64 research outputs found
Vanishing Scalar Invariant Spacetimes in Higher Dimensions
We study manifolds with Lorentzian signature and prove that all scalar
curvature invariants of all orders vanish in a higher-dimensional Lorentzian
spacetime if and only if there exists an aligned non-expanding, non-twisting,
geodesic null direction along which the Riemann tensor has negative boost
order.Comment: final versio
Quasi-Exact Solvability and the direct approach to invariant subspaces
We propose a more direct approach to constructing differential operators that
preserve polynomial subspaces than the one based on considering elements of the
enveloping algebra of sl(2). This approach is used here to construct new
exactly solvable and quasi-exactly solvable quantum Hamiltonians on the line
which are not Lie-algebraic. It is also applied to generate potentials with
multiple algebraic sectors. We discuss two illustrative examples of these two
applications: an interesting generalization of the Lam\'e potential which
posses four algebraic sectors, and a quasi-exactly solvable deformation of the
Morse potential which is not Lie-algebraic.Comment: 17 pages, 3 figure
Exceptional orthogonal polynomials and the Darboux transformation
We adapt the notion of the Darboux transformation to the context of
polynomial Sturm-Liouville problems. As an application, we characterize the
recently described Laguerre polynomials in terms of an isospectral
Darboux transformation. We also show that the shape-invariance of these new
polynomial families is a direct consequence of the permutability property of
the Darboux-Crum transformation.Comment: corrected abstract, added references, minor correction
Minimal tensors and purely electric or magnetic spacetimes of arbitrary dimension
We consider time reversal transformations to obtain twofold orthogonal
splittings of any tensor on a Lorentzian space of arbitrary dimension n.
Applied to the Weyl tensor of a spacetime, this leads to a definition of its
electric and magnetic parts relative to an observer (i.e., a unit timelike
vector field u), in any n. We study the cases where one of these parts vanishes
in particular, i.e., purely electric (PE) or magnetic (PM) spacetimes. We
generalize several results from four to higher dimensions and discuss new
features of higher dimensions. We prove that the only permitted Weyl types are
G, I_i and D, and discuss the possible relation of u with the WANDs; we provide
invariant conditions that characterize PE/PM spacetimes, such as Bel-Debever
criteria, or constraints on scalar invariants, and connect the PE/PM parts to
the kinematic quantities of u; we present conditions under which direct product
spacetimes (and certain warps) are PE/PM, which enables us to construct
explicit examples. In particular, it is also shown that all static spacetimes
are necessarily PE, while stationary spacetimes (e.g., spinning black holes)
are in general neither PE nor PM. Ample classes of PE spacetimes exist, but PM
solutions are elusive, and we prove that PM Einstein spacetimes of type D do
not exist, for any n. Finally, we derive corresponding results for the
electric/magnetic parts of the Riemann tensor. This also leads to first
examples of PM spacetimes in higher dimensions. We also note in passing that
PE/PM Weyl tensors provide examples of minimal tensors, and we make the
connection hereof with the recently proved alignment theorem. This in turn
sheds new light on classification of the Weyl tensors based on null alignment,
providing a further invariant characterization that distinguishes the types
G/I/D from the types II/III/N.Comment: 43 pages. v2: new proposition 4.10; some text reshuffled (former sec.
2 is now an appendix); references added; some footnotes cancelled, others
incorporated into the main text; some typos fixed and a few more minor
changes mad
Generalizations of pp-wave spacetimes in higher dimensions
We shall investigate -dimensional Lorentzian spacetimes in which all of
the scalar invariants constructed from the Riemann tensor and its covariant
derivatives are zero. These spacetimes are higher-dimensional generalizations
of -dimensional pp-wave spacetimes, which have been of interest recently in
the context of string theory in curved backgrounds in higher dimensions.Comment: 5 pages, RevTex, to appear in Physical Review
Realizations of Real Low-Dimensional Lie Algebras
Using a new powerful technique based on the notion of megaideal, we construct
a complete set of inequivalent realizations of real Lie algebras of dimension
no greater than four in vector fields on a space of an arbitrary (finite)
number of variables. Our classification amends and essentially generalizes
earlier works on the subject.
Known results on classification of low-dimensional real Lie algebras, their
automorphisms, differentiations, ideals, subalgebras and realizations are
reviewed.Comment: LaTeX2e, 39 pages. Essentially exetended version. Misprints in
Appendix are correcte
A Randomised Placebo-Controlled Trial of a Traditional Chinese Herbal Formula in the Treatment of Primary Dysmenorrhoea
BACKGROUND: Most traditional Chinese herbal formulas consist of at least four herbs. Four-Agents-Decoction (Si Wu Tang) is a documented eight hundred year old formula containing four herbs and has been widely used to relieve menstrual discomfort in Taiwan. However, no specific effect had been systematically evaluated. We applied Western methodology to assess its effectiveness and safety for primary dysmenorrhoea and to evaluate the compliance and feasibility for a future trial. METHODOLOGY/PRINCIPAL FINDINGS: A randomised, double-blind, placebo-controlled, pilot clinical trial was conducted in an ad hoc clinic setting at a teaching hospital in Taipei, Taiwan. Seventy-eight primary dysmenorrheic young women were enrolled after 326 women with self-reported menstrual discomfort in the Taipei metropolitan area of Taiwan were screened by a questionnaire and subsequently diagnosed by two gynaecologists concurrently with pelvic ultrasonography. A dosage of 15 odorless capsules daily for five days starting from the onset of bleeding or pain was administered. Participants were followed with two to four cycles for an initial washout interval, one to two baseline cycles, three to four treatment cycles, and three follow-up cycles. Study outcome was pain intensity measured by using unmarked horizontal visual analog pain scale in an online daily diary submitted directly by the participants for 5 days starting from the onset of bleeding or pain of each menstrual cycle. Overall-pain was the average pain intensity among days in pain and peak-pain was the maximal single-day pain intensity. At the end of treatment, both the overall-pain and peak-pain decreased in the Four-Agents-Decoction (Si Wu Tang) group and increased in the placebo group; however, the differences between the two groups were not statistically significant. The trends persisted to follow-up phase. Statistically significant differences in both peak-pain and overall-pain appeared in the first follow-up cycle, at which the reduced peak-pain in the Four-Agents-Decoction (Si Wu Tang) group did not differ significantly by treatment length. However, the reduced peak-pain did differ profoundly among women treated for four menstrual cycles (2.69 (2.06) cm, mean (standard deviation), for the 20 women with Four-Agents-Decoction and 4.68 (3.16) for the 22 women with placebo, p = .020.) There was no difference in adverse symptoms between the Four-Agents-Decoction (Si Wu Tang) and placebo groups. CONCLUSION/SIGNIFICANCE: Four-Agents-Decoction (Si Wu Tang) therapy in this pilot post-market clinical trial, while meeting the standards of conventional medicine, showed no statistically significant difference in reducing menstrual pain intensity of primary dysmenorrhoea at the end of treatment. Its use, with our dosage regimen and treatment length, was not associated with adverse reactions. The finding of statistically significant pain-reducing effect in the first follow-up cycle was unexpected and warrants further study. A larger similar trial among primary dysmenorrheic young women with longer treatment phase and multiple batched study products can determine the definitive efficacy of this historically documented formula. TRIAL REGISTRATION: Controlled-Trials.com ISRCTN23374750
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