Mathematicians prove open product-free subsets of (0,1) have measure below 1/3

wtgowers · x · 2026-07-24

Leonardo Franchi, Fredy Yip and the author solved a problem of Ben Green by proving that if A is an open product-free subset of the open interval (0,1), then its measure is less than 1/3.

They also note that the bound is sharp, and that the proof is not too hard to verify once the right structure is in place.

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