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SPLITTING NUMBER is NPcomplete
, 1997
"... We consider two graph invariants that are used as a measure of nonplanarity: the splitting number of a graph and the size of a maximum planar subgraph. The splitting number of a graph G is the smallest integer k 0, such that a planar graph can be obtained from G by k splitting operations. Such ope ..."
Abstract

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We consider two graph invariants that are used as a measure of nonplanarity: the splitting number of a graph and the size of a maximum planar subgraph. The splitting number of a graph G is the smallest integer k 0, such that a planar graph can be obtained from G by k splitting operations. Such operation replaces a vertex v by two nonadjacent vertices v 1 and v 2 , and attaches the neighbors of v either to v 1 or to v 2 . We prove that the splitting number decision problem is NPcomplete. We obtain as a consequence that planar subgraph remains NPcomplete when restricted to graphs with maximum degree 3, when restricted to graphs with no subdivision of K 5 , or when restricted to cubic graphs, problems that have been open since 1979.
The Splitting Number and Skewness of . . .
"... The skewness of a graph G is the minimum number of edges that need to be deleted from G to produce a planar graph. The splitting number of a graph G is the minimum number of splitting steps needed to turn G into a planar graph; where each step replaces some of the edges fu; vg incident to a sele ..."
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The skewness of a graph G is the minimum number of edges that need to be deleted from G to produce a planar graph. The splitting number of a graph G is the minimum number of splitting steps needed to turn G into a planar graph; where each step replaces some of the edges fu; vg incident to a selected vertex u by edges fu ; vg, where u is a new vertex. We show that the splitting number of the toroidal grid graph Cn \Theta Cm is minfn; mg \Gamma 2ffi n;3 ffi m;3 \Gamma ffi n;4 ffi m;3 \Gamma ffi n;3 ffi m;4 and its skewness is minfn; mg \Gamma ffi n;3 ffi m;3 \Gamma ffi n;4 ffi m;3 \Gamma ffi n;3 ffi m;4 . Here, ffi is the Kronecker symbol, i.e., ffi i;j is 1 if i = j, and 0 if i 6= j.