On the space of convex figures
V. V. Makeev,
N. Yu. Netsvetaev St. Petersburg State University, St. Petersburg, Russia
Abstract:
Let
$T$ be the set of convex bodies in
$\mathbb R^k$, and let
$\mathcal T$ be the set of classes of similar bodies in
$T$. We write
$F$ for
$T$ in the case
$k=2$. Define a metric
$d$ on
$\mathcal T$ by setting for classes
$\{K_1\},\{K_2\}$ (from
$\mathcal T$, of convex bodies
$K_1,K_2$)
$d(\{K_1\},\{K_2\}) =\inf\{\ln(b/a)\}$, where
$a$ and
$b$ are positive reals such that there is a similarity transformation
$A$ with
$aA(K_1)\subset K_2\subset bA(K_1)$. Let
$D_2$ be a planar unit disk. If
$x>0$, we denote by
$F_x$ the set of the planar convex figures
$K$ in
$F$ with
$d(\{D_2\},\{K\})\ge x$. We also equip the sets
$T$ and
$F$ with the usual Hausdorff metric.
We prove that if
$y>\ln(\operatorname{sec}(\pi/n))\ge x$ for some integer
$n>2$, then no mapping
$F_x\to F_y$ is
$\operatorname{SO}(2)$-equivariant.
Let
$M_k (n)$ be the space of
$k$-dimensional convex polyhedra with at most
$n$ hyperfaces (vertices), and let
$M_k$ denote the space of
$k$-dimensional convex polyhedra. We prove that there are no
$\operatorname{SO}(2)$-equivariant continuous mappings
$M_k(n+k)\to M_k(n)$.
Let
$T^s$ be the closed subspace of
$T$ formed by centrally symmetric bodies. Let
$T_x$ denote the closed subspace of
$T$ formed by the bodies
$K$ with
$d(T^s,\{K\})\ge x>0$. We prove that for every
$y>0$ there exists an
$x>0$ such that no mapping
$T_x\to T_y$ is
$\operatorname{SO}(2)$-equivariant.
Key words and phrases:
convex figure, convex body, orthogonal group, vector bundle, Grassmannian.
UDC:
514.172 Received: 31.12.2012