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991.
Matthias Schork 《Central European Journal of Mathematics》2006,4(2):304-318
Some aspects of duality triads introduced recently are discussed. In particular, the general solution for the triad polynomials
is given. Furthermore, a generalization of the notion of duality triad is proposed and some simple properties of these generalized
duality triads are derived. 相似文献
992.
Andreas Dress Katharina T. Huber Alice Lesser Vincent Moulton 《Annals of Combinatorics》2006,10(1):63-76
One of the main problems in phylogenetics is to find good approximations of metrics by weighted trees. As an aid to solving this problem, it could be tempting to consider
optimal realizations of metrics—the guiding principle being that, the (necessarily unique) optimal realization of a tree metric is the weighted tree that realizes this metric. And, although optimal realizations of arbitrary metrics are, in general,
not trees, but rather weighted networks, one could still hope to obtain a phylogenetically informative representation of a
given metric, maybe even more informative than the best approximating tree. However, optimal realizations are not only difficult
to compute, they may also be non-unique. Here we focus on one possible way out of this dilemma: hereditarily optimal realizations. These are essentially unique, and can be described in a rather explicit way. In this paper, we recall what a hereditarily
optimal realization of a metric is and how it is related to the 1-skeleton of the tight span of that metric, and we investigate under what conditions it coincides with this 1-skeleton. As a consequence, we will show
that hereditarily optimal realizations for consistent metrics, a large class of phylogentically relevant metrics, can be computed in a straight-forward fashion.
Received August 26, 2004 相似文献
993.
Given a function f : ℕ→ℝ, call an n-vertex graph f-connected if separating off k vertices requires the deletion of at least f(k) vertices whenever k≤(n−f(k))/2. This is a common generalization of vertex connectivity (when f is constant) and expansion (when f is linear). We show that an f-connected graph contains a cycle of length linear in n if f is any linear function, contains a 1-factor and a 2-factor if f(k)≥2k+1, and contains a Hamilton cycle if f(k)≥2(k+1)2. We conjecture that linear growth of f suffices to imply hamiltonicity. 相似文献
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995.
It is possible to consider two variants of cluster theory: Inaffine cluster theory, one considers collections ofsubsets of a given setX of objects or states, whereas inprojective cluster theory, one considers collections ofsplits (orbipartitions) of that set. In both contexts, it can be desirable to produce acontinuous model, that is, a spaceT encompassing the given setX which represents in a well-specified and more or less parsimonious way all possibleintermediate objects ortransition states compatible with certain restrictions derived from the given collection of subsets or splits. We investigate an interesting and intriguing relationship between two such constructions that appear in the context of projective cluster theory: TheBuneman construction and thetight-span (or justT)construction. 相似文献
996.
997.
Huber E. Frohlich K. Grill R. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1997,25(4):642-646
The recovery of a vacuum interrupter gap after short-circuit interruption was measured by application of an overshooting transient recovery voltage (TRV) several tens of microseconds after current zero. Copper chromium contact materials were employed varying in composition (25 and 50% chromium content), gas content, and production method. The gap failure was either pure dielectric or it was dominated by a significant postarc current. Therefore, postarc current phenomena were experimentally investigated focused on the relationship among the postarc current, the power frequency current amplitude, and the gap length. It was found that two postarc current maxima exist: the first strongly dependent on the power frequency current, and the second on the field strength. A correlation among postarc current facilitated failures, the ultimately dielectric recovery, and the erosion rate of the material was found. Strong indication is given that all of these effects are dominated by the metal vapor pressure rise given by the constricted rotating arc. A significant influence of the material properties can be drawn from these experiments, allowing a good estimation of the capability for short-circuit current interruption, thus providing a useful tool for material development 相似文献
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