37,206 research outputs found
Extensile actomyosin?
Living cells move thanks to assemblies of actin filaments and myosin motors
that range from very organized striated muscle tissue to disordered
intracellular bundles. The mechanisms powering these disordered structures are
debated, and all models studied so far predict that they are contractile. We
reexamine this prediction through a theoretical treatment of the interplay of
three well-characterized internal dynamical processes in actomyosin bundles:
actin treadmilling, the attachement-detachment dynamics of myosin and that of
crosslinking proteins. We show that these processes enable an extensive control
of the bundle's active mechanics, including reversals of the filaments'
apparent velocities and the possibility of generating extension instead of
contraction. These effects offer a new perspective on well-studied in vivo
systems, as well as a robust criterion to experimentally elucidate the
underpinnings of actomyosin activity.Comment: 5 pages, 5 figure
Singular spectrum of Lebesgue measure zero for one-dimensional quasicrystals
The spectrum of one-dimensional discrete Schr\"odinger operators associated
to strictly ergodic dynamical systems is shown to coincide with the set of
zeros of the Lyapunov exponent if and only if the Lyapunov exponent exists
uniformly. This is used to obtain Cantor spectrum of zero Lebesgue measure for
all aperiodic subshifts with uniform positive weights. This covers, in
particular, all aperiodic subshifts arising from primitive substitutions
including new examples as e.g. the Rudin-Shapiro substitution.
Our investigation is not based on trace maps. Instead it relies on an
Oseledec type theorem due to A. Furman and a uniform ergodic theorem due to the
author.Comment: 14 page
Homotopy (Pre-)Derivators of Cofibration Categories and Quasi-Categories
We prove that the homotopy prederivator of a cofibration category is
equivalent to the homotopy prederivator of its associated quasi-category of
frames, as introduced by Szumi\l{}o. We use this comparison result to deduce
various abstract properties of the obtained prederivators.Comment: Final version, incorporating suggestions by referee. Accepted for
publication in Algebraic & Geometric Topolog
Theoretical status of the lifetime predictions: and
We give a review of the theoretical status of the lifetime predictions in the
standard model. In case of we are already in a
rather advanced stage. We obtain
. It seems to be difficult to
improve these errors substantially. In addition now some experimental results
are available. For and
the theoretical status is much less advanced and the discrepancy between
experiment and theory still remains. We conclude with a what-to-do-list for
theorists.Comment: Invited talk given at the International Conference on Flavor Physics
in Zhang-Jia-Jie City, Hunan, China, May 31 - June 6,2001. 6 page
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