777 research outputs found
Calculating conjugacy classes in Sylow p-subgroups of finite Chevalley groups of rank six and seven
Let G(q) be a finite Chevalley group, where q is a power of a good prime p,
and let U(q) be a Sylow p-subgroup of G(q). Then a generalized version of a
conjecture of Higman asserts that the number k(U(q)) of conjugacy classes in
U(q) is given by a polynomial in q with integer coefficients. In an earlier
paper, the first and the third authors developed an algorithm to calculate the
values of k(U(q)). By implementing it into a computer program using GAP, they
were able to calculate k(U(q)) for G of rank at most 5, thereby proving that
for these cases k(U(q)) is given by a polynomial in q. In this paper we present
some refinements and improvements of the algorithm that allow us to calculate
the values of k(U(q)) for finite Chevalley groups of rank six and seven, except
E_7. We observe that k(U(q)) is a polynomial, so that the generalized Higman
conjecture holds for these groups. Moreover, if we write k(U(q)) as a
polynomial in q-1, then the coefficients are non-negative.
Under the assumption that k(U(q)) is a polynomial in q-1, we also give an
explicit formula for the coefficients of k(U(q)) of degrees zero, one and two.Comment: 16 page
On the coadjoint orbits of maximal unipotent subgroups of reductive groups
Let G be a simple algebraic group defined over an algebraically closed field
of characteristic 0 or a good prime for G. Let U be a maximal unipotent
subgroup of G and \u its Lie algebra. We prove the separability of orbit maps
and the connectedness of centralizers for the coadjoint action of U on (certain
quotients of) the dual \u* of \u. This leads to a method to give a
parametrization of the coadjoint orbits in terms of so-called minimal
representatives which form a disjoint union of quasi-affine varieties.
Moreover, we obtain an algorithm to explicitly calculate this parametrization
which has been used for G of rank at most 8, except E8.
When G is defined and split over the field of q elements, for q the power of
a good prime for G, this algorithmic parametrization is used to calculate the
number k(U(q), \u*(q)) of coadjoint orbits of U(q) on \u*(q). Since k(U(q),
\u*(q)) coincides with the number k(U(q)) of conjugacy classes in U(q), these
calculations can be viewed as an extension of the results obtained in our
earlier paper. In each case considered here there is a polynomial h(t) with
integer coefficients such that for every such q we have k(U(q)) = h(q).Comment: 14 pages; v2 23 pages; to appear in Transformation Group
Hybrid Local-Order Mechanism for Inversion Symmetry Breaking
Using classical Monte Carlo simulations, we study a simple statistical
mechanical model of relevance to the emergence of polarisation from local
displacements on the square and cubic lattices. Our model contains two key
ingredients: a Kitaev-like orientation-dependent interaction between nearest
neighbours, and a steric term that acts between next-nearest neighbours. Taken
by themselves, each of these two ingredients is incapable of driving long-range
symmetry breaking, despite the presence of a broad feature in the corresponding
heat capacity functions. Instead each component results in a "hidden"
transition on cooling to a manifold of degenerate states, the two manifolds are
different in the sense that they reflect distinct types of local order.
Remarkably, their intersection---\emph{i.e.} the ground state when both
interaction terms are included in the Hamiltonian---supports a spontaneous
polarisation. In this way, our study demonstrates how local ordering mechanisms
might be combined to break global inversion symmetry in a manner conceptually
similar to that operating in the "hybrid" improper ferroelectrics. We discuss
the relevance of our analysis to the emergence of spontaneous polarisation in
well-studied ferroelectrics such as BaTiO and KNbO.Comment: 8 pages, 8 figure
Help-seeking behaviors and mental well-being of first year undergraduate university students
University students demonstrate poor help-seeking behaviors for their mental health, despite often reporting low levels of mental well-being. The aims of this study were to examine the help-seeking intentions and experiences of first year university students in terms of their mental well-being, and to explore these students’ views on formal (e.g. psychiatrists) and informal (e.g. friends) help-seeking. Students from a university in the Republic of Ireland (n=220) completed an online questionnaire which focused on mental well-being and help-seeking behaviors. Almost a third of students had sought help from a mental health professional. Very few students reported availing of university/online supports. Informal sources of help were more popular than formal sources, and those who would avail and had availed of informal sources demonstrated higher well-being scores. Counselors were the source of professional help most widely used. General practitioners, chaplains, social workers, and family therapists were rated the most helpful. Those with low/average well-being scores were less likely to seek help than those with higher scores. Findings indicate the importance of enhancing public knowledge of mental health issues, and for further examination of students’ knowledge of help-seeking resources in order to improve the help-seeking behaviors and mental well-being of this population group
Orbital Dimer Model for Spin-Glass State in YMoO
The formation of a spin glass usually requires both structural disorder and
frustrated magnetic interactions. Consequently, the origin of spin-glass
behaviour in YMoO in which magnetic Mo ions occupy a
frustrated pyrochlore lattice with minimal compositional disorder has been
a longstanding question. Here, we use neutron and X-ray pair-distribution
function (PDF) analysis to develop a disorder model that resolves apparent
incompatibilities between previously-reported PDF, EXAFS and NMR studies and
provides a new and physical mechanism for spin-glass formation. We show that
Mo ions displace according to a local "2-in/2-out" rule on each Mo
tetrahedron, driven by orbital dimerisation of Jahn-Teller active Mo
ions. Long-range orbital order is prevented by the macroscopic degeneracy of
dimer coverings permitted by the pyrochlore lattice. Cooperative O
displacements yield a distribution of MoOMo angles, which in turn
introduces disorder into magnetic interactions. Our study demonstrates
experimentally how frustration of atomic displacements can assume the role of
compositional disorder in driving a spin-glass transition.Comment: 6 pages, 3 figure
Public attitudes to transport: Knowledge review of existing evidence
Report for Department for Transpor
Hubble Space Telescope Observations of the Oldest Star Clusters in the LMC
We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters
in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257 and Hodge 11. Our data
extend about 3 magnitudes below the main-sequence turnoff, allowing us to
determine accurate relative ages and the blue straggler frequencies. Based on a
differential comparison of the CMDs, any age difference between the three LMC
clusters is less than 1.5 Gyr. Comparing their CMDs to those of M 92 and M 3,
the LMC clusters, unless their published metallicities are significantly in
error, are the same age as the old Galactic globulars. The similar ages to
Galactic globulars are shown to be consistent with hierarchial clustering
models of galaxy formation. The blue straggler frequencies are also similar to
those of Galactic globular clusters. We derive a true distance modulus to the
LMC of (m-M)=18.46 +/- 0.09 (assuming (m-M)=14.61 for M 92) using these three
LMC clusters.Comment: 22 pages; to be published in Ap
Behaviour of infants with Down syndrome and their mothers in the still-face procedure
There has been limited study of how the constitutional characteristics of infants with Down syndrome (DS) influence the patterning of their relations with caregivers. To assess natural and perturbed interactions between infants with DS and their mothers, we tested ten 6-month-old infants with DS and 20 typically developing (TD) 4-month-old of similar mental age. Participants were videotaped with their mothers in a natural face-to-face interaction, a brief period when the mothers adopted a still-face, and a subsequent reengagement phase. There was little to distinguish the infants in the initial phase of natural interaction, but the mothers of infants with DS were more likely to show assertive warmth, and unlike in the case of mothers of TD infants, high maternal directiveness tended to be associated with lower levels of infant looking and lack of fussing. During the still-face episode, infants of both groups showed reduced looking and smiling, although infants with DS tended to show lower levels of fussing and fewer in this group showed fussing in the reengagement phase. Therefore DS infants were somewhat similar to TD infants of comparable mental age in being responsive to the still-face procedure, but showed indications of group differences in intense emotional reactivity
Exploration of Antiferromagnetic CoO and NiO using Reverse Monte Carlo Total Neutron Scattering Refinement
The atomic and magnetic structures of CoO and NiO have been probed using reverse Monte Carlo refinements of neutron total scattering data. The results obtained show that the known magnetic structure for NiO can be recovered by the reverse Monte Carlo process starting from random spin configurations, but it is insensitive to the spin direction in the {111} ferromagnetic planes. Refinements of the magnetic structure of CoO starting from random spin configurations result in collinear or non-collinear magnetic structure, consistent with those reported by other techniques. Starting from an ordered collinear spin structure for CoO and NiO leads to different results than when starting from a random arrangement of spins, evidence for configurational bias that highlights the need to take care when selecting a starting model for reverse Monte Carlo refinements of magnetic structures
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A near-field optical microscopy nanoarray
Multiplexing near-field scanning optical microscopy (NSOM) by the use of a nanoarray with parallel imaging is studied. The fabrication, characterization, and utilization of nanoarrays with {approximately} 100 nm diameter apertures spaced 500 nm center-to- center is presented. Extremely uniform nanoarrays with {approximately} 10{sup 8} apertures were fabricated by electron beam lithography and reactive ion etching. The nanoarrays were characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). In this paper we utilize these nanoarrays in a laser-illuminated microscope with parallel detection on a charge- coupled device (CCD). Detection of B-phycoerythrin (B-PE) molecules using near-field illumination is presented. In principle, our system can be used to obtain high lateral resolution NSOM images over a wide-field of view (e.g. 50-100 {mu}m) within seconds
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