Points of middle density in the real line

Csörnyei, Marianna; Grahl, Jack and O'Neil, Toby C. (2012). Points of middle density in the real line. Real Analysis Exchange, 37(2) pp. 243–248.


A Lebesgue measurable set in the real line has Lebesgue density 0 or 1 at almost every point. Kolyada showed that there is a positive constant $\delta$ such that for non-trivial measurable sets there is at least one point with upper and lower densities lying in the interval $(\delta, 1-\delta)$. Both Kolyada and later Szenes gave bounds for the largest possible value of this $\delta$. In this note we reduce the best known upper bound, disproving a conjecture of Szenes.

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