Abstract
A thrackle is a graph drawing in which every pair of edges meets exactly once. Conway's Thrackle Conjecture states that the number of edges of a thrackle cannot exceed the number of its vertices. Cairns et al. (2015) [1] prove that the Thrackle Conjecture holds for great-circle thrackles drawn on the sphere. They also posit that Conway's Thrackle Conjecture can be restated to say that a graph can be drawn as a thrackle drawing in the plane if and only if it admits a great-circle thrackle drawing. We demonstrate that the class of great-circle thrackleable graphs excludes some trees. Thus the informal conjecture from Cairns et al. (2015) [1] is not equivalent to the Thrackle Conjecture.
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