Generalized restricted Marstrand conjecture for analytic linear families

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Let Pt:Rn→RmP_t:\mathbb{R}^n\rightarrow\mathbb{R}^m be an analytic family of linear maps. Define f:[0,n]→Rf:[0,n]\rightarrow\mathbb{R} as follows: for an integer x∈[0,n]x\in[0,n], let f(x)f(x) be the largest integer such that every subspace A⊂RnA\subset\mathbb{R}^n with dim⁡A=x\dim A=x satisfies

dim⁡Pt(A)≥f(x)\dim P_t(A)\geq f(x)

for all but finitely many t∈[0,1]t\in[0,1]; for noninteger xx, define ff linearly on [⌊x⌋,⌊x⌋+1][\lfloor x\rfloor,\lfloor x\rfloor+1]. Generalized restricted Marstrand conjecture. For every Borel set A⊂RnA\subset\mathbb{R}^n,

dim⁡HPt(A)≥f(dim⁡HA)\dim_H P_t(A)\geq f(\dim_H A)

for almost every t∈[0,1]t\in[0,1]. This is a more general lower-bound formulation of the restricted Marstrand problem, allowing the expected projection dimension to be determined by the eventual behavior of the family on subspaces.

References

Primary source

Jiahan Du, “Restricted Marstrand's projection theorem for general families of linear subspaces”, arXiv:2510.16671 (2025).

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