4 vs 7 sparse undirected unweighted Diameter is SETH-hard at time n^4/3
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We show, assuming the Strong Exponential Time Hypothesis, that for every ε > 0, approximating undirected unweighted Diameter on n-vertex n^1+o(1)-edge graphs within ratio 7/4 - ε requires m^4/3 - o(1) time. This is the first result that conditionally rules out a near-linear time 5/3-approximation for undirected Diameter.
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