English

All projections of a typical Cantor set are Cantor sets

Geometric Topology 2022-12-07 v1 General Topology

Abstract

In 1994, John Cobb asked: given N>m>k>0N>m>k>0, does there exist a Cantor set in RN\mathbb R^N such that each of its projections into mm-planes is exactly kk-dimensional? Such sets were described for (N,m,k)=(2,1,1)(N,m,k)=(2,1,1) by L.Antoine (1924) and for (N,m,m)(N,m,m) by K.Borsuk (1947). Examples were constructed for the cases (3,2,1)(3,2,1) by J.Cobb (1994), for (N,m,m1)(N,m,m-1) and in a different way for (N,N1,N2)(N,N-1,N-2) by O.Frolkina (2010, 2019), for (N,N1,k)(N,N-1,k) by S.Barov, J.J.Dijkstra and M.van der Meer (2012). We show that such sets are exceptional in the following sense. Let C(RN)\mathcal C(\mathbb R^N) be a set of all Cantor subsets of RN\mathbb R^N endowed with the Hausdorff metric. It is known that C(RN)\mathcal C(\mathbb R^N) is a Baire space. We prove that there is a dense GδG_\delta subset PC(RN)\mathcal P \subset \mathcal C(\mathbb R^N) such that for each XPX\in \mathcal P and each non-zero linear subspace LRNL \subset \mathbb R^N, the orthogonal projection of XX into LL is a Cantor set. This gives a partial answer to another question of J.Cobb stated in the same paper (1994).

Keywords

Cite

@article{arxiv.2212.02982,
  title  = {All projections of a typical Cantor set are Cantor sets},
  author = {Olga Frolkina},
  journal= {arXiv preprint arXiv:2212.02982},
  year   = {2022}
}