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Given finitely many connected polygonal obstacles O_1,...,O_k in the plane and a set P of points in general position and not in any obstacle, the visibility graph of P with obstacles O_1,...,O_k is the (geometric) graph with vertex set P, where two vertices are adjacent if the straight line segment joining them intersects no obstacle. The obstacle number of a graph G is the smallest integer k such that G is the visibility graph of a set of points with k obstacles. If G is planar, we define thearXiv:1706.06992v3 fatcat:ucgmzezbazd6rhqwfborctcjtq