Compact Kakeya construction from small projections
ID: compact-kakeya-construction-from-small-projections
Choose a compact with full horizontal projection and zero-length projections for . Then the displayed set is compact, has zero area by the Fubini theorem, and contains a segment of every slope in . Such parameter sets exist by the Baire category theorem in the Hausdorff metric: fine parallel segments approximate any parameter set while making a fixed band of projections uniformly small. Finite grids and a countable intersection make every required projection null. Four rotated copies supply every planar direction.
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