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556.
Three classes of Petrov type-D nonrotating empty spaces, which we denote asA
+
*
,A
−
*
, anda
0
*
, can be seen to represent, respectively, a gravitational field of a uniformly accelerating source which moves with velocity
slower, faster than, or equal to that of light. The first two classes of space-time,A
+
*
andA
−
*
, approach to the last one,a
0
*
, as the acceleration becomes greater and greater. The inertial frame in a flat space-time, relative to which a faster-thanlight
particle moving along a spacelike geodesic behaves as an ordinary particle, is given and discussed. The behaviors of Killing
horizons in the classes,A
+
*
andA
−
*
are investigated. Comparisons are made between the analysis by Farhoosh and Zimmerman and ours. Our identification of the
acceleration parameter seems to be more appropriate than that by the above authors. 相似文献
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The well-known theorem of Erd?s-Pósa says that a graph G has either k disjoint cycles or a vertex set X of order at most f(k) for some function f such that G\X is a forest. Starting with this result, there are many results concerning packing and covering cycles in graph theory and combinatorial optimization. In this paper, we discuss packing disjoint S-cycles, i.e., cycles that are required to go through a set S of vertices. For this problem, Kakimura-Kawarabayashi-Marx (2011) and Pontecorvi-Wollan (2010) recently showed the Erd?s-Pósa-type result holds. We further try to generalize this result to packing S-cycles of odd length. In contrast to packing S-cycles, the Erd?s-Pósa-type result does not hold for packing odd S-cycles. We then relax packing odd S-cycles to half-integral packing, and show the Erd?s-Pósa-type result for the half-integral packing of odd S-cycles, which is a generalization of Reed (1999) when S=V. That is, we show that given an integer k and a vertex set S, a graph G has either 2k odd S-cycles so that each vertex is in at most two of these cycles, or a vertex set X of order at most f(k) (for some function f) such that G\X has no odd S-cycle. 相似文献