759 research outputs found
Twisted k-graph algebras associated to Bratteli diagrams
Given a system of coverings of k-graphs, we show that the cohomology of the
resulting (k+1)-graph is isomorphic to that of any one of the k-graphs in the
system. We then consider Bratteli diagrams of 2-graphs whose twisted
C*-algebras are matrix algebras over noncommutative tori. For such systems we
calculate the ordered K-theory and the gauge-invariant semifinite traces of the
resulting 3-graph C*-algebras. We deduce that every simple C*-algebra of this
form is Morita equivalent to the C*-algebra of a rank-2 Bratteli diagram in the
sense of Pask-Raeburn-R{\o}rdam-Sims.Comment: 28 pages, pictures prepared using tik
Comparison theory and smooth minimal C*-dynamics
We prove that the C*-algebra of a minimal diffeomorphism satisfies
Blackadar's Fundamental Comparability Property for positive elements. This
leads to the classification, in terms of K-theory and traces, of the
isomorphism classes of countably generated Hilbert modules over such algebras,
and to a similar classification for the closures of unitary orbits of
self-adjoint elements. We also obtain a structure theorem for the Cuntz
semigroup in this setting, and prove a conjecture of Blackadar and Handelman:
the lower semicontinuous dimension functions are weakly dense in the space of
all dimension functions. These results continue to hold in the broader setting
of unital simple ASH algebras with slow dimension growth and stable rank one.
Our main tool is a sharp bound on the radius of comparison of a recursive
subhomogeneous C*-algebra. This is also used to construct uncountably many
non-Morita-equivalent simple separable amenable C*-algebras with the same
K-theory and tracial state space, providing a C*-algebraic analogue of McDuff's
uncountable family of II_1 factors. We prove in passing that the range of the
radius of comparison is exhausted by simple C*-algebras.Comment: 30 pages, no figure
On embedding of the Bratteli diagram into a surface
We study C*-algebras O_{\lambda} which arise in dynamics of the interval
exchange transformations and measured foliations on compact surfaces. Using
Koebe-Morse coding of geodesic lines, we establish a bijection between Bratteli
diagrams of such algebras and measured foliations. This approach allows us to
apply K-theory of operator algebras to prove strict ergodicity criterion and
Keane's conjecture for the interval exchange transformations.Comment: final versio
Unstable solitons on noncommutative tori and D-branes
We describe a class of exact solutions of super Yang-Mills theory on
even-dimensional noncommutative tori. These solutions generalize the solitons
on a noncommutative plane introduced in hep-th/0009142 that are conjectured to
describe unstable D2p-D0 systems. We show that the spectrum of quadratic
fluctuations around our solutions correctly reproduces the string spectrum of
the D2p-D0 system in the Seiberg-Witten decoupling limit. In particular the
fluctuations correctly reproduce the 0-0 string winding modes. For p=1 and p=2
we match the differences between the soliton energy and the energy of an
appropriate SYM BPS state with the binding energies of D2-D0 and D4-D0 systems.
We also give an example of a soliton that we conjecture describes branes of
intermediate dimension on a torus such as a D2-D4 system on a four-torus.Comment: 22 pages, Latex; v.2: references adde
Circulating myostatin is reduced with aging in humans but not altered by short-term, high intensity training
Introduction: Ageing involves a loss of muscle mass and function. The rate of decline is associated with negative health outcomes and increased mortality (1). Muscle atrophy is observed at a predictable rate from 30 years of age (2), however maintenance of function is seen in masters athletes > 60 years of age (3). Myostatin acts as a negative regulator of muscle mass (4) and underlies hypertrophy with chronic resistance training (5) and atrophy in chronic conditions (4). Experiment 1: Declared healthy participants (n = 83, 18 - 75 years of age, 36 male, 47 female) were recruited. Body composition, metabolic rate, grip strength and 6-minute walk test were recorded. Venous blood was collected and total myostatin concentration (herein referred to as myostatin) quantified by enzyme-linked immunosorbent assay. Total myostatin was lower in females compared with males (2176.1 [135.3] vs. 2788.7 [180.2] pg.mL-1 [p = 0.007]). Stepwise regression observed that myostatin concentration is best predicted firstly by gender, then by age (r = 0.399, p = 0.02), and was not further improved by the addition of measures of metabolism, muscle mass or function. Experimental 2: A cohort of aged sedentary (SED) males (n = 14; 63.9 [5.6] years of age) and masters athletes (lifelong exerciser [LEX]; n = 10, 61.1 [5.8] years of age) completed 6 weeks of high intensity interview training (HITT). Two way ANOVA suggested no group (SED, LEX) × time (pre, post) interaction on myostatin concentration (p = 0.649), nor a main effect of time (p = 0.757), however there was a trend towards increased myostatin in the LEX group relative to SED (p = 0.083). Discussion: Loss of muscle mass and function occurs at a predictable rate from ~30 years of age, however the rate of loss differs between active and inactive populations. Here we demonstrate that total circulating myostatin decreases as age increases, and differs significantly between males and females. Total circulating myostatin negatively correlates with increasing age, however alterations in myostatin do not appear after short term training interventions. Longer term activity may alter myostatin, thus our next work will follow up experiment 2 with a 3 year longitudinal analysis
Electron Spin Relaxation in a Semiconductor Quantum Well
A fully microscopic theory of electron spin relaxation by the
D'yakonov-Perel' type spin-orbit coupling is developed for a semiconductor
quantum well with a magnetic field applied in the growth direction of the well.
We derive the Bloch equations for an electron spin in the well and define
microscopic expressions for the spin relaxation times. The dependencies of the
electron spin relaxation rate on the lowest quantum well subband energy,
magnetic field and temperature are analyzed.Comment: Revised version as will appear in Physical Review
Wilson function transforms related to Racah coefficients
The irreducible -representations of the Lie algebra consist of
discrete series representations, principal unitary series and complementary
series. We calculate Racah coefficients for tensor product representations that
consist of at least two discrete series representations. We use the explicit
expressions for the Clebsch-Gordan coefficients as hypergeometric functions to
find explicit expressions for the Racah coefficients. The Racah coefficients
are Wilson polynomials and Wilson functions. This leads to natural
interpretations of the Wilson function transforms. As an application several
sum and integral identities are obtained involving Wilson polynomials and
Wilson functions. We also compute Racah coefficients for U_q(\su(1,1)), which
turn out to be Askey-Wilson functions and Askey-Wilson polynomials.Comment: 48 page
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