39,715 research outputs found
The benefits of parenting: Government financial support for families with children since 1975
This commentary describes the changes to the structure of child-contingent support through the tax and benefit system since 1975. It also presents new results, which were produced to quantify explicitly the amount of government support for families with children, using representative samples of families from over the past three decades. With these data, it is possible to examine whether child-contingent support has become more or less progressive, or more or less slanted towards large families, lone-parents families or families with young children
Twinlike models with identical linear fluctuation spectra
Recently, the possibility of so-called twinlike field theories has been
demonstrated, that is, of different field theories which share the same
topological defect solution with the same energy density. Further, purely
algebraic conditions have been derived which the corresponding Lagrangians have
to obey in order that the field theories be twins of each other. A further
diagnostical tool which, in general, allows to distinguish the topological
defects of a given theory from the corresponding defects of its twins is the
spectrum of linear fluctuations about these defects. Very recently, however,
explicit examples of twin theories have been constructed such that not only
their shapes and energy densities coincide, but also their linear fluctuation
spectra are the same. Here we show that, again, there exist purely algebraic
conditions for the Lagrangian densities which imply that the corresponding
field theories are twins and that the fluctuation spectra about their defects
coincide. These algebraic conditions allow to construct an infinite number of
twins with coinciding fluctuation spectra for a given theory, and we provide
some explicit examples. The importance of this result is related to the fact
that coinciding defects with coinciding energy densities and identical
fluctuation spectra are almost indistinguishable physically, that is,
indistinguishable in a linear or semiclassical approximation. This implies that
the measurable physical properties of a kink, in general, do not allow to
determine the theory which provides the kink uniquely. Instead, in principle an
infinite number of possible theories has to be considered.Comment: Latex, 13 pages, no figure
Dirac lattice
We study the emergence of Dirac fermionic field in the low energy description
of non-relativistic dynamical models on graphs admitting continuum limit. The
Dirac fermionic field appears as the effective field describing the excitations
above point-like Fermi surface. Together with the Dirac fermionic field an
effective space-time metric is also emerging. We analyze the conditions for
such Fermi points to appear in general, paying special attention to the cases
of two and three spacial dimensions.Comment: 26 pages, 4 figures; typo and grammatical corrections, new
reference(s) added, version accepted for publicatio
- …