Open problems in Euclidean geometry ? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T15:18:21Z http://mathoverflow.net/feeds/question/53797 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry Open problems in Euclidean geometry ? Qfwfq 2011-01-30T16:15:35Z 2011-01-31T12:45:07Z <blockquote> <p>Which are some (research level) open problems in Euclidean geometry ?</p> </blockquote> <p>(Edit: I ask just out of curiosity, to understand how -and if- nowadays this is not a "dead" field yet)</p> <p>I should clarify a bit what I mean by "Euclidean geometry". By this term I mean, loosely, the study of the geometry of certain subsets of Euclidean space $\mathbb{E}^n$ from a point of view which is either the "classical" one (i.e. axiomatic), or one that involves more modern tools, <em>but</em> the problem in question has <em>not</em> to be "clearly" a problem within some other branch of maths such as differential or algebraic geometry, algebraic or general topology, analysis, or measure theory.</p> <p>Some examples to clarify:</p> <ul> <li>the study of configurations of lines or affine subspaces is EG; but the algebro-topological study of hyperplane arrangements is not.</li> <li>plane conics as defined via their metric property are objects of EG; but "algebraic curves" are not, unless they're difined by some "elementary enough property" (intentionally vague) involving the Euclidean metric.</li> <li>root systems of Lie algebras are EG. </li> <li>polyhedral cones are EG.</li> <li>polytopes are EG.</li> <li>tassellations of space with polytopes or analogous objects are in EG.</li> <li>minimal surfaces in $\mathbb{E}^3$ are not EG.</li> <li>fractal geometry (Julia sets, self-affine fractals...) is not EG.</li> <li>not sure about convex bodies. If they're polyhedral I'd say their study fits in EG.</li> <li>packings of spheres are EG.</li> </ul> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53798#53798 Answer by Andres Caicedo for Open problems in Euclidean geometry ? Andres Caicedo 2011-01-30T16:27:12Z 2011-01-30T16:27:12Z <p>In recent years there have been a good amount of surveys and publications on "computational" or "combinatorial" geometry, and looking at them may give you a good idea of current questions. Specifically, there is the excellent recent book "Research Problems in Discrete Geometry" by Brass, Moser, and Pach. You may want to start by looking there and at the references it provides. Besides a good deal of information on classical questions, among many other topics, you find:</p> <ul> <li>Density problems for packings and coverings.</li> <li>Distance problems.</li> <li>Lattice point problems.</li> <li>Graph drawings and geometric graphs.</li> <li>Geometric inequalities.</li> </ul> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53802#53802 Answer by Gerhard Paseman for Open problems in Euclidean geometry ? Gerhard Paseman 2011-01-30T16:50:56Z 2011-01-30T16:50:56Z <p>Among the many choices one might get from an Internet search, I suggest Unsolved Problems in Geometry by Hallard Croft, Kenneth Falconer, and Richard Guy (Springer-Verlag, 1991). It may include references to non-Euclidean geometries.</p> <p>As an aside, I would like to see a geometric proof that the configuration of Pappus is implied by that of Desargues for finite geometric spaces.</p> <p>Gerhard "Ask Me About System Design" Paseman, 2011.01.30</p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53813#53813 Answer by David Eppstein for Open problems in Euclidean geometry ? David Eppstein 2011-01-30T19:28:21Z 2011-01-30T19:28:21Z <p>An important open problem in combinatorial Euclidean geometry is the question of how many different <a href="http://en.wikipedia.org/wiki/K-set_%28geometry%29" rel="nofollow">halving lines</a> a set of $2n$ points in the Euclidean plane may have, in the worst case. A halving line is a line through two of the points such that $n-1$ of the points are on each of its sides. The number of halving lines is known to be $O(n^{4/3})$, and there are examples of point sets for which this number is $n2^{\sqrt{\Omega(\log n)}}$, but there remains a large gap between these upper and lower bounds.</p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53815#53815 Answer by Matthew Kahle for Open problems in Euclidean geometry ? Matthew Kahle 2011-01-30T19:40:57Z 2011-01-30T22:33:51Z <p>The <a href="http://en.wikipedia.org/wiki/Unit_distance_graph#Counting_unit_distances" rel="nofollow">Unit Distance Problem</a> asks:</p> <blockquote> <p>For a set of $n$ points in the plane, what is the maximal number $g(n)$ of unit distances realized among the ${n \choose 2}$ pairs?</p> </blockquote> <p>A properly scaled square grid gives a lower bound of something like $g(n) \ge n^{1 + \frac{c}{\log \log{n}}}$, and a beautiful application of the <a href="http://terrytao.wordpress.com/2007/09/18/the-crossing-number-inequality/" rel="nofollow">crossing number lemma</a> gives that $g(n) = O(n^{4/3})$.</p> <p>A closely related problem where great progress was made very recently is the Distinct Distance problem, asking for the minimum number $f(n)$ of distinct distances among $n$ points in the plane. (Clearly $f(n)g(n) \ge {n \choose 2}$.)</p> <p>Guth and Katz <a href="http://arxiv.org/abs/1011.4105" rel="nofollow">recently obtained a sharp exponent</a> for $f(n)$. <a href="http://terrytao.wordpress.com/2010/11/20/the-guth-katz-bound-on-the-erdos-distance-problem/" rel="nofollow">Terence Tao</a> and <a href="http://gilkalai.wordpress.com/2010/11/20/janos-pach-guth-and-katzs-solution-of-erdos-distinct-distances-problem/" rel="nofollow">János Pach</a> wrote nice summaries of this work. </p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53817#53817 Answer by Greg Kuperberg for Open problems in Euclidean geometry ? Greg Kuperberg 2011-01-30T20:25:03Z 2011-01-30T20:25:03Z <p>I might as well air this question (first posed by Keith Ball) that has sweeping ramifications in convex geometry in high dimensions if the answer is yes:</p> <p>Let $K$ be a centrally symmetric convex body in $\mathbb{R}^n$, and let $K^\circ$ be the polar or dual convex body. Define a statistic $e(K)$ as the expected value of $(\vec{x} \cdot \vec{y})^2$, where $\vec{x}$ is chosen randomly from $K$ and $\vec{y}$ is chosen randomly from $K^\circ$. Then for each fixed $n$, is $e(K)$ maximized when $K$ is an ellipsoid? The question is even open in two dimensions.</p> <p>A much weaker conjecture is that the integral of $(\vec{x} \cdot \vec{y})^2$ over $K \times K^\circ$, as opposed to the average value, is maximized when $K$ is an ellipsoid. It is known that $K \times K^\circ$ has the most volume when $K$ is an ellipsoid; this fact is called Santaló's inequality.</p> <p>It is known that the answer to the first conjecture is no if $K$ is not centrally symmetric, even if the origin is the only points fixed by the symmetries of $K$. (Central symmetry means specifically that $K = -K$.)</p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53832#53832 Answer by Gerry Myerson for Open problems in Euclidean geometry ? Gerry Myerson 2011-01-30T22:18:17Z 2011-01-30T22:18:17Z <p>In addition to the Croft-Falconer-Guy and Brass-Moser-Pach books others have mentioned, there's Victor Klee and Stan Wagon, Old and New Unsolved Problems in Plane Geometry and Number Theory, No. 11 in the MAA series, Dolciani Mathematical Expositions, from 1991. </p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53835#53835 Answer by Gerald Edgar for Open problems in Euclidean geometry ? Gerald Edgar 2011-01-30T23:14:54Z 2011-01-30T23:14:54Z <p>Let $L_0,L_1,L_2,L_3,L_4$ be five lines in general position on the Euclidean plane---think of the subscripts mod $5$ and draw $L_i$ as the consecutive lines of a (not necessarily regular!) pentagram. Let $C_i$ be the circle inscribed about the triangle formed by $L_i$, $L_{i-2}$, and $L_{i+2}$. Then $C_{i-1}$ and $C_{i+1}$ meet at the intersection of $L_{i-1}$ and $L_{i+1}$, and again at some other point $P_i$ (which we take to be the same point if the two circles are tangent there). Show that these five points $P_i$ are concyclic.<br> .<br> Actually, it isn't open, but some years ago presented to me as open, with computer-graphical evidence for it...</p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53871#53871 Answer by Benoît Kloeckner for Open problems in Euclidean geometry ? Benoît Kloeckner 2011-01-31T09:30:05Z 2011-01-31T09:30:05Z <p>The happy ending problem ( <a href="http://en.wikipedia.org/wiki/Happy_Ending_problem" rel="nofollow">http://en.wikipedia.org/wiki/Happy_Ending_problem</a> ) says that any set of five points in the plane in general position has a subset of four points that form the vertices of a convex quadrilateral. More generally, Erdös and Szekeres proved that for any positive integer $N$, there is a minimal integer $f(N)$ such that any set of $f(N)$ points in the plane in general position has a subset of $N$ points that form the vertices of a convex polygon, and it is known that $f(N)$ is at least $1+2^{N-2}$.</p> <p>An open question is: does $f(N)=1+2^{N-2}$ hold?</p> http://mathoverflow.net/questions/53797/open-problems-in-euclidean-geometry/53880#53880 Answer by XX for Open problems in Euclidean geometry ? XX 2011-01-31T12:45:07Z 2011-01-31T12:45:07Z <p>Polynomial Hirsch Conjecture?</p>