On the shape of a convex body with respect to its second projection body

Abstract

We prove results relative to the problem of finding sharp bounds for the affine invariant P(K)=V(ΠK)/Vd−1(K). Namely, we prove that if K is a 3-dimensional zonoid of volume 1, then its second projection body Π2K is contained in 8K, while if K is any symmetric 3-dimensional convex body of volume 1, then Π2K contains 6K. Both inclusions are sharp. Consequences of these results include a stronger version of a reverse isoperimetric inequality for 3-dimensional zonoids established by the author in a previous work, a reduction for the 3-dimensional Petty conjecture to another isoperimetric problem and the best known lower bound up to date for P(K) in 3 dimensions. As byproduct of our methods, we establish an almost optimal lower bound for high-dimensional bodies of revolution.