Kakeya set

Needle shown rotating inside a deltoid. At every stage of its rotation, the needle is in contact with the deltoid at three points: two endpoints (blue) and one tangent point (black). The needle's midpoint (red) describes a circle with diameter equal to half the length of the needle.

In mathematics, a Kakeya set, or Besicovitch set, is any set of points in Euclidean space which contains a unit line segment in every direction. While many types of objects satisfy this property, several interesting results and questions are motivated by considering how small such sets can be. Besicovitch showed that there are Besicovitch sets of measure zero.

A Kakeya needle set (sometimes also known as a Kakeya set) is a (Besicovitch) set within which a unit line segment can be rotated continuously through 180 degrees, returning to its original position with reversed orientation. Besicovitch showed that there are Kakeya needle sets of arbitrarily small positive measure.

Kakeya needle problem

The Kakeya needle problem asks whether there is a minimum area of a region D in the plane, in which a needle can be turned through 360°. This question was first posed, for convex regions, in 1917 by Soichi Kakeya (1886–1947), a Japanese mathematician who worked mainly in mathematical analysis.

He seems to have suggested that D of minimum area, without the convexity restriction, would be a three-pointed deltoid shape. The original problem was solved by Pal. The early history of this question has been subject to some discussion, though.

Besicovitch sets

A 'sprouting' method for constructing a Kakeya set of small measure. Shown here are two possible ways of dividing our triangle and overlapping the pieces to get a smaller set, the first if we just use two triangles, and the second if we use eight. Notice how small the sizes are of the final figures are in comparison to each other.

Besicovitch was able to show that there is no lower bound > 0 for the area of such a region D, in which a needle of unit length can be turned round. This built on earlier work of his, on plane sets which contain a unit segment in each orientation. Such a set is now called a Besicovitch set. Besicovitch's work showing such a set could have arbitrarily small measure was from 1919. The problem may have been considered by analysts before that.

One method of constructing a Besicovitch set can be described as follows (see figure for corresponding illustrations). The following is known as a "Perron tree" after O. Perron who was able to simplify Besicovitch's original construction: take a triangle with height 1, divide it in two, and translate both pieces over each other so that their bases overlap on some small interval. Then this new figure will have a reduced total area.

Now, suppose we divide our triangle into eight subtriangles. For each consecutive pair of triangles, perform the same overlapping operation we described before to get four new shapes, each consisting of two overlapping triangles. Next, overlap consecutive pairs of these new shapes by shifting their bases over each other partially, so we're left with two shapes, and finally overlap these two in the same way. In the end, we get a shape looking somewhat like a tree, but with an area much smaller than our original triangle.

To construct an even smaller set, subdivide your triangle into, say, $2^{n}$ triangles each of base length $2^{-n}$, and perform the same operations as we did before when we divided our triangle twice and eight times. If the amount of overlap we do on each triangle is small enough and the size $n$ of the subdivision of our triangle is large enough, we can form a tree of area as small as we like. A Besicovitch set can be created by combining three rotations of a Perron tree created from an equilateral triangle. Adapting this method further, we may actually construct a sequence of sets whose intersection is a Besicovitch set of measure zero.

There are other methods for constructing Besicovitch sets of measure zero aside from the 'sprouting' method. For example, Kahane uses Cantor sets to construct a Besicovitch set of measure zero in the two-dimensional plane.

A Kakeya needle set constructed from Perron trees.

Kakeya needle sets

By using a trick of Pàl, known as Pàl joins (given two parallel lines, any unit line segment can be moved continuously from one to the other on a set of arbitrary small measure), a set in which a unit line segment can be rotated continuously through 180 degrees can be created from a Besicovitch set consisting of Perron trees.

In 1941, H. J. Van Alphen showed that there are arbitrary small Kakeya needle sets inside a circle with radius $2+\epsilon$ (arbitrary $\epsilon >0$). Simply connected Kakeya needle sets with smaller area than the deltoid were found in 1965. Melvin Bloom and I. J. Schoenberg independently presented Kakeya needle sets with areas approaching to $(5-2{\sqrt {2}})\pi /24$, the Bloom-Schoenberg number. Schoenberg conjectured that this number is the lower bound for the area of simply connected Kakeya needle sets. However, in 1971, F. Cunningham showed that, given $\epsilon >0$, there is a simply connected Kakeya needle set of area less than $\epsilon$ contained in a circle of radius 1.

Kakeya conjecture

Statement

The same question of how small these Besicovitch sets could be was then posed in higher dimensions, giving rise to a number of conjectures known collectively as the Kakeya conjectures, and have helped initiate the field of mathematics known as geometric measure theory. In particular, if there exist Besicovitch sets of measure zero, how 'measure zero' are they? More explicitly put, could they also have s-dimensional Hausdorff measure zero for some dimension s less than the dimension of the space in which they lie? This question gives rise to the following conjecture:

Kakeya set conjecture: Define a Besicovitch set in ${\mathbb {R}}^{n}$ to be a set which contains a unit line segment in every direction. Is it true that such sets necessarily have Hausdorff dimension and Minkowski dimension equal to n? This is known to be true for n = 1, 2 but only partial results are known in higher dimensions.

Kakeya maximal function

A modern way of approaching this problem is to consider a particular type of maximal function, which we construct as follows: Denote $S^{n-1}\subset \mathbb {R} ^{n}$ to be the unit sphere in n-dimensional space. Define $T_{e}^{\delta }(a)$ to be the cylinder of length 1, radius $\delta >0$, centered at the point $a\in \mathbb {R} ^{n}$, and whose long side is parallel to the direction of the unit vector $e\in S^{n-1}$. Then for a locally integrable function $f$, we define the Kakeya maximal function of $f$ to be

$f_{*}^{\delta }(e)=\sup _{a\in \mathbb {R} ^{n}}{\frac {1}{m(T_{e}^{\delta }(a))}}\int _{T_{e}^{\delta }(a)}|f(y)|dm(y)$

where $m$ denotes the n-dimensional Lebesgue measure. Notice that $f_{*}^{\delta }$ is defined for vectors $e$ in the sphere $S^{n-1}$.

Then there is a conjecture for these functions that, if true, will imply the Kakeya set conjecture for higher dimensions:

Kakeya maximal function conjecture: For all $\epsilon >0$, there exists a constant $C_{\epsilon }>0$ such that for any function $f$ and all $\delta >0$,

$||f_{*}^{\delta }||_{L^{n}(S^{n-1})}\leq C_{\epsilon }\delta ^{-\epsilon }||f||_{L^{n}(\mathbb {R} ^{n})}.$

(see lp space for notation).

Results

Some results toward proving the Kakeya conjecture are the following:

• The Kakeya conjecture is true for n = 1 (trivially) and n = 2 (Davies).
• In any n-dimensional space, Wolff showed that the dimension of a Kakeya set must be at least ${\frac {n+2}{2}}$.

• In 2002, Katz and Tao improved this bound to $(2-{\sqrt {2}})(n-4)+3$, which is better for n>4.

Applications to analysis

Somewhat surprisingly, these conjectures have been shown to be connected to a number of questions in other fields, notably in harmonic analysis. For instance, in 1971, Charles Fefferman was able to use the Besicovitch set construction to show that Fourier transform in dimensions greater than 2, when integral is over balls centered at the origin whose radius tend to infinity, does not necessarily converge in the L^p norm when p ≠ 2.

Analogues and generalizations of the Kakeya problem

Sets containing circles and spheres

Analogues of the Kakeya problem include considering sets containing more general shapes than lines, such as circles.

• In 1997 and 1999, Wolff proved that sets containing a sphere of every radius must have full dimension, that is, the dimension is equal to the dimension of the space it is lying in, and proved this by proving bounds on a circular maximal function analogous to the Kakeya maximal function.

• It was conjectured that there existed sets containing a sphere around every point of measure zero. Results of Elias Stein proved all such sets must have positive measure when $n\geq 3$, and Marstrand proved the same for the case n=2.

Sets containing k-dimensional disks

A generalization of the Kakeya conjecture is to consider sets that contain, instead of segments of lines in every direction, but, say, portions of k-dimensional subspaces. Define an (n,k) -Besicovitch set K to be a compact set in $\mathbb {R} ^{n}$ containing a translate of‘ every k-dimensional unit disk which has Lebesgue measure zero. That is, if B denotes the unit ball centered at zero, for every k-dimensional subspace P, there exists $x\in \mathbb {R} ^{n}$ such that $(P\cap B)+x\subseteq K$. Hence, a (n,1)-Besicovitch set is the standard Besicovitch set described earlier.

The (n,k)-Besicovitch conjecture: There are no (n,k)-Besicovitch sets for k>1.

In 1979, Marstrand proved that there were no (3,2)-Besicovitch sets. At around the same time, however, Falconer proved that there were no (n,k)-Besicovitch sets for $k>{\frac {n}{2}}$. The best bound to date is by Bourgain, who proved in that no such sets exist when $2^{k-1}+k>n$.

Kakeya sets in vector spaces over finite fields

In 1999, Wolff posed the finite field analogue to the Kakeya problem, in hopes that the techniques for solving this simpler conjecture could be carried over to the Euclidean case.

Finite Field Kakeya Conjecture: Let $F$ be a finite field, let $K\subseteq F^{n}$ be a Kakeya set, i.e. for each vector $y\in F^{n}$ $K$ contains a line $\{x+ty:t\in F\}$ for some $x\in F$. Then the set $K$ has size at least $c_{n}|F|^{n}$ where $c_{n}>0$ is a constant that only depends on $n$.

This conjecture was proved in 2008 by Zeev Dvir ("On the size of Kakeya sets in finite fields".) using what Terence Tao called a "beautifully simple argument". It is not clear whether the techniques will extend to proving the original Kakeya conjecture but this proof does lend credence to the original conjecture by making essentially algebraic counterexamples unlikely. Dvir has written a survey article on recent (as of 2009) progress on the finite field Kakeya problem and its relationship to randomness extractors.

See also

• Nikodym set

References

• Besicovitch, Abram (1963). "The Kakeya Problem". American Mathematical Monthly. 70: 697–706. doi:10.2307/2312249.
• Falconer, K. J. (1985). The Geometry of Fractal Sets. Cambridge University Press.

• Katz, Nets Hawk (2000). "An improved bound on the Minkowski dimension of Besicovitch sets in R³" (PDF). Ann. of Math. 152: 383–446. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Tao, Terence (2001). "From Rotating Needles to Stability of Waves: Emerging Connections between Combinatorics, Analysis and PDE" (PDF). Notices of the AMS. 48 (3): 297–303. {{cite journal}}: Unknown parameter |month= ignored (help)

• Wolff, Thomas (1999). "Recent work connected with the Kakeya problem". In H. Rossi (ed.) (ed.). Prospects in Mathematics. AMS. {{cite book}}: |editor= has generic name (help)
• Wolff, Thomas (2003). Lectures in Harmonic Analysis. AMS.

External links

• Kakeya at University of British Columbia
• Besicovitch at UCLA
• Kakeya needle problem at mathworld