Fixing numbers for matroids

Motivated by work in graph theory, we define the fixing number for a matroid. We give upper and lower bounds for fixing numbers for a general matroid in terms of the size and maximum orbit size (under the action of the matroid automorphism group). We prove the fixing numbers for the cycle matroid and bicircular matroid associated with 3-connected graphs are identical. Many of these results have interpretations through permutation groups, and we make this connection explicit.Comment: This is a major revision of a previous versio

Planned improvements to the Owens Valley frequency-agile interferometer

Three small antennas will be added to the OVRO interferometer to form a five-element solar-dedicated array. This would provide up to 7 or 10 baselines (compared to the present 1 or 3). This would be sufficient to apply microwave diagnostics to most active region and burst sources. By using frequency-synthesis it would also provide an imaging capability comparable to that of an approximately 100 baseline interferometer. Expansion of the array is discussed

Series-parallel posets and the Tutte polynomial

AbstractWe investigate the Tutte polynomial f(P; t, z) of a series-parallel partially ordered set P. We show that f(P) can be computed in polynomial-time when P is series-parallel and that series-parallel posets having isomorphic deletions and contractions are themselves isomorphic. A formula for f(P∗) in terms of f(P) is obtained and shows these two polynomials factor over Z[t, z] the same way. We examine several subclasses of the class of series-parallel posets, proving that f(P) ≠ f(Q) for non-isomorphic posets P and Q in the largest of these classes. We also give excluded subposet characterizations of the various subclasses

Impulsive phase solar flare X-ray polarimetry

The pioneering observational work in solar flare X-ray polarimetry was done in a series of satellite experiments by Tindo and his collaborators in the Soviet Union; initial results showed high levels of polarization in X-ray flares (up to 40%), although of rather low statistical significance, and these were generally interpreted as evidence for strong beaming of suprathermal electrons in the flare energy release process. However, the results of the polarimeter flown by the Columbia Astrophysics Laboratory as part of the STS-3 payload on the Space Shuttle by contrast showed very low levels of polarization. The largest value (observed during the impulsive phase of a single event) was 3.4% + or - 2.2%. At the same time but independent of the observational work, Leach and Petrosian (1983) showed that the high levels of polarization in the Tindo results were difficult to understand theoretically, since the electron beam is isotropized on an energy loss timescale. A subsequent comparison by Leach, Emslie, and Petrosian (1985) of the impulsive phase STS-3 result and the above theoretical treatment shows that the former is consistent with several current models and that a factor of approximately 3 improvement in sensitivity is needed to distinguish properly among the possibilities

HOPE Longitudinal Study: Year 2 Results

Abstract pending

United States Security Assistance 1977-1980: Human Rights Issues Affecting Arms Transfers

The 1970s were a period of great transition for the United States. Plagued by the Vietnam conflict, a conflict which we could never win because of poor planning and lack of resolve by government officials in the highest office, and the Watergate debacle, where our government veered out of control, the time was ripe for change. Which is precisely why the ideals and philosophies of one James Earl Carter enabled him to ascend to become the 39th President of the United States. President Carter entered office pledging the United States would lead the way in the restriction of weapons exports and increase world awareness on the issue of human rights. He kept his word by passing a six-point plan, known as Presidential Directive 13, which imposed a variety of restrictions on the transfer of arms from the United States to other countries. He also attempted to accomplish his human rights objective by linking the transfer of arms with the recipient country\u27s acceptable treatment of its populace. By protesting human rights violations he hoped to secure increased rights for the politically oppressed around the world. Though the Carter effort did succeed in raising the level of concern for human rights around the globe, due to its uneven application and inconsistent implementation, it was discarded as a policy in 1979. The six-point plan to reduce the proliferation of arms around the globe was also ineffective, primarily because countries friendly to the United States did not agree with the merits of the plan