Planar self-similar set projections and fibres

Determine whether every line projection of a planar self-similar set with dense rotations and Hausdorff dimension $s>1$ has positive one-dimensional Lebesgue measure and fibres of Hausdorff dimension $s-1$ at Lebesgue-almost every point of the projected image.

Background

The main set theorem proves positive projection volume and almost-everywhere fibre dimension in dimensions d≥3d\ge3 under dense rotations. The planar analogue for the uniform measure is false because there are examples with singular projections and discrete conditional measures. However, singularity of a projected measure does not by itself imply that the support has zero Lebesgue measure, so the corresponding set statement remains unresolved. The group-mixing argument used in the paper cannot handle SO(2)SO(2) because it lacks the required compact semisimple structure.

References

Does every line projection of a planar self-similar set with dense rotations and dimension $s>1$ have positive length and fibres of dimension $s-1$ at Lebesgue-almost every point of its image?

— Absolute continuity and dimension conservation for self-similar sets and measures  (2609.29301 - Jin et al., 24 Sep 2026) in Section 7, “Prospects and open questions,” subsection “The planar set theorem”