Falconer's Conjecture
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In
geometric measure theory In mathematics, geometric measure theory (GMT) is the study of geometric properties of sets (typically in Euclidean space) through measure theory. It allows mathematicians to extend tools from differential geometry to a much larger class of surfac ...
, Falconer's conjecture, named after Kenneth Falconer, is an
unsolved problem List of unsolved problems may refer to several notable List of conjectures, conjectures or open problems in various academic fields: Natural sciences, engineering and medicine * List of unsolved problems in astronomy, Unsolved problems in astronom ...
concerning the sets of
Euclidean distance In mathematics, the Euclidean distance between two points in Euclidean space is the length of the line segment between them. It can be calculated from the Cartesian coordinates of the points using the Pythagorean theorem, and therefore is o ...
s between points in
compact Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to: * Interstate compact, a type of agreement used by U.S. states * Blood compact, an ancient ritual of the Philippines * Compact government, a t ...
d-dimensional spaces. Intuitively, it states that a set of points that is large in its
Hausdorff dimension In mathematics, Hausdorff dimension is a measure of ''roughness'', or more specifically, fractal dimension, that was introduced in 1918 by mathematician Felix Hausdorff. For instance, the Hausdorff dimension of a single point is zero, of a line ...
must determine a set of distances that is large in measure. More precisely, if S is a
compact set In mathematics, specifically general topology, compactness is a property that seeks to generalize the notion of a closed and bounded subset of Euclidean space. The idea is that a compact space has no "punctures" or "missing endpoints", i.e., i ...
of points in d-dimensional
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are ''Euclidean spaces ...
whose Hausdorff dimension is strictly greater than d/2, then the
conjecture In mathematics, a conjecture is a conclusion or a proposition that is proffered on a tentative basis without proof. Some conjectures, such as the Riemann hypothesis or Fermat's conjecture (now a theorem, proven in 1995 by Andrew Wiles), ha ...
states that the set of distances between pairs of points in S must have nonzero
Lebesgue measure In measure theory, a branch of mathematics, the Lebesgue measure, named after French mathematician Henri Lebesgue, is the standard way of assigning a measure to subsets of higher dimensional Euclidean '-spaces. For lower dimensions or , it c ...
.


Formulation and motivation

proved that
Borel set In mathematics, a Borel set is any subset of a topological space that can be formed from its open sets (or, equivalently, from closed sets) through the operations of countable union, countable intersection, and relative complement. Borel sets ...
s with Hausdorff dimension greater than (d+1)/2 have distance sets with nonzero measure. He motivated this result as a multidimensional generalization of the Steinhaus theorem, a previous result of
Hugo Steinhaus Hugo Dyonizy Steinhaus ( , ; 14 January 1887 – 25 February 1972) was a Polish mathematician and educator. Steinhaus obtained his PhD under David Hilbert at Göttingen University in 1911 and later became a professor at the Jan Kazimierz Univers ...
proving that every set of
real number In mathematics, a real number is a number that can be used to measure a continuous one- dimensional quantity such as a duration or temperature. Here, ''continuous'' means that pairs of values can have arbitrarily small differences. Every re ...
s with nonzero measure must have a
difference set In combinatorics, a (v,k,\lambda) difference set is a subset D of cardinality, size k of a group (mathematics), group G of order of a group, order v such that every non-identity element, identity element of G can be expressed as a product d_1d_2^ o ...
that contains an interval of the form (-\varepsilon,\varepsilon) for some \varepsilon>0. It may also be seen as a continuous analogue of the
Erdős distinct distances problem In discrete geometry, the Erdős distinct distances problem states that every set of points in the plane has a nearly-linear number of distinct distances. It was posed by Paul Erdős in 1946 and almost proven by Larry Guth and Nets Katz in 2015. ...
, which states that large finite sets of points must have large numbers of distinct distances. Based on this result, Falconer's conjecture states that if S is a compact set of points in d-dimensional
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are ''Euclidean spaces ...
whose Hausdorff dimension is strictly greater than d/2, then the set of distances between pairs of points in S must have nonzero
Lebesgue measure In measure theory, a branch of mathematics, the Lebesgue measure, named after French mathematician Henri Lebesgue, is the standard way of assigning a measure to subsets of higher dimensional Euclidean '-spaces. For lower dimensions or , it c ...
. That is, the threshold dimension of (d+1)/2 from Falconer's result can be reduced to d/2, while restricting the class of sets in the result to compact sets rather than Borel sets.


Partial results

proved that compact sets of points whose Hausdorff dimension is greater than \tfrac + \tfrac have distance sets with nonzero measure; for large values of d this approximates the threshold on Hausdorff dimension given by the Falconer conjecture. For points in the
Euclidean plane In mathematics, a Euclidean plane is a Euclidean space of Two-dimensional space, dimension two, denoted \textbf^2 or \mathbb^2. It is a geometric space in which two real numbers are required to determine the position (geometry), position of eac ...
, Borel sets of Hausdorff dimension greater than 5/4 (or \tfrac + \tfrac with d=2) have distance sets with nonzero measure and, more strongly, they have a point such that the Lebesgue measure of the distances from the set to this point is positive. For d>3 the best known bound is \tfrac + \tfrac-\tfrac according to a preprint by Du, Ou, Ren and
Zhang Zhang may refer to: Chinese culture, etc. * Zhang (surname) (張/张), common Chinese surname ** Zhang (surname 章), a rarer Chinese surname * Zhang County (漳县), of Dingxi, Gansu * Zhang River (漳河), a river flowing mainly in Henan * ''Zha ...
A variant of Falconer's conjecture states that, for points in the plane, a compact set whose Hausdorff dimension is greater than or equal to one must have a distance set of Hausdorff dimension one. As a partial result in this direction, the existence of a distance set of Hausdorff dimension one follows for sets of Hausdorff dimension greater than 5/4, from the results on measure. Another partial result is that a compact planar set with Hausdorff dimension at least one, the distance set must have Hausdorff dimension at least 1/2.


Related conjectures

Proving a bound strictly greater than 1/2 for the dimension of the distance set in the case of compact planar sets with Hausdorff dimension at least one would be equivalent to resolving several other unsolved conjectures. These include a conjecture of
Paul Erdős Paul Erdős ( ; 26March 191320September 1996) was a Hungarian mathematician. He was one of the most prolific mathematicians and producers of mathematical conjectures of the 20th century. pursued and proposed problems in discrete mathematics, g ...
on the existence of
Borel Borel may refer to: People * Antoine Borel (1840–1915), a Swiss-born American businessman * Armand Borel (1923–2003), a Swiss mathematician * Borel (author), 18th-century French playwright * Borel (1906–1967), pseudonym of the French actor ...
subring In mathematics, a subring of a ring is a subset of that is itself a ring when binary operations of addition and multiplication on ''R'' are restricted to the subset, and that shares the same multiplicative identity as .In general, not all s ...
s of the real numbers with fractional Hausdorff dimension, and a variant of the
Kakeya set In mathematics, a Kakeya set, or Besicovitch set, is a set of points in Euclidean space which contains a unit line segment in every direction. For instance, a disk of radius 1/2 in the Euclidean plane, or a ball of radius 1/2 in three-dimensiona ...
problem on the Hausdorff dimension of sets such that, for every possible direction, there is a line segment whose intersection with the set has high Hausdorff dimension. These conjectures were solved by Bourgain.


Other distance functions

For non-Euclidean distance functions in the plane defined by polygonal norms, the analogue of the Falconer conjecture is false: there exist sets of Hausdorff dimension two whose distance sets have measure zero..


References

{{reflist Measure theory Dimension theory Metric geometry Conjectures Unsolved problems in geometry