Van Engelen's Borel classification conjecture for zero-dimensional spaces

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Let XX be a zero-dimensional Borel space that is not locally compact. A meager topological group is a topological group whose underlying space is meager, and a filter is the corresponding topological space notion used in the paper. Theorem van Engelen asserts that, for zero-dimensional Δ30\mathbf{\Delta}^0_3 spaces that are not locally compact, these two conditions are equivalent: being a meager topological group and being homeomorphic to a filter.

Van Engelen's conjecture. The same equivalence holds for every zero-dimensional Borel space that is not locally compact.

This extends van Engelen's theorem from zero-dimensional Δ30\mathbf{\Delta}^0_3 spaces to all zero-dimensional Borel spaces. The source explicitly describes the Borel version as open.

References

Primary source

Andrea Medini, “A complete classification of the zero-dimensional homogeneous spaces under determinacy”, arXiv:2312.10735 (2025).

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