Mean Field Theory for Sigmoid Belief Networks

We develop a mean field theory for sigmoid belief networks based on ideas from statistical mechanics. Our mean field theory provides a tractable approximation to the true probability distribution in these networks; it also yields a lower bound on the likelihood of evidence. We demonstrate the utility of this framework on a benchmark problem in statistical pattern recognition---the classification of handwritten digits.Comment: See http://www.jair.org/ for any accompanying file

Effects of calcium carbonate particulate releasing surgical anchors on bone and tendon healing

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.Cataloged from PDF version of thesis.Includes bibliographical references (p. 31-32).The Calaxo ® screw, developed by Smith and Nephew, is a novel biomedical composite composed of poly-DL-lactide-co-glycolide (PLLA:PGA) 85:15 and calcium carbonate particulates. Comparisons to an identical surgical anchor composed of PLLA, largely considered an industry standard, shows increased mean failure strength in surgical reconstructions of the patellar-tibia complex in sheep. SEM imaging confirms the osteoconductivity of the Calaxo ® screw proposed in previous studies and also suggest a positive effect on the differentiation of tissues along the tendon-bone interface. These findings support the intended use of the Calaxo ® screw as a fixation device for rotator cuff repair procedures.by Jordan-Ryan J.I.K. Medeiros.S.B

Fast Learning by Bounding Likelihoods in Sigmoid Type Belief Networks

Sigmoid type belief networks, a class of probabilistic neural networks, provide a natural framework for compactly representing probabilistic information in a variety of unsupervised and supervised learning problems. Often the parameters used in these networks need to be learned from examples. Unfortunately, estimating the parameters via exact probabilistic calculations (i.e, the EM-algorithm) is intractable even for networks with fairly small numbers of hidden units. We propose to avoid the infeasibility of the E step by bounding likelihoods instead of computing them exactly. We introduce extended and complementary representations for these networks and show that the estimation of the network parameters can be made fast (reduced to quadratic optimization) by performing the estimation in either of the alternative domains. The complementary networks can be used for continuous density estimation as well

A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721

We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721. Fitting the STIS data with magnetic white dwarf model spectra, we derive a field strength of B~144MG and an effective temperature of 17000K<Teff<23000K. This measurement makes RX J1554.2+2721 only the third cataclysmic variable containing a white dwarf with a field exceeding 100MG. Similar to the other high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble mass transfer, which suggests that a considerable number of high-field polars may still remain undiscovered.Comment: 4 pages, accepted for ApJ Letter

Neutrino Dark Energy and Moduli Stabilization in a BPS Braneworld Scenario

A braneworld model for neutrino Dark Energy (DE) is presented. We consider a five dimensional two-branes set up with a bulk scalar field motivated by supergravity. Its low-energy effective theory is derived with a moduli space approximation (MSA). The position of the two branes are parametrized by two scalar degrees of freedom (moduli). After detuning the brane tensions a classical potential for the moduli is generated. This potential is unstable for dS branes and we suggest to consider as a stabilizing contribution the Casimir energy of bulk fields. In particular we add a massive spinor (neutrino) field in the bulk and then evaluate the Casimir contribution of the bulk neutrino with the help of zeta function regularization techniques. We construct an explicit form of the 4D neutrino mass as function of the two moduli. To recover the correct DE scale for the moduli potential the usual cosmological constant fine-tuning is necessary, but, once accepted, this model suggests a stronger connection between DE and neutrino physics.Comment: 26 pages, 1 EPS figur

AUSSAT battery life test program

AUSSAT Pty. Ltd., the Australian National Satellite organization, has contracted with the Hughes Aircraft Company (HAC) for the construction of 3 satellites based on the now familiar HS-376 product line. As part of the AUSSAT contract, HAC is conducting an extensive NiCd battery life test program. The life test program, objectives and test results to date are described. Particular emphasis is given to the evaluation of the FS2117 separator as a future replacement for the Pellon 2505 separator of which only a very limited quantity remains

Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS

We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000 K and 50000 K. Through a least-squares minimization scheme with an evolutionary algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95 previously published plus 14 newly discovered within SDSS, and five discovered by SEGUE). Since we were limited to a single spectrum for each object we used only centered magnetic dipoles or dipoles which were shifted along the magnetic dipole axis. We also statistically investigated the distribution of magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White Dwarfs, Barcelona, 200

Preeclampsia is associated with compromized maternal synthesis of long chain polyunsaturated fatty acids leading to offspring deficiency

Obesity and excessive lipolysis are implicated in preeclampsia (PE). Intrauterine growth restriction is associated with low maternal body mass index and decreased lipolysis. Our aim was to assess how maternal and offspring fatty acid metabolism is altered in mothers in the third trimester of pregnancy with PE (n=62) or intrauterine growth restriction (n=23) compared with healthy pregnancies (n=164). Markers of lipid metabolism and erythrocyte fatty acid concentrations were measured. Maternal adipose tissue fatty acid composition and mRNA expression of adipose tissue fatty acid–metabolizing enzymes and placental fatty acid transporters were compared. Mothers with PE had higher plasma triglyceride (21%, P&#60;0.001) and nonesterified fatty acid (50%, P&#60;0.001) concentrations than controls. Concentrations of major n−6 and n−3 long-chain polyunsaturated fatty acids in erythrocytes were 23% to 60% lower (all P&#60;0.005) in PE and intrauterine growth restriction mothers and offspring compared with controls. Subcutaneous adipose tissue Δ−5 and Δ−6 desaturase and very long-chain fatty acid elongase mRNA expression was lower in PE than controls (respectively, mean [SD] control 3.38 [2.96] versus PE 1.83 [1.91], P=0.030; 3.33 [2.25] versus 1.03 [0.96], P&#60;0.001; 0.40 [0.81] versus 0.00 [0.00], P=0.038 expression relative to control gene [square root]). Low maternal and fetal long-chain polyunsaturated fatty acid concentrations in PE may be the result of decreased maternal synthesis