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Stronger bounds for generalized degrees and Menger path systems

100%
Faudree R., Tuza Z.
Discussiones Mathematicae Graph Theory
|
1995
|
tom 15
|
nr 2
167-177
EN
For positive integers d and m, let $P_{d,m}(G)$ denote the property that between each pair of vertices of the graph G, there are m internally vertex disjoint paths of length at most d. For a positive integer t a graph G satisfies the minimum generalized degree condition δₜ(G) ≥ s if the cardinality of the union of the neighborhoods of each set of t vertices of G is at least s. Generalized degree conditions that ensure that $P_{d,m}(G)$ is satisfied have been investigated. In particular, it has been shown, for fixed positive integers t ≥ 5, d ≥ 5t², and m, that if an m-connected graph G of order n satisfies the generalized degree condition δₜ(G) > (t/(t+1))(5n/(d+2))+(m-1)d+3t², then for n sufficiently large G has property $P_{d,m}(G)$. In this note, this result will be improved by obtaining corresponding results on property $P_{d,m}(G)$ using a generalized degree condition δₜ(G), except that the restriction d ≥ 5t² will be replaced by the weaker restriction d ≥ max{5t+28,t+77}. Also, it will be shown, just as in the original result, that if the order of magnitude of δₜ(G) is decreased, then $P_{d,m}(G)$ will not, in general, hold; so the result is sharp in terms of the order of magnitude of δₜ(G).
2
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A proof of menger's theorem by contraction

80%
Göring F.
Discussiones Mathematicae Graph Theory
|
2002
|
tom 22
|
nr 1
111-112
EN
A short proof of the classical theorem of Menger concerning the number of disjoint AB-paths of a finite graph for two subsets A and B of its vertex set is given. The main idea of the proof is to contract an edge of the graph.
3
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Some weak covering properties and infinite games

61%
Sakai M.
Open Mathematics
|
2014
|
tom 12
|
nr 2
322-329
EN
We show that (I) there is a Lindelöf space which is not weakly Menger, (II) there is a Menger space for which TWO does not have a winning strategy in the game Gfin(O,Do). These affirmatively answer questions posed in Babinkostova, Pansera and Scheepers [Babinkostova L., Pansera B.A., Scheepers M., Weak covering properties and infinite games, Topology Appl., 2012, 159(17), 3644–3657]. The result (I) automatically gives an affirmative answer of Wingers’ problem [Wingers L., Box products and Hurewicz spaces, Topology Appl., 1995, 64(1), 9–21], too. The selection principle S1(Do,Do) is also discussed in view of a special base. We show that every subspace of a hereditarily Lindelöf space with an ortho-base satisfies S1(Do,Do).
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