ZFC counterexamples to Marstrand’s projection theorem

Establish whether a counterexample to conclusion (1) of Marstrand’s Projection Theorem, namely a set whose projections fail to have the expected Hausdorff dimension for almost every direction, can be constructed within ZFC without additional set-theoretic assumptions.

Background

Marstrand’s Projection Theorem states that if B is an analytic subset of the Euclidean plane, then for almost every line through the origin, the projection of B onto that line has Hausdorff dimension equal to the minimum of 1 and the Hausdorff dimension of B. The introduction explains that Davies constructed a set violating this conclusion under the Continuum Hypothesis, while stronger assumptions such as V=L were used to obtain co-analytic counterexamples.

The unresolved issue is whether any counterexample of this Davies type can be produced in the ordinary axiomatic framework of ZFC itself. This problem concerns the precise set-theoretic strength required to violate the almost-everywhere projection conclusion when the analyticity assumption is weakened or removed.

References

It remains open whether any counterexamples of this sort can be built within ZFC.

Hyperarithmetic directions can all be exceptional for Marstrand's projection theorem  (2608.26904 - Greenberg et al., 27 Aug 2026) in Section 1, Introduction

Is there a Davies example provably in ZFC?

Open Problems in Mathematical Logic  (2608.26628 - Barmpalias et al., 27 Aug 2026) in Section 7