In convex geometry, the projection body of a convex body in n-dimensional Euclidean space is the convex body such that for any vector , the support function of in the direction u is the (n – 1)-dimensional volume of the projection of K onto the hyperplane orthogonal to u.
For a convex body, let denote the polar body of its projection body. There are two remarkable affine isoperimetric inequality for this body. Petty (1971) proved that for all convex bodies ,
where denotes the n-dimensional unit ball and is n-dimensional volume, and there is equality precisely for ellipsoids. Zhang (1991) proved that for all convex bodies ,
where denotes any -dimensional simplex, and there is equality precisely for such simplices.
The intersection bodyIK of K is defined similarly, as the star body such that for any vector u the radial function of IK from the origin in direction u is the (n – 1)-dimensional volume of the intersection of K with the hyperplane u⊥.
Equivalently, the radial function of the intersection body IK is the Funk transform of the radial function of K.
Intersection bodies were introduced by Lutwak (1988).
Koldobsky (1998a) showed that a centrally symmetric star-shaped body is an intersection body if and only if the function 1/||x|| is a positive definite distribution, where ||x|| is the homogeneous function of degree 1 that is 1 on the boundary of the body, and Koldobsky (1998b) used this to show that the unit balls lp n, 2 < p ≤ ∞ in n-dimensional space with the lp norm are intersection bodies for n=4 but are not intersection bodies for n ≥ 5.
Petty, Clinton M. (1967), "Projection bodies", Proceedings of the Colloquium on Convexity (Copenhagen, 1965), Kobenhavns Univ. Mat. Inst., Copenhagen, pp. 234–241, MR0216369
Petty, Clinton M. (1971), "Isoperimetric problems", Proceedings of the Conference on Convexity and Combinatorial Geometry (Univ. Oklahoma, Norman, Okla., 1971). Dept. Math., Univ. Oklahoma, Norman, Oklahoma, pp. 26–41, MR0362057