Mixed volume

In mathematics, more specifically, in convex geometry, the mixed volume is a way to associate a non-negative number to an $n$-tuple of convex bodies in $n$-dimensional space. This number depends on the size and shape of the bodies and on their relative orientation to each other.

Definition

Let $K_{1},K_{2},\dots ,K_{r}$ be convex bodies in $\mathbb {R} ^{n}$ and consider the function

$f(\lambda _{1},\ldots ,\lambda _{r})=\mathrm {Vol} _{n}(\lambda _{1}K_{1}+\cdots +\lambda _{r}K_{r}),\qquad \lambda _{i}\geq 0,$

where ${\text{Vol}}_{n}$ stands for the $n$-dimensional volume and its argument is the Minkowski sum of the scaled convex bodies $K_{i}$. One can show that $f$ is a homogeneous polynomial of degree $n$, therefore it can be written as

$f(\lambda _{1},\ldots ,\lambda _{r})=\sum _{j_{1},\ldots ,j_{n}=1}^{r}V(K_{j_{1}},\ldots ,K_{j_{n}})\lambda _{j_{1}}\cdots \lambda _{j_{n}},$

where the functions $V$ are symmetric. For a particular index function $j\in \{1,\ldots ,r\}^{n}$, the coefficient $V(K_{j_{1}},\dots ,K_{j_{n}})$ is called the mixed volume of $K_{j_{1}},\dots ,K_{j_{n}}$.

Properties

• The mixed volume is uniquely determined by the following three properties:

1. $V(K,\dots ,K)={\text{Vol}}_{n}(K)$;
2. $V$ is symmetric in its arguments;
3. $V$ is multilinear: $V(\lambda K+\lambda 'K',K_{2},\dots ,K_{n})=\lambda V(K,K_{2},\dots ,K_{n})+\lambda 'V(K',K_{2},\dots ,K_{n})$ for $\lambda ,\lambda '\geq 0$.

• The mixed volume is non-negative and monotonically increasing in each variable: $V(K_{1},K_{2},\ldots ,K_{n})\leq V(K_{1}',K_{2},\ldots ,K_{n})$ for $K_{1}\subseteq K_{1}'$.
• The Alexandrov–Fenchel inequality, discovered by Aleksandr Danilovich Aleksandrov and Werner Fenchel:

$V(K_{1},K_{2},K_{3},\ldots ,K_{n})\geq {\sqrt {V(K_{1},K_{1},K_{3},\ldots ,K_{n})V(K_{2},K_{2},K_{3},\ldots ,K_{n})}}.$

Numerous geometric inequalities, such as the Brunn–Minkowski inequality for convex bodies and Minkowski's first inequality, are special cases of the Alexandrov–Fenchel inequality.

Quermassintegrals

Let $K\subset \mathbb {R} ^{n}$ be a convex body and let $B=B_{n}\subset \mathbb {R} ^{n}$ be the Euclidean ball of unit radius. The mixed volume

$W_{j}(K)=V({\overset {n-j{\text{ times}}}{\overbrace {K,K,\ldots ,K} }},{\overset {j{\text{ times}}}{\overbrace {B,B,\ldots ,B} }})$

is called the j-th quermassintegral of $K$.

The definition of mixed volume yields the Steiner formula (named after Jakob Steiner):

$\mathrm {Vol} _{n}(K+tB)=\sum _{j=0}^{n}{\binom {n}{j}}W_{j}(K)t^{j}.$

Intrinsic volumes

The j-th intrinsic volume of $K$ is a different normalization of the quermassintegral, defined by

$V_{j}(K)={\binom {n}{j}}{\frac {W_{n-j}(K)}{\kappa _{n-j}}},$ or in other words $\mathrm {Vol} _{n}(K+tB)=\sum _{j=0}^{n}V_{j}(K)\,\mathrm {Vol} _{n-j}(tB_{n-j}).$

where $\kappa _{n-j}={\text{Vol}}_{n-j}(B_{n-j})$ is the volume of the $(n-j)$-dimensional unit ball.

Hadwiger's characterization theorem

Main article: Hadwiger's theorem

Hadwiger's theorem asserts that every valuation on convex bodies in $\mathbb {R} ^{n}$ that is continuous and invariant under rigid motions of $\mathbb {R} ^{n}$ is a linear combination of the quermassintegrals (or, equivalently, of the intrinsic volumes).

External links

Burago, Yu.D. (2001) [1994], "Mixed volume theory", Encyclopedia of Mathematics, EMS Press