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    Pach, János
Odd-cycle transversal in triangle-free graphs ★★
Author(s): Erdos; Faudree; Pach; Spencer
Conjecture   If  is a simple triangle-free graph, then there is a set of at most
 is a simple triangle-free graph, then there is a set of at most  edges whose deletion destroys every odd cycle.
 edges whose deletion destroys every odd cycle. 
 is a simple triangle-free graph, then there is a set of at most
 is a simple triangle-free graph, then there is a set of at most  edges whose deletion destroys every odd cycle.
 edges whose deletion destroys every odd cycle. Keywords:
Are different notions of the crossing number the same? ★★★
Problem   Does the following equality hold for every graph  ?
? ![\[ \text{pair-cr}(G) = \text{cr}(G) \]](/files/tex/8cece1e00bb0e9fc122e0a5cad0dab2681cf33a4.png) 
 
 ?
? ![\[ \text{pair-cr}(G) = \text{cr}(G) \]](/files/tex/8cece1e00bb0e9fc122e0a5cad0dab2681cf33a4.png) 
 The crossing number  of a graph
 of a graph  is the minimum number of edge crossings in any drawing of
 is the minimum number of edge crossings in any drawing of  in the plane. In the pairwise crossing number
 in the plane. In the pairwise crossing number  , we minimize the number of pairs of edges that cross.
, we minimize the number of pairs of edges that cross. 
Keywords: crossing number; pair-crossing number
 
   
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