80,341 research outputs found
On explicit solutions to the stationary axisymmetric Einstein-Maxwell equations describing dust disks
We review explicit solutions to the stationary axisymmetric Einstein-Maxwell
equations which can be interpreted as disks of charged dust. The disks of
finite or infinite extension are infinitesimally thin and constitute a surface
layer at the boundary of an electro-vacuum. The Einstein-Maxwell equations in
the presence of one Killing vector are obtained by using a projection
formalism. The SU(2,1) invariance of the stationary Einstein-Maxwell equations
can be used to construct solutions for the electro-vacuum from solutions to the
pure vacuum case via a so-called Harrison transformation. It is shown that the
corresponding solutions will always have a non-vanishing total charge and a
gyromagnetic ratio of 2. Since the vacuum and the electro-vacuum equations in
the stationary axisymmetric case are completely integrable, large classes of
solutions can be constructed with techniques from the theory of solitons. The
richest class of physically interesting solutions to the pure vacuum case due
to Korotkin is given in terms of hyperelliptic theta functions. The Harrison
transformed hyperelliptic solutions are discussed.Comment: 44 pages, 11 figures, revie
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Psy-expertise, therapeutic culture and the politics of the personal in development
Expertise stemming from the psy disciplines is increasingly and explicitly shaping international development policy and practice. Whilst some policy makers see the use of psy expertise as a new way to reduce poverty, increase economic efficiency, and promote wellbeing, others raise concerns that psychocentric development promotes individual over structural change, pathologises poverty, and depoliticises development. This paper specifically analyses four aspects of psy knowledge used in contemporary development policy: child development/developmental psychology, behavioural economics, positive psychology, and global mental health. This analysis illuminates the co-constitutive intellectual and colonial histories of development and psy-expertise: a connection that complicates claims that development has been psychologized; the uses and coloniality of both within a neoliberal project; and the potential for psychopolitics to inform development
Hyperelliptic Theta-Functions and Spectral Methods
A code for the numerical evaluation of hyperelliptic theta-functions is
presented. Characteristic quantities of the underlying Riemann surface such as
its periods are determined with the help of spectral methods. The code is
optimized for solutions of the Ernst equation where the branch points of the
Riemann surface are parameterized by the physical coordinates. An exploration
of the whole parameter space of the solution is thus only possible with an
efficient code. The use of spectral approximations allows for an efficient
calculation of all quantities in the solution with high precision. The case of
almost degenerate Riemann surfaces is addressed. Tests of the numerics using
identities for periods on the Riemann surface and integral identities for the
Ernst potential and its derivatives are performed. It is shown that an accuracy
of the order of machine precision can be achieved. These accurate solutions are
used to provide boundary conditions for a code which solves the axisymmetric
stationary Einstein equations. The resulting solution agrees with the
theta-functional solution to very high precision.Comment: 25 pages, 12 figure
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