39 research outputs found
Remarks on "Resolving isospectral `drums' by counting nodal domains"
In [3] the authors studied the 4-parameter family of isospectral flat 4-tori
T^\pm(a,b,c,d) discovered by Conway and Sloane. With a particular method of
counting nodal domains they were able to distinguish these tori (numerically)
by computing the corresponding nodal sequences relative to a few explicit
tuples (a,b,c,d). In this note we confirm the expectation expressed in [3] by
proving analytically that their nodal count distinguishes any 4-tuple of
distinct positive real numbers.Comment: 5 page
On the Ricci tensor in type II B string theory
Let be a metric connection with totally skew-symmetric torsion \T
on a Riemannian manifold. Given a spinor field and a dilaton function
, the basic equations in type II B string theory are \bdm \nabla \Psi =
0, \quad \delta(\T) = a \cdot \big(d \Phi \haken \T \big), \quad \T \cdot \Psi
= b \cdot d \Phi \cdot \Psi + \mu \cdot \Psi . \edm We derive some relations
between the length ||\T||^2 of the torsion form, the scalar curvature of
, the dilaton function and the parameters . The main
results deal with the divergence of the Ricci tensor \Ric^{\nabla} of the
connection. In particular, if the supersymmetry is non-trivial and if
the conditions \bdm (d \Phi \haken \T) \haken \T = 0, \quad \delta^{\nabla}(d
\T) \cdot \Psi = 0 \edm hold, then the energy-momentum tensor is
divergence-free. We show that the latter condition is satisfied in many
examples constructed out of special geometries. A special case is . Then
the divergence of the energy-momentum tensor vanishes if and only if one
condition \delta^{\nabla}(d \T) \cdot \Psi = 0 holds. Strong models (d \T =
0) have this property, but there are examples with \delta^{\nabla}(d \T) \neq
0 and \delta^{\nabla}(d \T) \cdot \Psi = 0.Comment: 9 pages, Latex2
The odd side of torsion geometry
We introduce and study a notion of `Sasaki with torsion structure' (ST) as an
odd-dimensional analogue of K\"ahler with torsion geometry (KT). These are
normal almost contact metric manifolds that admit a unique compatible
connection with 3-form torsion. Any odd-dimensional compact Lie group is shown
to admit such a structure; in this case the structure is left-invariant and has
closed torsion form.
We illustrate the relation between ST structures and other generalizations of
Sasaki geometry, and explain how some standard constructions in Sasaki geometry
can be adapted to this setting. In particular, we relate the ST structure to a
KT structure on the space of leaves, and show that both the cylinder and the
cone over an ST manifold are KT, although only the cylinder behaves well with
respect to closedness of the torsion form. Finally, we introduce a notion of
`G-moment map'. We provide criteria based on equivariant cohomology ensuring
the existence of these maps, and then apply them as a tool for reducing ST
structures.Comment: 34 pages; v2: added a small generalization (Proposition 3.6) of the
cone construction; two references added. To appear on Ann. Mat. Pura App
Party-group relations in new southern European democracies in the crisis era
UID/CPO/04627/2013
PTDC/IVC-CPO/1864/2014publishersversionpublishe
Agricultural Interests and the Origins of Capitalism: A Parallel Comparative History of Germany, Denmark, New Zealand, and the USA
Addressing the literature on the historical origins of capitalism, this study analyses the role agriculture played in the formation of non-market economic coordination in economic and social affairs around 1900. It argues that the dominant rural production profile dictated whether farmers did exert a significant impact on socio-economic institution and policy formation outside the rural sector. By applying the method of parallel demonstration of theory, we illustrate the plausibility of these theoretical considerations through juxtaposing the historical record of Germany, Denmark, New Zealand and the USA. The article highlights the limits of a dichotomous view on the origins of capitalism because the coordination effect of rural economies varies within the later coordinated and the later liberal cluster of market economies