNX-08AE38A, NNX13AB03GAlfred P. Sloan FoundationU.S. Department of Energy Oce of ScienceUniversity of ArizonaBrazilian Participation GroupBrookhaven National LaboratoryUniversity of CambridgeCarnegie Mellon UniversityUniversity of FloridaFrench Participation GroupGerman Participation GroupHarvard UniversityInstituto de Astrosica de CanariasMichigan State/Notre Dame/JINA Participation GroupJohns Hopkins UniversityLawrence Berkeley National LaboratoryMax Planck Institute for AstrophysicsMax Planck Institute for Extraterrestrial PhysicsNew Mexico State UniversityNew York UniversityOhio State UniversityUniversity of PortsmouthPrinceton UniversitySpanish Participation GroupUniversity of TokyoUniversity of UtahVanderbilt UniversityUniversity of VirginiaUniversity of WashingtonYale UniversityMcDonald Observator
The SDSS-III APOGEE Radial Velocity Survey of M dwarfs I: Description of Survey and Science Goals
We are carrying out a large ancillary program with the SDSS-III, using the
fiber-fed multi-object NIR APOGEE spectrograph, to obtain high-resolution
H-band spectra of more than 1200 M dwarfs. These observations are used to
measure spectroscopic rotational velocities, radial velocities, physical
stellar parameters, and variability of the target stars. Here, we describe the
target selection for this survey and results from the first year of scientific
observations based on spectra that is publicly available in the SDSS-III DR10
data release. As part of this paper we present RVs and vsini of over 200 M
dwarfs, with a vsini precision of ~2 km/s and a measurement floor at vsini = 4
km/s. This survey significantly increases the number of M dwarfs studied for
vsini and RV variability (at ~100-200 m/s), and will advance the target
selection for planned RV and photometric searches for low mass exoplanets
around M dwarfs, such as HPF, CARMENES, and TESS. Multiple epochs of radial
velocity observations enable us to identify short period binaries, and AO
imaging of a subset of stars enables the detection of possible stellar
companions at larger separations. The high-resolution H-band APOGEE spectra
provide the opportunity to measure physical stellar parameters such as
effective temperatures and metallicities for many of these stars. At the
culmination of this survey, we will have obtained multi-epoch spectra and RVs
for over 1400 stars spanning spectral types of M0-L0, providing the largest set
of NIR M dwarf spectra at high resolution, and more than doubling the number of
known spectroscopic vsini values for M dwarfs. Furthermore, by modeling
telluric lines to correct for small instrumental radial velocity shifts, we
hope to achieve a relative velocity precision floor of 50 m/s for bright M
dwarfs. We present preliminary results of this telluric modeling technique in
this paper.Comment: Submitted to Astronomical Journa
Colorful Borsuk--Ulam theorems and applications
We prove a colorful generalization of the Borsuk--Ulam theorem and derive
colorful consequences from it, such as a colorful generalization of the ham
sandwich theorem. Even in the uncolored case this specializes to a
strengthening of the ham sandwich theorem, which given an additional condition,
contains a result of B\'{a}r\'{a}ny, Hubard, and Jer\'{o}nimo on well-separated
measures as a special case. We prove a colorful generalization of Fan's
antipodal sphere covering theorem, we derive a short proof of Gale's colorful
KKM theorem, and we prove a colorful generalization of Brouwer's fixed point
theorem. Our results also provide an alternative between Radon-type
intersection results and KKM-type covering results. Finally, we prove colorful
Borsuk--Ulam theorems for higher symmetry.Comment: 15 page
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