Results for "A. Reiners"

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Differential Rotation in F StarsOct 12 2007Differential rotation can be detected in single line profiles of stars rotating more rapidly than about $v \sin{i} = 10$ km s$^{-1}$ with the Fourier transform technique. This allows to search for differential rotation in large samples to look for correlations ... More
Exact sequences, Hochschild cohomology, and the Lie module structure over the $M$-relative centerJul 09 2014Jan 29 2016In this article, we present actions by central elements on Hochschild cohomology groups with arbitrary bimodule coefficients, as well as an interpretation of these actions in terms of exact sequences. Since our construction utilises the monoidal structure ... More
Spectral line lists of a nitrogen gas discharge for wavelength calibration in the range $4500-11000$cm$^{-1}$Sep 10 2015A discharge of nitrogen gas, as created in a microwave-induced plasma, exhibits a very dense molecular emission line spectrum. Emission spectra of this kind could serve as wavelength calibrators for high-resolution astrophysical spectrographs in the near-infrared, ... More
Fiber polytopes for the projections between cyclic polytopesDec 17 1997The cyclic polytope $C(n,d)$ is the convex hull of any $n$ points on the moment curve ${(t,t^2,...,t^d):t \in \reals}$ in $\reals^d$. For $d' >d$, we consider the fiber polytope (in the sense of Billera and Sturmfels) associated to the natural projection ... More
First signatures of strong differential rotation in A-type starsNov 14 2003We reanalyzed high quality spectra of 158 stars of spectral types A0--F1 and $v \sin{i}$ between 60 and 150 km s$^{-1}$. Using a Least Squares Deconvolution technique we extracted high $S/N$ broadening profiles and determined the loci of the Fourier transform ... More
A Volume-limited Sample of 63 M7--M9.5 Dwarfs I. Space Motion, Kinematic Age, and LithiumSep 25 2009In a volume-limited sample of 63 ultracool dwarfs of spectral type M7--M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this first paper we introduce our volume-complete sample ... More
The First Direct Measurements of Magnetic Fields on Very Low-Mass StarsOct 12 2006We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the effects of a field on the FeH band near 1 micron, we obtain information on whether the integrated surface magnetic ... More
Diameter of reduced wordsJun 25 2009Oct 07 2011For finite reflection groups of types A and B, we determine the diameter of the graph whose vertices are reduced words for the longest element and whose edges are braid relations. This is deduced from a more general theorem that applies to supersolvable ... More
Cyclotomic and simplicial matroidsFeb 12 2004Two naturally occurring matroids representable over Q are shown to be dual: the {\it cyclotomic matroid} $\mu_n$ represented by the $n^{th}$ roots of unity $1,\zeta,\zeta^2,...,\zeta^{n-1}$ inside the cyclotomic extension $Q(\zeta)$, and a direct sum ... More
Isometry invariant permutation codes and mutually orthogonal Latin squaresDec 17 2018Mar 05 2019Commonly the direct construction and the description of mutually orthogonal Latin squares (MOLS) makes use of difference or quasi-difference matrices. Now there exists a correspondence between MOLS and separable permutation codes. We like to present separable ... More
Fake degrees for reflection actions on rootsDec 29 2011Jan 27 2012A finite irreducible real reflection group of rank l and Coxeter number h has root system of cardinality h*l. It is shown that the fake degree for the permutation action on its roots is divisible by [h]_q = 1+q+q^2+...+q^{h-1}, and that in simply-laced ... More
Control of the isoperimetric deficit by the Willmore deficitJun 30 2011In the class of smoothly embedded surfaces of sphere type we prove that the isoperimetric deficit can be controlled by the Willmore deficit.
Koszul incidence algebras, affine semigroups, and Stanley-Reisner idealsApr 10 2009Feb 07 2010We prove a theorem unifying three results from combinatorial homological and commutative algebra, characterizing the Koszul property for incidence algebras of posets and affine semigroup rings, and characterizing linear resolutions of squarefree monomial ... More
Invariant derivations and differential forms for reflection groupsDec 03 2016Feb 01 2019Classical invariant theory of a complex reflection group $W$ highlights three beautiful structures: -- the $W$-invariant polynomials constitute a polynomial algebra, over which -- the $W$-invariant differential forms with polynomial coefficients constitute ... More
Teff and logg dependence of FeH in M-dwarfsDec 11 2008We present synthetic FeH band spectra in the z-filter range for several M-dwarf models with logg=3.0-5.0 [cgs] and Teff=2800K -3450K. Our aim is to characterize convective velocities in M-dwarfs and to give a rough estimate of the range in which 3D-atmosphere ... More
3D simulations of M star atmosphere velocities and their influence on molecular FeH linesOct 19 2009We present an investigation of the velocity fields in early to late M-type star hydrodynamic models, and we simulate their influence on FeH molecular line shapes. The M star model parameters range between log g of 3.0 - 5.0 and Teff of 2500 K and 4000 ... More
Flat-relative optimal extraction. A quick and efficient algorithm for stabilised spectrographsNov 20 2013Optimal extraction is a key step in processing the raw images of spectra as registered by two-dimensional detector arrays to a one-dimensional format. Previously reported algorithms reconstruct models for a mean one-dimensional spatial profile to assist ... More
Teff and log g dependence of velocity fields in M-starsAug 06 2009We present an investigation of velocity fields in early to late M-type hydrodynamic stellar atmosphere models. These velocities will be expressed in classical terms of micro- and macro-turbulent velocities for usage in 1D spectral synthesis. The M-star ... More
A search for star-planet interactions in chromospheric linesDec 08 2010Massive planets in very close orbits around their central stars can induce so-called star-planet interactions (SPI), which may be of magnetic or gravitational nature. In both cases, SPI can potentially cause recurring chromospheric emission on the host ... More
Evidence for Magnetic Flux Saturation in Rapidly Rotating M StarsOct 28 2008We present magnetic flux measurements in seven rapidly rotating M dwarfs. Our sample stars have X-ray and H-alpha emission indicative of saturated emission, i.e., emission at a high level independent of rotation rate. Our measurements are made using near-infrared ... More
Generalized investigation of the rotation-activity relation: Favouring rotation period instead of Rossby numberAug 26 2014Magnetic activity in Sun-like and low-mass stars causes X-ray coronal emission, which is stronger for more rapidly rotating stars. This relation is often interpreted in terms of the Rossby number, i.e., the ratio of rotation period to convective overturn ... More
The Radio Activity-Rotation Relation of Ultracool DwarfsAug 01 2011[Abridged] We present a new radio survey of about 100 late-M and L dwarfs undertaken with the VLA. The sample was chosen to explore the role of rotation in the radio activity of ultracool dwarfs. Combining the new sample with results from our previous ... More
Poset edge densities, nearly reduced words, and barely set-valued tableauxMar 31 2016Mar 14 2018In certain finite posets, the expected down-degree of their elements is the same whether computed with respect to either the uniform distribution or the distribution weighting an element by the number of maximal chains passing through it. We show that ... More
Toric partial ordersNov 18 2012We define toric partial orders, corresponding to regions of graphic toric hyperplane arrangements, just as ordinary partial orders correspond to regions of graphic hyperplane arrangements. Combinatorially, toric posets correspond to finite posets under ... More
Alternating subgroups of Coxeter groupsFeb 07 2007We study combinatorial properties of the alternating subgroup of a Coxeter group, using a presentation of it due to Bourbaki.
Bimahonian distributionsMar 16 2007Jan 07 2008Motivated by permutation statistics, we define for any complex reflection group W a family of bivariate generating functions. They are defined either in terms of Hilbert series for W-invariant polynomials when W acts diagonally on two sets of variables, ... More
The Koszul homology algebra of the second Veronese is generated by the lowest strandOct 11 2017We show that the Koszul homology algebra of the second Veronese subalgebra of a polynomial ring over a field of characteristic zero is generated, as an algebra, by the homology classes corresponding to the syzygies of the lowest linear strand.
A refined count of Coxeter element factorizationsAug 21 2017For well-generated complex reflection groups, Chapuy and Stump gave a simple product for a generating function counting reflection factorizations of a Coxeter element by their length. This is refined here to record the number of reflections used from ... More
Critical groups for Hopf algebra modulesApr 12 2017Dec 24 2017This paper considers an invariant of modules over a finite-dimensional Hopf algebra, called the critical group. This generalizes the critical groups of complex finite group representations studied by Benkart, Klivans, Reiner and Gaetz. A formula is given ... More
A quasisymmetric function for matroidsJun 26 2006Jan 12 2009A new isomorphism invariant of matroids is introduced, in the form of a quasisymmetric function. This invariant (1) defines a Hopf morphism from the Hopf algebra of matroids to the quasisymmetric functions, which is surjective if one uses rational coefficients, ... More
CRIRES spectroscopy and empirical line-by-line identification of FeH molecular absorption in an M dwarfJul 23 2010Molecular FeH provides a large number of sharp and isolated absorption lines that can be used to measure radial velocity, rotation, or magnetic field strength with high accuracy. Our aim is to provide an FeH atlas for M-type stars in the spectral region ... More
The activity and rotation limit in the HyadesDec 08 2010We conduct a study of K to M type stars to investigate the activity and the rotation limit in the Hyades. We use a sample of 40 stars in this intermediate-age cluster (~625 Myr) to probe stellar rotation in the threshold region where stellar activity ... More
Rotation, magnetism, and metallicity of M dwarf systemsAug 17 2011Close M-dwarf binaries and higher multiples allow the investigation of rotational evolution and mean magnetic flux unbiased from scatter in inclination angle and age since the orientation of the spin axis of the components is most likely parallel and ... More
Rapid magnetic flux variability on the flare star CN LeonisMar 08 2007We present UVES/VLT observations of the nearby flare star CN Leo covering the Wing-Ford FeH band near 1mu with high spectral resolution. Some of the FeH absorption lines in this band are magnetically sensitive and allow a measurement of the mean magnetic ... More
Differential rotation in rapidly rotating F-starsSep 23 2003We obtained high quality spectra of 135 stars of spectral types F and later and derived ``overall'' broadening functions in selected wavelength regions utilizing a Least Squares Deconvolution (LSD) procedure. Precision values of the projected rotational ... More
Exploring the magnetic field complexity in M dwarfs at the boundary to full convectionJan 21 2014Based on detailed spectral synthesis we carry out quantitative measurements of the strength and complexity of surface magnetic fields in the four well-known M-dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 populating the mass regime around the boundary between ... More
Detection of differential rotation in psi Cap with profile analysisJul 18 2001We report detection of differential rotation on the F5 dwarf psi Cap using line profile analysis. The Fourier transform of both FeI lambda 5775 and SiI lambda 5772 are used to obtain a projected rotational velocity of v sini = (42+-1)km/s. Modelling of ... More
Periodic Radio Emission from the M7 Dwarf 2MASS J13142039+1320011: Implications for the Magnetic Field TopologyJul 07 2011We present multi-epoch radio and optical observations of the M7 dwarf 2MASS J13142039+1320011. We detect a ~1 mJy source at 1.43, 4.86, 8.46 and 22.5 GHz, making it the most luminous radio emission over the widest frequency range detected from an ultracool ... More
A laser-lock concept to reach cm/s-precision in Doppler experiments with Fabry-Perot wavelength calibratorsAug 26 2014State-of-the-art Doppler experiments require wavelength calibration with precision at the cm/s level. A low-finesse Fabry-Perot interferometer (FPI) can provide a wavelength comb with a very large bandwidth as required for astronomical experiments, but ... More
On the possibility of detecting extrasolar planets' atmospheres with the Rossiter-McLaughlin-effectMar 12 2009The detection of extrasolar planets' atmospheres requires very demanding observations. For planets that can not be spatially separated from their host stars, i.e. the vast majority of planets, the transiting planets are the only ones allowing to probe ... More
On the kinematic age of brown dwarfs: Radial velocities and space motions of 43 nearby L dwarfsJan 12 2010We present radial velocity measurements of a sample of L0-L8 dwarfs observed with VLT/UVES and Keck/HIRES. We combine these measurements with distance and proper motion from the literature to determine space motions for 43 of our targets. We identify ... More
A high resolution spectral atlas of brown dwarfsJul 18 2007We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf system, spectral classes at which most of the objects are brown dwarfs. Our atlas covers the optical region from H$\alpha$ up to the near infrared at 1 $\mu$m. We present spectral ... More
Investigation of transit-selected exoplanet candidates from the MACHO surveyApr 11 2007Context: Planets outside our solar system transiting their host star, i. e. those with an orbital inclination near 90 degree, are of special interest to derive physical properties of extrasolar planets. With the knowledge of the host star's physical parameters, ... More
Equivariant Bifurcation and Absolute Irreducibility in $\R^8$ A contribution to the Ize conjectureOct 08 2012Sep 06 2013M. Field [5] refers to an unpublished work by J. Ize for a result that loss of stability through an absolutely irreducible representation of a compact Lie group leads to bifurcation of steady states. The main ingredient of the proof is the hypotheses, ... More
Magnetic Fields in Low-mass Stars: An Overview of Observational BiasesOct 22 2013Stellar magnetic dynamos are driven by rotation, rapidly rotating stars produce stronger magnetic fields than slowly rotating stars do. The Zeeman effect is the most important indicator of magnetic fields, but Zeeman broadening must be disentangled from ... More
Evidence for accretion in a nearby, young brown dwarfAug 24 2009We report on the discovery of the young, nearby, brown dwarf 2MASS J0041353$-$562112. The object has a spectral type of M7.5, it shows Li absorption and signatures of accretion, which implies that it still has a disk and suggests an age below 10 Myr. ... More
Pseudodeterminants and perfect square spanning tree countsNov 26 2013Jan 05 2015The pseudodeterminant $\textrm{pdet}(M)$ of a square matrix is the last nonzero coefficient in its characteristic polynomial; for a nonsingular matrix, this is just the determinant. If $\partial$ is a symmetric or skew-symmetric matrix then $\textrm{pdet}(\partial\partial^t)=\textrm{pdet}(\partial)^2$. ... More
Observations of Cool-Star Magnetic FieldsMar 01 2012Cool stars like the Sun harbor convection zones capable of producing substantial surface magnetic fields leading to stellar magnetic activity. The influence of stellar parameters like rotation, radius, and age on cool-star magnetism, and the importance ... More
The effects of inclination, gravity darkening and differential rotation on absorption profiles of fast rotatorsJun 24 2003Mechanisms influencing absorption line profiles of fast rotating stars can be sorted into two groups; (i) intrinsic variations sensitive to temperature and pressure, and (ii) global effects common to all spectral lines. I present a detailed study on the ... More
Rotation- and temperature-dependence of stellar latitudinal differential rotationSep 14 2005Sep 30 2005More than 600 high resolution spectra of stars with spectral type F and later were obtained in order to search for signatures of differential rotation in line profiles. In 147 stars, the rotation law could be measured, 28 of them are found to be differentially ... More
Homological epimorphisms, recollements and Hochschild cohomology - with a conjecture by Snashall-Solberg in viewNov 04 2014Jun 02 2016We show that recollements of module categories give rise to homomorphisms between the associated Hochschild cohomology algebras which preserve the strict Gerstenhaber structure, i.e., the cup product, the graded Lie bracket and the squaring map. We review ... More
The Narrowest M-dwarf Line Profiles and the Rotation-Activity Connection at Very Slow RotationFeb 23 2007The rotation-activity connection explains stellar activity in terms of rotation and convective overturn time. It is well established in stars of spectral types F--K and in M-type stars of young clusters. It is not established in field M-dwarfs, because ... More
Magnetic Fields on Cool StarsDec 06 2010Magnetic fields are an important ingredient to cool star physics, and there is great interest in measuring fields and their geometry in order to understand stellar dynamos and their influence on star formation and stellar evolution. During the last few ... More
At the Bottom of the Main Sequence: Activity and magnetic fields beyond the threshold to complete convectionDec 14 2007Dec 18 2007The bottom of the main sequence hosts objects with fundamentally different properties. At masses of about 0.3 M$_{\odot}$, stars become fully convective and at about 0.08 M$_{\odot}$ the hydrogen-burning main sequence ends; less massive objects are brown ... More
Ultra-Cool and Extra-Vigorous: Rotation and Activity in M and L dwarfsOct 29 2007The study of rotation and activity in low-mass stars or brown dwarfs of spectral classes M and L has seen enormous progress during the last years. I summarize the results from different works that measured activity, rotation, and sometimes magnetic fields. ... More
Monoidal categories and the Gerstenhaber bracket in Hochschild cohomologyMar 14 2014Feb 27 2015In this monograph, we extend S. Schwede's exact sequence interpretation of the Gerstenhaber bracket in Hochschild cohomology to certain exact and monoidal categories. Therefore we establish an explicit description of an isomorphism by A. Neeman and V. ... More
Coincidences among skew Schur functionsFeb 27 2006Jan 14 2009New sufficient conditions and necessary conditions are developed for two skew diagrams to give rise to the same skew Schur function. The sufficient conditions come from a variety of new operations related to ribbons (also known as border strips or rim ... More
Calibrating models of ultralow-mass starsOct 17 2005Evolutionary and atmospheric models have become available for young ultralow-mass objects. These models are being used to determine fundamental parameters from observational properties. TiO bands are used to determine effective temperatures in ultralow-mass ... More
Activity-induced radial velocity jitter in a flaring M dwarfMar 15 2009We investigate the effect of stellar activity and flares on short-term radial velocity measurements in the mid-M flare star CN Leo. Radial velocity variations are calculated from 181 UVES spectra obtained during three nights. We searched for spectral ... More
Magnetic field observations of low-mass starsJan 13 2009Direct measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and ... More
The CARMENES search for exoplanets around M dwarfs. Activity indicators at visible and near-infrared wavelengthsJan 25 2019The CARMENES survey is searching for Earth-like planets orbiting M dwarfs using the radial velocity method. Studying the stellar activity of the target stars is important to avoid false planet detections and to improve our understanding of the atmospheres ... More
Detection of strong activity in the eclipsing binary brown dwarf 2MASSJ05352184-0546085 - A possible explanation for the temperature reversalNov 04 2007We show high resolution spectra of the eclipsing brown dwarf binary 2MASSJ05352184-0546085 taken at the two opposite radial velocity maxima. Comparisons of the TiO bands to model and template spectra are fully consistent with the temperatures previously ... More
Magnetic fields in M-dwarfs: quantitative results from detailed spectral synthesis in FeH linesDec 07 2010Strong surface magnetic fields are ubiquitously found in M-dwarfs with mean intensities on the order of few thousand Gauss-three orders of magnitude higher than the mean surface magnetic field of the Sun. These fields and their interaction with photospheric ... More
Detecting Planets Around Very Low Mass Stars with the Radial Velocity MethodAug 31 2009Dec 11 2009The detection of planets around very low-mass stars with the radial velocity method is hampered by the fact that these stars are very faint at optical wavelengths where the most high-precision spectrometers operate. We investigate the precision that can ... More
Strong dipole magnetic fields in fast rotating fully convective starsJan 25 2018M dwarfs are the most numerous stars in our Galaxy with masses between approximately 0.5 and 0.1 solar mass. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes, and large-scale magnetic fields. Such ... More
Modelling the molecular Zeeman effect in M-dwarfs: methods and first resultsAug 15 2010We present first quantitative results of the surface magnetic field measurements in selected M-dwarfs based on detailed spectra synthesis conducted simultaneously in atomic and molecular lines of the FeH Wing-Ford $F^4\,\Delta-X^4\,\Delta$ transitions. ... More
Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-starDec 09 2004We have derived the mean broadening profile of the star V102 in the region of the open cluster IC4665 from high resolution spectroscopy. At a projected equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong deviation from classical rotation. ... More
From solar-like to anti-solar differential rotation in cool starsNov 13 2013Stellar differential rotation can be separated into two main regimes: solar-like when the equator rotates faster than the poles and anti-solar when the polar regions rotate faster than the equator. We investigate the transition between these two regimes ... More
A search for star-planet interactions in the upsilon Andromedae system at X-ray and optical wavelengthsOct 27 2010Feb 08 2011Close-in, giant planets are expected to influence their host stars via tidal or magnetic interaction. But are these effects strong enough in suitable targets known so far to be observed with today's instrumentation? The upsilon And system, an F8V star ... More
Capture and release of a conditional state of a cavity QED system by quantum feedbackFeb 12 2002Detection of a single photon escaping an optical cavity QED system prepares a non-classical state of the electromagnetic field. The evolution of the state can be modified by changing the drive of the cavity. For the appropriate feedback, the conditional ... More
What controls the large-scale magnetic fields of M dwarfs?Sep 24 2013Observations of active M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. We detail the analogy between some anelastic dynamo simulations and spectropolarimetric observations of 23 M stars. ... More
Multiwavelength observations of a giant flare on CN Leonis I. The chromosphere as seen in the optical spectraJul 13 2008Flares on dM stars contain plasmas at very different temperatures and thus affect a wide wavelength range in the electromagnetic spectrum. While the coronal properties of flares are studied best in X-rays, the chromosphere of the star is observed best ... More
Mechanically Reinforced MgB2 Wires and Tapes with High Transport CurrentsApr 29 2002In this contribution, different concepts for mechanically reinforced MgB2 wires with high transport currents are presented and characterised. It is shown that the achieved critical transport currents depend strongly on the conductor concept, the choice ... More
Time evolution and squeezing of the field amplitude in cavity QEDDec 18 2000We present the conditional time evolution of the electromagnetic field produced by a cavity QED system in the strongly coupled regime. We obtain the conditional evolution through a wave-particle correlation function that measures the time evolution of ... More
What controls the magnetic geometry of M dwarfs?Dec 01 2012Context: observations of rapidly rotating M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. In dynamo models, the relative importance of inertia in the force balance -- quantified by ... More
Transport spin polarisation in SrRuO3 measured through Point Contact Andreev reflectionAug 02 2002Nov 05 2002We report a study in which Andreev reflection using a Nb point contact is used to measure the transport spin polarisation of the 4d itinerant ferromagnet SrRuO3. By performing the study in high quality thin films with residual resistivities less than ... More
Can stellar activity make a planet seem misaligned?Jul 11 2016Several studies have shown that the occultation of stellar active regions by the transiting planet can generate anomalies in the high-precision transit light curves, and these anomalies may lead to an inaccurate estimate of the planetary parameters (e.g., ... More
A Monte Carlo study of surface sputtering by dual and rotated ion beamsSep 27 2011Several, recently proposed methods of surface manufacturing based on ion beam sputtering, which involve dual beam setups, sequential application of ion beams from different directions, or sample rotation, are studied with the method of kinetic Monte Carlo ... More
Weyl group $q$-Kreweras numbers and cyclic sievingMay 30 2016The paper concerns a definition for $q$-Kreweras numbers for finite Weyl groups $W$, refining the $q$-Catalan numbers for $W$, and arising from work of the second author. We give explicit formulas in all types for the $q$-Kreweras numbers. In the classical ... More
Wavepacket-current <--> supercurrent conversion in closed N/S/N circuitsSep 26 2000The rise and decay in space and time of a supercurrent in a superconductor between two normal current leads connected to a reservoir is computed with the help of quasiparticle wavepackets which suffer Andreev scattering in the interfaces between the normal ... More
Estimation of Radio Channel Parameters in Case of an Unknown TransmitterDec 11 2015This paper investigates the estimation of radio channel parameters from receiver data, whereby the transmitter is fully unknown. We use a multipath model to describe the radio channel between transmitter and receiver. According to this model, we discuss ... More
A Volume-limited Sample of 63 M7-M9.5 Dwarfs II. Activity, magnetism, and the fade of the rotation-dominated dynamoDec 21 2009In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, ... More
Conjectures on the cohomology of the GrassmannianSep 17 2003We give a series of successively weaker conjectures on the cohomology ring of the Grassmannian, starting with the Hilbert series of a certain natural filtration.
(q,t)-analogues and GL_n(F_q)Apr 18 2008Jun 16 2009We start with a (q,t)-generalization of a binomial coefficient. It can be viewed as a polynomial in t that depends upon an integer q, with combinatorial interpretations when q is a positive integer, and algebraic interpretations when q is the order of ... More
The cyclotomic polynomial topologicallyDec 08 2010Apr 16 2012We interpret the coefficients of the cyclotomic polynomial in terms of simplicial homology.
A universal coefficient theorem for Gauss's LemmaSep 27 2012Oct 24 2012We prove a version of Gauss's Lemma. It recursively constructs polynomials {c_k} for k=0,1,...,m+n, in Z[a_i,A_i,b_j,B_j] for i=0,...,m, and j=0,1,...,n, having degree at most (m+n choose m) in each of the four variable sets, such that whenever {A_i},{B_j},{C_k} ... More
P-partitions revisitedJun 30 2011Feb 15 2012We compare a traditional and non-traditional view on the subject of P-partitions, leading to formulas counting linear extensions of certain posets.
The lattice of closure relations of a posetSep 17 1994In this paper we show that the set of closure relations on a finite poset P forms a supersolvable lattice, as suggested by Rota. Furthermore this lattice is dually isomorphic to the lattice of closed sets in a convex geometry (in the sense of Edelman ... More
Improved orbital solution and masses for the very low-mass multiple system LHS 1070Mar 05 2008Jun 04 2008We present a refined orbital solution for the components A, B, and C of the nearby late-M type multiple system LHS 1070. By combining astrometric datapoints from NACO/VLT, CIAO/SUBARU, and PUEO/CFHT, as well as a radial velocity measurement from the newly ... More
Spreading of EGF Receptor Activity into EGF-free Regions and Molecular Therapies of CancerMar 18 2008The primary activation of the epidermal growth factor receptor (EGFR) has become a prominent target for molecular therapies against several forms of cancer. But despite considerable progress during the last years, many of its aspects remain poorly understood. ... More
Differential posets and Smith normal formsNov 12 2008We conjecture a strong property for the up and down maps U and D in an r-differential poset: DU+tI and UD+tI have Smith normal forms over Z[t]. In particular, this would determine the integral structure of the maps U, D, UD, DU, including their ranks ... More
Do absolutely irreducible group actions have odd dimensional fixed point spaces?Nov 17 2010In his volume [5] on "Symmetry Breaking for Compact Lie Groups" Mike Field quotes a private communication by Jorge Ize claiming that any bifurcation problem with absolutely irreducible group action would lead to bifurcation of steady states. The proof ... More
Representation stability for cohomology of configuration spaces in $\mathbf{R}^d$May 15 2015Dec 11 2015This paper studies representation stability in the sense of Church and Farb for representations of the symmetric group $S_n$ on the cohomology of the configuration space of $n$ ordered points in $\mathbf{R}^d$. This cohomology is known to vanish outside ... More
Is it possible to detect planets around young active G and K dwarfs?Nov 14 2013Theoretical predictions suggest that the distribution of planets in very young stars could be very different to that typically observed in Gyr old systems that are the current focus of radial velocity surveys. However, the detection of planets around ... More
Ground-based detection of an extended helium atmosphere in the Saturn-mass exoplanet WASP-69bDec 07 2018Hot gas giant exoplanets can lose part of their atmosphere due to strong stellar irradiation, affecting their physical and chemical evolution. Studies of atmospheric escape from exoplanets have mostly relied on space-based observations of the hydrogen ... More
The rotation-magnetic field relationSep 26 2008Today, the generation of magnetic fields in solar-type stars and its relation to activity and rotation can coherently be explained, although it is certainly not understood in its entirety. Rotation facilitates the generation of magnetic flux that couples ... More
Plyades: A Python Library for Space Mission DesignJul 01 2016Plyades: A Python Library for Space Mission Design Designing a space mission is a computation-heavy task. Software tools that conduct the necessary numerical simulations and optimizations are therefore indispensable. The usability of existing software, ... More
Dynamical Transitions in a Dragged Growing Polymer ChainJul 22 2016We extend the Rouse model of polymer dynamics to situations of non-stationary chain growth. For a dragged polymer chain of length $N(t) = t^\alpha$, we find two transitions in conformational dynamics. At $\alpha= 1/2$, the propagation of tension and the ... More
Implementation of the Hierarchical Reference Theory for simple one-component fluidsDec 03 2001Apr 12 2002Combining renormalization group theoretical ideas with the integral equation approach to fluid structure and thermodynamics, the Hierarchical Reference Theory is known to be successful even in the vicinity of the critical point and for sub-critical temperatures. ... More
The First High-Resolution Spectra of 1.3 L SubdwarfsDec 06 2005We present the first high-resolution (R ~ 31,000) spectra of the cool sdL 2MASS0532, and what was originally identified as an early-type L subdwarf (sdL) LSR1610-0040. Our work, in combination with contemporaneous work by Cushing and Vacca, makes it clear ... More