Displaying blocking pairs in signed graphs

A signed graph is a pair (G, S) where G is a graph and S is a subset of the edges of G. A circuit of G is even (resp. odd) if it contains an even (resp. odd) number of edges of S. A blocking pair of (G, S) is a pair of vertices s, t such that every odd circuit intersects at least one of s or t. In this paper, we characterize when the blocking pairs of a signed graph can be represented by 2-cuts in an auxiliary graph. We discuss the relevance of this result to the problem of recognizing even cycle matroids and to the problem of characterizing signed graphs with no odd-K5 minor

Blood Pressure Measurement Validation Off the Cuff? Comment on A New Cuffless Device for Measuring Blood Pressure: A Real-Life Validation Study .

Contains fulltext : 199952.pdf (publisher's version ) (Open Access

U(3) and Pseudo-U(3) Symmetry of the Relativistic Harmonic Oscillator

We show that a Dirac Hamiltonian with equal scalar and vector harmonic oscillator potentials has not only a spin symmetry but an U(3) symmetry and that a Dirac Hamiltonian with scalar and vector harmonic oscillator potentials equal in magnitude but opposite in sign has not only a pseudospin symmetry but a pseudo-U(3) symmetry. We derive the generators of the symmetry for each case.Comment: 8 pages, 0 figures, pusblished in Physical Review Letters 95, 252501 (2005

The Dwarf Nova PQ Andromedae

We report a photometric study of the WZ Sagittae-type dwarf nova PQ Andromedae. The light curve shows strong (0.05 mag full amplitude) signals with periods of 1263(1) and 634(1) s, and a likely double-humped signal with P=80.6(2) min. We interpret the first two as nonradial pulsation periods of the underlying white dwarf, and the last as the orbital period of the underlying binary. We estimate a distance of 150(50) pc from proper motions and the two standard candles available: the white dwarf and the dwarf-nova outburst. At this distance, the K magnitude implies that the secondary is probably fainter than any star on the main sequence -- indicating a mass below the Kumar limit at 0.075 M_sol. PQ And may be another "period bouncer", where evolution now drives the binary out to longer period.Comment: PDF, 13 pages, 2 figures; accepted, in press, to appear September 2005, PASP; more info at http://cba.phys.columbia.edu

Warm dust in the terrestrial planet zone of a sun-like Pleiad: collisions between planetary embryos?

Only a few solar-type main sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD+20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades cluster. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the non-standard wavelength of ~9 micron. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (>~1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD+20 307/HD 23514 phenomenon indicates that the mass equivalent of Earth's Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.Comment: ApJ in press, 19 pages including 3 figures and 2 tables, minor changes to the tables and figure

Which Way Was I Going? Contextual Retrieval Supports the Disambiguation of Well Learned Overlapping Navigational Routes

Groundbreaking research in animals has demonstrated that the hippocampus contains neurons that distinguish betweenoverlapping navigational trajectories. These hippocampal neurons respond selectively to the context of specific episodes despite interference from overlapping memory representations. The present study used functional magnetic resonanceimaging in humans to examine the role of the hippocampus and related structures when participants need to retrievecontextual information to navigate well learned spatial sequences that share common elements. Participants were trained outside the scanner to navigate through 12 virtual mazes from a ground-level first-person perspective. Six of the 12 mazes shared overlapping components. Overlapping mazes began and ended at distinct locations, but converged in the middle to share some hallways with another maze. Non-overlapping mazes did not share any hallways with any other maze. Successful navigation through the overlapping hallways required the retrieval of contextual information relevant to thecurrent navigational episode. Results revealed greater activation during the successful navigation of the overlapping mazes compared with the non-overlapping mazes in regions typically associated with spatial and episodic memory, including thehippocampus, parahippocampal cortex, and orbitofrontal cortex. When combined with previous research, the current findings suggest that an anatomically integrated system including the hippocampus, parahippocampal cortex, and orbitofrontal cortexis critical for the contextually dependent retrieval of well learned overlapping navigational routes

Detection of bondline delaminations in multilayer structures with lossy components

The detection of bondline delaminations in multilayer structures using ultrasonic reflection techniques is a generic problem in adhesively bonded composite structures such as the Space Shuttles's Solid Rocket Motors (SRM). Standard pulse echo ultrasonic techniques do not perform well for a composite resonator composed of a resonant layer combined with attenuating layers. Excessive ringing in the resonant layer tends to mask internal echoes emanating from the attenuating layers. The SRM is made up of a resonant steel layer backed by layers of adhesive, rubber, liner and fuel, which are ultrasonically attenuating. The structure's response is modeled as a lossy ultrasonic transmission line. The model predicts that the acoustic response of the system is sensitive to delaminations at the interior bondlines in a few narrow frequency bands. These predictions are verified by measurements on a fabricated system. Successful imaging of internal delaminations is sensitive to proper selection of the interrogating frequency. Images of fabricated bondline delaminations are presented based on these studies