Baire-property reduction for completely Ramsey families

ID: baire-property-reduction-for-completely-ramsey-families

In the Ellentuck topology, suppose every open set is completely Ramsey and every meagre set is completely Ramsey-null. If is contained in a meagre set for some open , first refine a basic neighbourhood without changing its stem to avoid that exceptional set, and then refine again to be homogeneous for . The resulting neighbourhood is homogeneous for . Sets with such an open representative form a sigma-algebra: countable unions collect the meagre errors, and complements replace by its interior, differing only on the nowhere-dense boundary of . This proves that all star-Borel sets are completely Ramsey.

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