The iteration time and the general position number in graph convexities
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In this paper, we study two graph convexity parameters: iteration time and general position number. The iteration time was defined in 1981 in the geodesic convexity, but its computational complexity was still open. The general position number was defined in the geodesic convexity and proved NP-hard in 2018. We extend these parameters to any graph convexity and prove that the iteration number is NP-hard in the P_3 convexity and, with this result, we can prove that the iteration time is also NP-hard in the geodesic convexity even in graphs with diameter two, a very natural question which was unsolved since 1981. These results are also important, since they are the last two missing NP-hardness results regarding the ten most studied graph convexity parameters in the geodesic and P_3 convexities. Finally, we also prove that the general position number of the monophonic convexity is NP-hard, W[1]-hard (parameterized by the size of the solution) and n^1-ε-inapproximable in polynomial time for any ε>0 unless P=NP, even in graphs with diameter two.
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