6,479 research outputs found
Tiki Casts a Spell on Wnt
Negative feedback is a widespread feature of signaling pathways. In an unexpected twist described in this issue, He and colleagues identify a membrane-tethered metalloprotease named Tiki that inhibits Wnt signaling by removing an essential eight-residue fragment from Wnt itself
On primitive formulation in fluid mechanics and fluid-structure interaction with constant piecewise properties in velocity-potentials of acceleration
Discrete mechanics makes it possible to formulate any problem of fluid
mechanics or fluid-structure interaction in velocity and potentials of
acceleration; the equation system consists of a single vector equation and
potentials updates. The scalar potential of the acceleration represents the
pressure stress and the vector potential is related to the rotational-shear
stress. The formulation of the equation of motion can be expressed in the form
of a splitting which leads to an exact projection method; the application of
the divergence operator to the discrete motion equation exhibits, without any
approximation, a Poisson equation with constant coefficients on the scalar
potential whatever the variations of the physical properties of the media. The
a posteriori calculation of the pressure is made explicitly by introducing at
this stage the local density. Two first examples show the interest of the
formulation presented on classical solutions of Navier-Stokes equations;
similarly as other results obtained with this formulation, the convergence is
of order two in space and time for all the quantities, velocity and potentials.
This formulation is then applied to a two-phase flow driven by surface tension
and partial wettability. The last case corresponds to a fluid-structure
interaction problem for which an analytical solution exists.Comment: 18 pages, 7 figures, article. Acta Mechanica, 202
The Continuous Skolem-Pisot Problem: On the Complexity of Reachability for Linear Ordinary Differential Equations
We study decidability and complexity questions related to a continuous
analogue of the Skolem-Pisot problem concerning the zeros and nonnegativity of
a linear recurrent sequence. In particular, we show that the continuous version
of the nonnegativity problem is NP-hard in general and we show that the
presence of a zero is decidable for several subcases, including instances of
depth two or less, although the decidability in general is left open. The
problems may also be stated as reachability problems related to real zeros of
exponential polynomials or solutions to initial value problems of linear
differential equations, which are interesting problems in their own right.Comment: 14 pages, no figur
Price Discrimination and EU Dairy Policy: An Economic Evaluation of Policy Options
In a period of market liberalization and multilateral trade negotiations, price discrimination for commodities with distinct markets provides additional policy options to support farm income. While both the USA and Canada have implemented price discrimination policies in their domestic dairy sector, so far the European Union (EU) has not. This paper evaluates the options of developing a price discrimination policy in the EU dairy sector. The analysis is based on an interregional model of the EU dairy sector, involving milk production, dairy processing, and consumption of ten dairy commodities in nine regions. The paper shows that a price discrimination policy that increases prices for commodities with more inelastic demand (fluid milk, soft dairy products) would generate income that can be redistributed to dairy farmers. The results suggest that, while such a price discrimination policy can be a WTO compatible way to support dairy farm income, the efficiency of the associated income transfers declines in the presence of significant supply response.EU, price discrimination, pooling, dairy policy, Demand and Price Analysis,
La parada de los monstruos
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La venjança d'Abel
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