72 research outputs found
Carnot-Caratheodory metric and gauge fluctuation in Noncommutative Geometry
Gauge fields have a natural metric interpretation in terms of horizontal
distance. The latest, also called Carnot-Caratheodory or subriemannian
distance, is by definition the length of the shortest horizontal path between
points, that is to say the shortest path whose tangent vector is everywhere
horizontal with respect to the gauge connection. In noncommutative geometry all
the metric information is encoded within the Dirac operator D. In the classical
case, i.e. commutative, Connes's distance formula allows to extract from D the
geodesic distance on a riemannian spin manifold. In the case of a gauge theory
with a gauge field A, the geometry of the associated U(n)-vector bundle is
described by the covariant Dirac operator D+A. What is the distance encoded
within this operator ? It was expected that the noncommutative geometry
distance d defined by a covariant Dirac operator was intimately linked to the
Carnot-Caratheodory distance dh defined by A. In this paper we precise this
link, showing that the equality of d and dh strongly depends on the holonomy of
the connection. Quite interestingly we exhibit an elementary example, based on
a 2 torus, in which the noncommutative distance has a very simple expression
and simultaneously avoids the main drawbacks of the riemannian metric (no
discontinuity of the derivative of the distance function at the cut-locus) and
of the subriemannian one (memory of the structure of the fiber).Comment: published version with additional figures to make the proof more
readable. Typos corrected in this ultimate versio
On Pythagoras' theorem for products of spectral triples
We discuss a version of Pythagoras theorem in noncommutative geometry. Usual
Pythagoras theorem can be formulated in terms of Connes' distance, between pure
states, in the product of commutative spectral triples. We investigate the
generalization to both non pure states and arbitrary spectral triples. We show
that Pythagoras theorem is replaced by some Pythagoras inequalities, that we
prove for the product of arbitrary (i.e. non-necessarily commutative) spectral
triples, assuming only some unitality condition. We show that these
inequalities are optimal, and provide non-unital counter-examples inspired by
K-homology.Comment: Paper slightly shortened to match the published version; Lett. Math.
Phys. 201
Using the Wigner-Ibach Surmise to Analyze Terrace-Width Distributions: History, User's Guide, and Advances
A history is given of the applications of the simple expression generalized
from the surmise by Wigner and also by Ibach to extract the strength of the
interaction between steps on a vicinal surface, via the terrace width
distribution (TWD). A concise guide for use with experiments and a summary of
some recent extensions are provided.Comment: 11 pages, 4 figures, reformatted (with revtex) version of refereed
paper for special issue of Applied Physics A entitled "From Surface Science
to Device Physics", in honor of the retirements of Prof. H. Ibach and Prof.
H. L\"ut
The E8 geometry from a Clifford perspective
This paper considers the geometry of from a Clifford point of view in three complementary ways. Firstly, in earlier work, I had shown how to construct the four-dimensional exceptional root systems from the 3D root systems using Clifford techniques, by constructing them in the 4D even subalgebra of the 3D Clifford algebra; for instance the icosahedral root system gives rise to the largest (and therefore exceptional) non-crystallographic root system . Arnold's trinities and the McKay correspondence then hint that there might be an indirect connection between the icosahedron and . Secondly, in a related construction, I have now made this connection explicit for the first time: in the 8D Clifford algebra of 3D space the elements of the icosahedral group are doubly covered by 8-component objects, which endowed with a `reduced inner product' are exactly the root system. It was previously known that splits into -invariant subspaces, and we discuss the folding construction relating the two pictures. This folding is a partial version of the one used for the construction of the Coxeter plane, so thirdly we discuss the geometry of the Coxeter plane in a Clifford algebra framework. We advocate the complete factorisation of the Coxeter versor in the Clifford algebra into exponentials of bivectors describing rotations in orthogonal planes with the rotation angle giving the correct exponents, which gives much more geometric insight than the usual approach of complexification and search for complex eigenvalues. In particular, we explicitly find these factorisations for the 2D, 3D and 4D root systems, as well as , whose Coxeter versor factorises as . This explicitly describes 30-fold rotations in 4 orthogonal planes with the correct exponents arising completely algebraically from the factorisation
Simulating rewetting events in intermittent rivers and ephemeral streams: A global analysis of leached nutrients and organic matter
Climate change and human pressures are changing the global distribution and the exâ
tent of intermittent rivers and ephemeral streams (IRES), which comprise half of the
global river network area. IRES are characterized by periods of flow cessation, during
which channel substrates accumulate and undergo physicoâchemical changes (preconâ
ditioning), and periods of flow resumption, when these substrates are rewetted and
release pulses of dissolved nutrients and organic matter (OM). However, there are no
estimates of the amounts and quality of leached substances, nor is there information
on the underlying environmental constraints operating at the global scale. We experiâ
mentally simulated, under standard laboratory conditions, rewetting of leaves, riverâ
bed sediments, and epilithic biofilms collected during the dry phase across 205 IRES
from five major climate zones. We determined the amounts and qualitative characterâ
istics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds.
In addition, we evaluated the variance in leachate characteristics in relation to selected
environmental variables and substrate characteristics. We found that sediments, due
to their large quantities within riverbeds, contribute most to the overall flux of disâ
solved substances during rewetting events (56%â98%), and that flux rates distinctly
differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contribâ
uted most to the areal fluxes. The largest amounts of leached substances were found
in the continental climate zone, coinciding with the lowest potential bioavailability of
the leached OM. The opposite pattern was found in the arid zone. Environmental variâ
ables expected to be modified under climate change (i.e. potential evapotranspiration,
aridity, dry period duration, land use) were correlated with the amount of leached subâ
stances, with the strongest relationship found for sediments. These results show that
the role of IRES should be accounted for in global biogeochemical cycles, especially
because prevalence of IRES will increase due to increasing severity of drying event
New genetic loci implicated in fasting glucose homeostasis and their impact on type 2 diabetes risk
Levels of circulating glucose are tightly regulated. To identify new loci influencing glycemic traits, we performed meta-analyses of 21 genome-wide association studies informative for fasting glucose, fasting insulin and indices of beta-cell function (HOMA-B) and insulin resistance (HOMA-IR) in up to 46,186 nondiabetic participants. Follow-up of 25 loci in up to 76,558 additional subjects identified 16 loci associated with fasting glucose and HOMA-B and two loci associated with fasting insulin and HOMA-IR. These include nine loci newly associated with fasting glucose (in or near ADCY5, MADD, ADRA2A, CRY2, FADS1, GLIS3, SLC2A2, PROX1 and C2CD4B) and one influencing fasting insulin and HOMA-IR (near IGF1). We also demonstrated association of ADCY5, PROX1, GCK, GCKR and DGKB-TMEM195 with type 2 diabetes. Within these loci, likely biological candidate genes influence signal transduction, cell proliferation, development, glucose-sensing and circadian regulation. Our results demonstrate that genetic studies of glycemic traits can identify type 2 diabetes risk loci, as well as loci containing gene variants that are associated with a modest elevation in glucose levels but are not associated with overt diabetes
Cellular context shapes cyclic nucleotide signaling in neurons through multiple levels of integration
International audienceIntracellular signaling with cyclic nucleotides are ubiquitous signaling pathways, yet the dynamics of these signals profoundly differ in different cell types. Biosensor imaging experiments, by providing direct measurements in intact cellular environment, reveal which receptors are activated by neuromodulators and how the coincidence of different neuromodulators is integrated at various levels in the signaling cascade. Phosphodiesterases appear as one important determinant of cross-talk between different signaling pathways. Finally, analysis of signal dynamics reveal that striatal medium-sized spiny neuron obey a different logic than other brain regions such as cortex, probably in relation with the function of this brain region which efficiently detects transient dopamine
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