59,191 research outputs found
Hard Lefschetz for Chow groups of generalized Kummer varieties
The main result of this note is a hard Lefschetz theorem for the Chow groups
of generalized Kummer varieties. The same argument also proves hard Lefschetz
for Chow groups of Hilbert schemes of abelian surfaces. As a consequence, we
obtain new information about certain pieces of the Chow groups of generalized
Kummer varieties, and Hilbert schemes of abelian surfaces. The proofs are based
on work of Shen-Vial and Fu-Tian-Vial on multiplicative Chow-K\"unneth
decompositions.Comment: 9 pages, to appear in Abh. Math. Semin. Univ. Hambg., comments
welcome
Cyclic cycle systems of the complete multipartite graph
In this paper, we study the existence problem for cyclic -cycle
decompositions of the graph , the complete multipartite graph with
parts of size , and give necessary and sufficient conditions for their
existence in the case that
Greenhouse gas balance over thaw-freeze cycles in discontinuous zone permafrost
Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost-thaw cycles since the end of the mid-Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost-thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short-term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long-term (centuries) net C deposition provides a negative feedback to climate warming
Mixed motivic sheaves (and weights for them) exist if 'ordinary' mixed motives do
The goal of this paper is to prove: if certain 'standard' conjectures on
motives over algebraically closed fields hold, then over any 'reasonable'
there exists a motivic -structure for the category of Voevodsky's
-motives (as constructed by Cisinski and Deglise). If is 'very
reasonable' (for example, of finite type over a field) then the heart of this
-structure (the category of mixed motivic sheaves over ) is endowed with
a weight filtration with semi-simple factors. We also prove a certain 'motivic
decomposition theorem' (assuming the conjectures mentioned) and characterize
semi-simple motivic sheaves over in terms of those over its residue fields.
Our main tool is the theory of weight structures. We actually prove somewhat
more than the existence of a weight filtration for mixed motivic sheaves: we
prove that the motivic -structure is transversal to the Chow weight
structure for -motives (that was introduced previously and independently by
D. Hebert and the author; weight structures and their transversality with
t-structures were also defined by the author in recent papers). We also deduce
several properties of mixed motivic sheaves from this fact. Our reasoning
relies on the degeneration of Chow-weight spectral sequences for 'perverse
'etale homology' (that we prove unconditionally); this statement also yields
the existence of the Chow-weight filtration for such (co)homology that is
strictly restricted by ('motivic') morphisms.Comment: a few minor corrections mad
Block Motion Changes in Japan Triggered by the 2011 Great Tohoku Earthquake
Plate motions are governed by equilibrium between basal and edge forces.
Great earthquakes may induce differential static stress changes across tectonic
plates, enabling a new equilibrium state. Here we consider the torque balance
for idealized circular plates and find a simple scalar relationship for changes
in relative plate speed as a function of its size, upper mantle viscosity, and
coseismic stress changes. Applied to Japan, the 2011
Tohoku earthquake generated coseismic stresses of
~Pa that could have induced changes in motion of small (radius
~km) crustal blocks within Honshu. Analysis of time-dependent GPS
velocities, with corrections for earthquake cycle effects, reveals that plate
speeds may have changed by up to mm/yr between -year epochs
bracketing this earthquake, consistent with an upper mantle viscosity of Pas, suggesting that great earthquakes may modulate
motions of proximal crustal blocks at frequencies as high as ~Hz
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