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Definition 1. Given a body $V$ in $\mathbb R^n$, the function $$p_V(r)=\mathop{\rm vol} [V\cap B_r(0)]$$ will be called profile of $V$.

Definition 2. Define Voronoi cell of lattice $L$ in $\mathbb R^n$ as $$V_L=\{\,x\in \mathbb R^n;\,|x|\le |x+\ell| \ \text{for any}\ \ell\in L\}.$$

Question. Can it happen that Voronoi cells of a pair of lattices have the same profile, but not isometric?

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The question is inspired by this one.

edited Jul 20 2011 at 20:13

asked May 20 2011 at 0:48

Anton Petrunin
16.4k●41●110

Presumably $B_r(0)$ is the ball of radius $r$ centered at $0$? If so, it seems, at least, profile $\Rightarrow$ isometric for $n=1$! – Joseph O'Rourke May 20 2011 at 1:19

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The $II_{16,0}$ and $E_8 + E_8$ lattices in dimension 16 have different Voronoi domains, since the reflection planes for the roots yield different Dynkin diagrams. I think the domains have the same profile, because the theta constants of the lattices are equal. – S. Carnahan♦ May 20 2011 at 2:21

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Equality of theta-functions is equivalent to the fact that both latices have the same number of points in any ball centered at the origin (?). I do not see why this property related to the one I want --- they sound similar, but I do not see a bridge between them. – Anton Petrunin May 20 2011 at 3:15

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@Will Jagy: There are quite a few polytopes with the same profile, even centrally symmetric ones. You can stellate two pairs of opposite sides of an icosahedron (or octagon), getting the same profile regardless of the pairs of opposite sides you choose. The symmetries do not act transitively on the possibilities. However, it's much easier to motivate considering the profile of the Voronoi domain of a lattice since it tells you the distribution of distances to the lattice in $\mathbb{R}^n / L$. – Douglas Zare May 20 2011 at 17:24

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@Scott Carnahan: Given a lattice, one can consider the norm of the "most distant point to the lattice" that is closest to the origin. Is that invariant the same for E_8⊕E_8 and for II_{16,0}? Let me rephrase because I might have said things in a confusing way: I'm asking for the diameters (or maybe "radius") of the Voronoi cells: are they the same for those two lattices? – André Henriques Jul 20 2011 at 23:41

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Voronoi cell of lattices with the same profile.

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