Covering shadows with a smaller volume

TL;DR

This paper explores how convex bodies with certain projection properties relate to their volumes, introducing classes called i-cylinder bodies that connect shadow covering conditions to volume inequalities.

Contribution

It introduces the class of i-cylinder bodies, linking projection shadow conditions to volume comparisons and establishing a hierarchy of these classes.

Findings

C{n,1} is the set of centrally symmetric convex bodies.

C{n,n} includes all convex bodies in n-dimensional space.

Members of C{n,i} satisfy specific geometric inequalities.

Abstract

For each i = 1, ..., n constructions are given for convex bodies K and L in n-dimensional Euclidean space such that each rank i orthogonal projection of K can be translated inside the corresponding projection of L, even though K has strictly larger m-th intrinsic volumes (i.e. V_m(K) > V_m(L)) for all m > i. It is then shown that, for each i = 1, ..., n, there is a class of bodies C{n,i}, called i-cylinder bodies of R^n, such that, if the body L with i-dimensional covering shadows is an i-cylinder body, then K will have smaller n-volume than L. The families C{n,i} are shown to form a strictly increasing chain of subsets C{n,1} < C{n,2} < ... < C{n,n-1} < C{n,n}, where C{n,1} is precisely the collection of centrally symmetric compact convex sets in n-dimensional space, while C{n,n} is the collection of all compact convex sets in n-dimensional space. Members of each family C{n,i} are seen to play a fundamental role in relating covering conditions for projections to the theory of mixed volumes, and members of C{n,i} are shown to satisfy certain geometric inequalities. Related open questions are also posed.