1,357,279 research outputs found
Ramkumar et al. (2016) Premotor and motor cortices encode reward
No full textA dataset that was used in a paper by Ramkumar et al. (2016), PLoS One,
to report the effect of reward on firing rates in the macaque premotor
and motor cortices
On the smoothness of lexicographic points on Hilbert schemes
No full textWe study the geometry of standard graded Hilbert schemes of polynomial rings and exterior algebras. Our investigation is motivated by a famous theorem of Reeves–Stillman for the Grothendieck Hilbert scheme, which states that the lexicographic point is smooth. By contrast, we show that, in standard graded Hilbert schemes of polynomial rings and exterior algebras, the lexicographic point can be singular, and it can lie in multiple irreducible components. We answer questions of Peeva–Stillman and of Maclagan–Smith
Rational singularities of nested Hilbert schemes
No full textThe Hilbert scheme of points Hilb^n(S) of a smooth surface S is a well-studied parameter space, lying at the interface of algebraic geometry, commutative algebra, representation theory, combinatorics, and mathematical physics. The foundational result is a classical theorem of Fogarty, stating that Hilb^n(S) is a smooth variety of dimension 2n. In recent years there has been growing interest in a natural generalization of Hilb^n(S), the nested Hilbert scheme Hilb^(n1,n2)(S), which parametrizes nested pairs of zero-dimensional subschemes Z1 ⊇ Z2 of S with deg Zi = ni. In contrast to Fogarty’s theorem, Hilb^(n1,n2)(S) is almost always singular, and very little is known about its singularities. In this paper, we aim to advance the knowledge of the geometry of these nested Hilbert schemes. Work by Fogarty in the 70’s shows that Hilb(n,1)(S) is a normal Cohen–Macaulay variety, and Song more recently proved that it has rational singularities. In our main result, we prove that the nested Hilbert scheme Hilb(n,2)(S) has rational singularities. We employ an array of tools from commutative algebra to prove this theorem. Using Gröbner bases, we establish a connection between Hilb(n,2)(S) and a certain variety of matrices with an action of the general linear group. This variety of matrices plays a central role in our work, and we analyze it by various algebraic techniques, including the Kempf–Lascoux–Weyman technique of calculating syzygies, square-free Gröbner degenerations, and the Stanley–Reisner correspondence. Along the way, we also obtain results on classes of irreducible and reducible nested Hilbert schemes, dimension of singular loci, and F-singularities in positive characteristic
On Rees algebras of 2-determinantal ideals
No full textLet I be the ideal of minors of a 2 by n matrix of linear forms with the
expected codimension. In this paper we prove that the Rees algebra of I and its
special fiber ring are Cohen-Macaulay and Koszul; in particular, they are
quadratic algebras. The main novelty in our approach is the analysis of a
stratification of the Hilbert scheme of determinantal ideals. We study
degenerations of Rees algebras along this stratification, and combine it with
certain squarefree Groebner degenerations.Comment: Final version. To appear on the Journal of the London Mathematical
Societ
On the tangent space to the Hilbert scheme of points in P3
No full textIn this paper we study the tangent space to the Hilbert scheme
, motivated by Haiman's work on and by a long-standing conjecture of Brian\c{c}on and Iarrobino
on the most singular point in . For points
parametrizing monomial subschemes, we consider a decomposition of the tangent
space into six distinguished subspaces, and show that a fat point exhibits an
extremal behavior in this respect. This decomposition is also used to
characterize smooth monomial points on the Hilbert scheme. We prove the first
Brian\c{c}on-Iarrobino conjecture up to a factor of 4/3, and improve the known
asymptotic bound on the dimension of .
Furthermore, we construct infinitely many counterexamples to the second
Brian\c{c}on-Iarrobino conjecture, and we also settle a weaker conjecture of
Sturmfels in the negative.Comment: 20 pages. Final version; to appear on Transactions of the American
Mathematical Societ
Experimental evaluation of long term evolution-based NC OFDM secondary-to-secondary interference
No full textScarcity of spectrum resources, inefficient spectrum usage and the inflexibility of the current spectrum assignment are few of the major roadblocks in the development of new wireless communication standards. Secondary spectrum sharing has become a viable solution to alleviate this problem. Secondary users are unlicensed devices that use opportunistic spectrum access to identify vacant frequency bins and thereby utilize the spectrum. For advanced wireless communication standards like the Long Term Evolution (LTE) which primarily calls for higher data rates, evaluation of design parameters for ensuring efficient coexistence of heterogeneous secondary users and guaranteeing acceptable minimum level of performance becomes essential. Additionally, the understanding of the interference between secondary users occupying adjacent frequency bands for their transmission is imperative. This thesis focuses on the coexistence of secondary users in the same band assuming that the primary spectrum is found available. By Implementing two Non Contiguous Orthogonal Frequency Division Multiplexing ( NC-OFDM) based secondary transmitters on a real time platform, the design parameters that need to be considered to ensure efficient coexistence have been identified and investigated. The performance degradations observed at a particular secondary link due to presence of another interfering secondary link occupying adjacent frequency bands for its transmission have also been studied. This thesis also focuses on implementation of algorithms to modify the existing NC-OFDM transmission at the secondary transmitter end to reduce its Interference effects on the other secondary links operating within the same band. The focus is on an LTE-based Secondary Non Contiguous Orthogonal Frequency Division Multiplexing Transceiver on a Real Time Platform developed by National Instruments. The various blocks needed to design a real time LTE based communications links are discussed. An experimental LTE-to-LTE interference analysis based on the Real Time Platform and the designed system is presented.M.S.Includes bibliographical referencesby Ajay Ramkumar Iye
Premotor and motor cortices encode reward
No full textA dataset that was used in a paper by Ramkumar et al. (2016), PLoS One, to report the effect of reward on firing rates in the macaque premotor and motor cortices<br
Regulation of senescence in cancer and aging
No full textCo-author Charusheila Ramkumar is a student in the Cell Biology program in the Graduate School of Biomedical Sciences (GSBS) at UMass Medical School.Senescence is regarded as a physiological response of cells to stress, including telomere dysfunction, aberrant oncogenic activation, DNA damage, and oxidative stress. This stress response has an antagonistically pleiotropic effect to organisms: beneficial as a tumor suppressor, but detrimental by contributing to aging. The emergence of senescence as an effective tumor suppression mechanism is highlighted by recent demonstration that senescence prevents proliferation of cells at risk of neoplastic transformation. Consequently, induction of senescence is recognized as a potential treatment of cancer. Substantial evidence also suggests that senescence plays an important role in aging, particularly in aging of stem cells. In this paper, we will discuss the molecular regulation of senescence its role in cancer and aging. The potential utility of senescence in cancer therapeutics will also be discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The influence of contaminants and their interactions of diesel engine oil tribology using electrostatic condition monitoring
No full textIn modern automotive engines, especially in diesel engines, consumer demand for ever
increasing service intervals for vehicles has led to longer oil drain periods.
Consequently this has increased contamination levels in lubricating oils that will in turn
reduce engine efficiency and increase the possibility of system failure due to increases
in viscosity and the potential of oil starvation leading to scuffing and catastrophic
failure of the engine. Therefore it is necessary to investigate the effects of diesel engine
oil contaminants on the tribological performance of tribocontacts and also the possible
interaction between the contaminants. The thesis aims to investigate the influence of
contaminants and their interactions on diesel engine oil using Electro sensing (ES)
monitoring.
ES monitoring suggest sensitivity to tribocharging, triboemission, surface charging, and
wear debris generated in the tribocontact. However, ES monitoring is still in its initial
stages of development but the relationships between the ES charge signal and
corresponding wear mechanisms are established.
Initial experiments showed the sensitivities between charge and wear to changes in
additive chemistry, as well as contamination (specifically a soot stimulant). The results
indicated some interesting findings which were difficult to elucidate but paved the way
for a parametric study.
The parametric matrix examined the effect of contaminants and it was observed that
wear rate was mainly influenced by acid and soot additions. The best correlation of
steady state charge with the other measured tribological parameters of wear rate, friction
and temperature is seen for the series of oxidised oils. The multi-contaminated oil
(L4×4) shows remarkably little degradation in tribological performance with a reduced
friction level compared with the uncontaminated oil and only a modest increase in wear
rate, slight increase in charge levels. This clearly showed that there were interactions
between the contaminants.
The factorial matrix studied interactions between the diesel engine oil contaminants on
various tribo-couple materials, including silicon nitride, using ES monitoring. For
charge, pin material – sulphuric acid and oxidation –sulphuric acid, interactions were
statistically identified as having significant influence on performance. For friction, both
pin and disc materials were identified as main influences along with pin material –
sulphuric acid interactions. For wear, the presence of soot and the influence of pin
material were identified statistically as the main effects.
To conclude, the ES monitoring was sensitive to the presence and levels of
contaminants in diesel lubricating oil, particularly diesel soot. The change in charge
levels indicated the concentration of soot level present in the contact, which was directly
related to wear. ES monitoring also detected interactions between the contaminants
through statistical analysis. ES monitoring has shown that monitoring lubricant
performance and the effects of contamination are feasible under laboratory conditions
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