Homometric $\Rightarrow$ isometric? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-20T07:00:51Z http://mathoverflow.net/feeds/question/65279 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/65279/homometric-rightarrow-isometric Homometric $\Rightarrow$ isometric? Joseph O'Rourke 2011-05-17T23:07:04Z 2012-08-18T20:13:46Z <p>Suppose you know that there is a mapping between two Riemmanian manifolds $M_1$ and $M_2$ such that, for each $x_1 \in M_1$, the (codimension-1) measure of the set of points at distance $d$ from $x_1$ is the same as the measure of the set of points at distance $d$ from the corresponding $x_2 \in M_2$, for all $d>0$. (Theo J-F provides in the comments a precise definition of the appropriate "measure.") Say then the two manifolds are <em>homometric</em>. Does this imply that they are isometric?</p> <p>On a 2-manifold, the homometric condition means that the circumference of radius-$d$ circles matches. When only a discrete set of points are at distance $d$ from $x_1$, I would like the cardinality of that set to match that from $x_2$, although perhaps this is too much to ask.</p> <p>The notion of a <a href="http://en.wikipedia.org/wiki/Homometric_structures" rel="nofollow">homometric set</a> comes from discrete geometry (e.g., in crystallography), where indeed there are homometric but incongruent sets. See, e.g., (1), or the earlier MO question (2).</p> <p>It may be that for manifolds, the two concepts coincide. However, I cannot see a direct implication. I'd prefer to make as few assumptions as possible (e.g., my manifolds are not smooth), but any insight under any assumptions would be welcomed. Thanks!</p> <p><b>Addendum.</b> James Cranch suggested that perhaps discrete examples of pairs of homometric sets can be mimicked by pairs of manifolds, which would answer my question negatively. But he subsequently deleted his sketch, as it was not clear it could be turned into a proof. As I mentioned in the comments, there is a follow-up question by Anton Petrunin, "<a href="http://mathoverflow.net/questions/65491" rel="nofollow">Voronoi cell of lattices with the same profile</a>," also currently (<em>18 Aug 2012</em>) unanswered.</p> <p><hr> (1) Joseph Rosenblatt and Paul D. Seymour. "<a href="http://epubs.siam.org/sima/resource/1/sjmael/v3/i3/p343_s1" rel="nofollow">The Structure of Homometric Sets</a>." <em>SIAM. J. on Algebraic and Discrete Methods</em>, <b>3</b>, 343-350, 1982.</p> <p>(2) MO: "<a href="http://mathoverflow.net/questions/31741/" rel="nofollow">Largest pair of homometric Golomb rulers?</a>"</p>