2020/06/26 by Balka, Richárd, Elekes, Márton, Kiss, Viktor +2
#05D40 #51F99 #54E52 #Classical Analysis and ODEs (math.CA) #FOS: Mathematics #Metric Geometry (math.MG)
paper · doi:10.48550/arxiv.2006.15206
We answer a question of Banakh, Jabłońska and Jabłoński by showing that for d≥ 2 there exists a compact set K ⊆ ℝd such that the projection of K onto each hyperplane is of non-empty interior, but K+K is nowhere dense. The proof relies on a random construction. A natural approach in the proofs is to construct such a K in the unit cube with full projections, that is, such that the projections of K agree with that of the unit cube. We investigate the generalization of these problems for projections onto various dimensional subspaces as well as for ℓ-fold sumsets. We obtain numerous positive and negative results, but also leave open many interesting cases. We also show that in most cases if we have a specific example of such a compact set then actually the generic (in the sense of Baire category) compact set in a suitably chosen space is also an example. Finally, utilizing a computer-aided construction, we show that the compact set in the plane with full projections and nowhere dense sumset can be self-similar.