7,164 research outputs found
False‑positive technetium‑99m methylene diphosphonate bone scan activity in the orbit in a patient with a history of breast carcinoma
Metastasis of breast carcinoma to the orbit is an uncommon entity and carries a poor prognosis. This case report presents false-positive technetium-99m methylene diphosphonate activity in the right orbit of a patient with a history of a primary breast neoplasm. Orbital computed tomography imaging was obtained to further characterize the radiotracer uptake identified on the bone scan and demonstrated diffuse right globe intraocular calcifications secondary to degenerative intraocular changes. A brief literature review of orbital metastasis from breast carcinoma and causes of intraocular calcification in the context of chronic vision loss are provided
QPTAS and Subexponential Algorithm for Maximum Clique on Disk Graphs
A (unit) disk graph is the intersection graph of closed (unit) disks in the plane. Almost three decades ago, an elegant polynomial-time algorithm was found for Maximum Clique on unit disk graphs [Clark, Colbourn, Johnson; Discrete Mathematics '90]. Since then, it has been an intriguing open question whether or not tractability can be extended to general disk graphs. We show the rather surprising structural result that a disjoint union of cycles is the complement of a disk graph if and only if at most one of those cycles is of odd length. From that, we derive the first QPTAS and subexponential algorithm running in time 2^{O~(n^{2/3})} for Maximum Clique on disk graphs. In stark contrast, Maximum Clique on intersection graphs of filled ellipses or filled triangles is unlikely to have such algorithms, even when the ellipses are close to unit disks. Indeed, we show that there is a constant ratio of approximation which cannot be attained even in time 2^{n^{1-epsilon}}, unless the Exponential Time Hypothesis fails
Comparison of fragment partitions production in peripheral and central collisions
Ensembles of single-source events, produced in peripheral and central
collisions and correponding respectively to quasi-projectile and quasi-fusion
sources, are analyzed. After selections on fragment kinematic properties,
excitation energies of the sources are derived using the calorimetric method
and the mean behaviour of fragments of the two ensembles are compared.
Differences observed in their partitions, especially the charge asymmetry, can
be related to collective energy deposited in the systems during the collisions.Comment: 7 pages, 2 figures, presented at the International Workshop on
Multifragmentation and Related Topics, Caen France, 4-7th november 2007
(IWM2007
Coherent imaging of extended objects
When used with coherent light, optical imaging systems, even
diffraction-limited, are inherently unable to reproduce both the amplitude and
the phase of a two-dimensional field distribution because their impulse
response function varies slowly from point to point (a property known as
non-isoplanatism). For sufficiently small objects, this usually results in a
phase distortion and has no impact on the measured intensity. Here, we show
that the intensity distribution can also be dramatically distorted when objects
of large extension or of special shapes are imaged. We illustrate the problem
using two simple examples: the pinhole camera and the aberration-free thin
lens. The effects predicted by our theorical analysis are also confirmed by
experimental observations.Comment: 10 pages, 9 figures, submitted to Optics Communication
Designing RNA secondary structures is hard
An RNA sequence is a word over an alphabet on four elements {A, C, G, U} called bases. RNA sequences fold into secondary structures where some bases match one another while others remain unpaired. Pseudoknot-free secondary structures can be represented as well-parenthesized expressions with additional dots, where pairs of matching parentheses symbolize paired bases and dots, unpaired bases. The two fundamental problems in RNA algorithmic are to predict how sequences fold within some model of energy and to design sequences of bases which will fold into targeted secondary structures. Predicting how a given RNA sequence folds into a pseudoknot-free secondary structure is known to be solvable in cubic time since the eighties and in truly subcubic time by a recent result of Bringmann et al. (FOCS 2016), whereas Lyngsø has shown it is NP-complete if pseudoknots are allowed (ICALP 2004). As a stark contrast, it is unknown whether or not designing a given RNA secondary structure is a tractable task; this has been raised as a challenging open question by Anne Condon (ICALP 2003). Because of its crucial importance in a number of fields such as pharmaceutical research and biochemistry, there are dozens of heuristics and software libraries dedicated to RNA secondary structure design. It is therefore rather surprising that the computational complexity of this central problem in bioinformatics has been unsettled for decades.
In this paper we show that, in the simplest model of energy which is the Watson-Crick model the design of secondary structures is NP-complete if one adds natural constraints of the form: index i of the sequence has to be labeled by base b. This negative result suggests that the same lower bound holds for more realistic models of energy. It is noteworthy that the additional constraints are by no means artificial: they are provided by all the RNA design pieces of software and they do correspond to the actual practice (see for example the instances of the EteRNA project). Our reduction from a variant of 3-Sat has as main ingredients: arches of parentheses of different widths, a linear order interleaving variables and clauses, and an intended rematching strategy which increases the number of pairs iff the three literals of a same clause are not satisfied. The correctness of the construction is also quite intricate; it relies on the polynomial algorithm for the design of saturated structures – secondary structures without dots – by Haleš et al. (Algorithmica 2016), counting arguments, and a concise case analysis
The eclipsing bursting X-ray binary EXO 0748-676 revisited by XMM-Newton
The bright eclipsing and bursting low-mass X-ray binary EXO 0748-676 has been
observed at several occasions by XMM-Newton during the initial calibration and
performance verification (CAL/PV) phase. We present here the results obtained
from observations with the EPIC cameras. Apart from several type-I X-ray
bursts, the source shows a high degree of variability with the presence of soft
flares. The wide energy coverage and high sensitivity of XMM-Newton allows for
the first time a detailed description of the spectral variability.
The source is found to be the superposition of a central (~2 10^8 cm)
Comptonized emission, most probably a corona surrounding the inner edge of an
accretion disk, associated with a more extended (~3 10^10 cm) thermal halo at a
typical temperature of ~0.6 keV with an indication of non-solar abundances.
Most of the variations of the source can be accounted for by a variable
absorption affecting only the central comptonized component and reaching up to
NH ~1.3 10^23 cm^{-2}. The characteristics of the surrounding halo are found
compatible with an irradiated atmosphere of an accretion disc which intercepts
the central emission due to the system high inclination.Comment: 6 pages, 4 figures, accepted for publication in A&A Letters, XMM
special issu
Break-up fragments excitation and the freeze-out volume
We investigate, in microcanonical multifragmentation models, the influence of
the amount of energy dissipated in break-up fragments excitation on freeze-out
volume determination. Assuming a limiting temperature decreasing with nuclear
mass, we obtain for the Xe+Sn at 32 MeV/nucleon reaction [J. D. Frankland et
al., Nucl. Phys. A689, 905 (2001); A689, 940 (2001)] a freeze-out volume almost
half the one deduced using a constant limiting temperature.Comment: 11 pages, 6 figure
Direct observation of a hydrophobic bond in loop-closure of a capped (-OCH2CH2-)n oligomer in water
The small r variation of the probability density P(r) for end-to-end
separations of a -CH2CH3 capped (-OCH2CH2-)n oligomer in water is computed to
be closely similar to the CH4 ... CH4 potential of mean force under the same
circumstances. Since the aqueous solution CH4 ... CH4 potential of mean force
is the natural physical definition of a primitive hydrophobic bond, the present
result identifies an experimentally accessible circumstance for direct
observation of a hydrophobic bond which has not been observed previously
because of the low solubility of CH4 in water. The physical picture is that the
soluble chain molecule carries the capping groups into aqueous solution, and
permits them to find one another with reasonable frequency. Comparison with the
corresponding results without the solvent shows that hydration of the solute
oxygen atoms swells the chain molecule globule. This supports the view that the
chain molecule globule might have a secondary effect on the hydrophobic
interaction which is of first interest here. The volume of the chain molecule
globule is important for comparing the probabilities with and without solvent
because it characterizes the local concentration of capping groups. Study of
other capping groups to enable X-ray and neutron diffraction measurements of
P(r) is discussed.Comment: 4 pages, 3 figure
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