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JOURNALS // Zapiski Nauchnykh Seminarov POMI // Archive

Zap. Nauchn. Sem. POMI, 2013 Volume 415, Pages 24–28 (Mi znsl5682)

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


 English version:
Journal of Mathematical Sciences (New York), 2016, 212:5, 533–535

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