Abstract
We prove that on a closed, orientable surface of genus g, the maximum cardinality of a set of simple loops with the property that no two are homotopic or intersect in more than k points grows as a function of g like \(g^{k+1}\), up to a factor of \(\log g\). The proof of the upper bound uses arguments from probabilistic combinatorics and a theorem of Scott related to the fact that surface groups are LERF.
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Notes
For functions f and g of several variables including k, we write \(f \lesssim _k g\) to mean that there exists a function C of k alone that satisfies the inequality \(f \le C \cdot g\) for all values of the variables. We write \(f \sim _k g\) if \(f \lesssim _k g\) and \(g \lesssim _k f\). Similarly, we write \(f \lesssim g\) and \(f \sim g\) if C can be taken to be an absolute constant.
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Acknowledgements
I thank Ravi Boppana, Jacob Caudell, Jonah Gaster, and Larry Guth for fun and helpful conversations. In particular, Larry laid the groundwork for constructing the “enemy graphs" of Theorem 8. I thank Dan Margalit for drawing my attention to [SS00] and the referee for unpacking the proof of Lemma 10. This work was supported by NSF CAREER Award DMS-1455132.
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