Hadwiger conjecture (combinatorial geometry): Difference between revisions

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{{see also|Hadwiger conjecture (graph theory)}}

[[File:Hadwiger covering.svg|thumb|300px|A triangle can be covered by three smaller copies of itself; a square requires four smaller copies]]

{{unsolved|mathematics|Can every <math>n</math>-dimensional convex body be covered by <math>2^n</math> smaller copies of itself?}}

In [[combinatorial geometry]], the '''Hadwiger conjecture''' states that any [[convex body]] in ''n''-dimensional [[Euclidean space]] can be covered by 2^''n'' or fewer smaller bodies [[Homothetic transformation|homothetic]] with the original body, and that furthermore, the upper bound of 2^''n'' is necessary if and only if the body is a [[parallelepiped]]. There also exists an equivalent formulation in terms of the number of floodlights needed to illuminate the body.

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: <math>K\subseteq\bigcup_{i=1}^{2^n} s_i K + v_i.</math>

Furthermore, the upper bound is necessary iffif and only if ''K'' is a parallelepiped, in which case all 2^''n'' of the scalars may be chosen to be equal to 1/2.

===Alternate formulation with illumination===

As shown by [[Vladimir Boltyansky|Boltyansky]], the problem is equivalent to [[illumination problem|one of illumination]]: how many floodlights must be placed outside of an opaque convex body in order to completely illuminate its exterior? For the purposes of this problem, a body is only considered to be illuminated if for each point of the boundary of the body, there is at least one floodlight that is separated from the body by all of the [[tangent plane]]s intersecting the body on this point; thus, although the faces of a cube may be lit by only two floodlights, the planes tangent to its vertices and edges cause it to need many more lights in order for it to be fully illuminated. For any convex body, the number of floodlights needed to completely illuminate it turns out to equal the number of smaller copies of the body that are needed to cover it.<ref name="BMP"/>

==Examples==

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==Known results==

The two-dimensional case was settled by {{harvtxt|Levi|1955}}: every two-dimensional bounded convex set may be covered with four smaller copies of itself, with the fourth copy needed only in the case of parallelograms. However, the conjecture remains open in higher dimensions except for some special cases. The best known asymptotic upper bound on the number of smaller copies needed to cover a given body is<ref>{{harvtxt|HuangCampos|Slomkavan Hintum|TkoczMorris|VritsiouTiba|20222023}}.</ref>

:<math>\displaystyle 4^n\binomexp\frac{2n-cn}{n}\exp(-c\sqrt{log^8 n})</math>.

where <math>c</math> is a positive constant. For small <math>n</math> the upper bound of <math>(n+1)n^{n-1}-(n-1)(n-2)^{n-1}</math> established by {{harvtxt|Lassak|1988}} is better than the asymptotic one. In three dimensions it is known that 16 copies always suffice, but this is still far from the conjectured bound of 8 copies.<ref name="BMP">{{harvtxt|Brass|Moser|Pach|2005}}.</ref>

The conjecture is known to hold for certain special classes of convex bodies, including, in dimension three, centrally symmetric polyhedra and [[Surface of constant width|bodies of constant width]] in three dimensions.<ref name="BMP"/> The number of copies needed to cover any [[zonotope]] (other than a parallelepiped) is at most <math>(3/4)2^n</math>, while for bodies with a smooth surface (that is, having a single tangent plane per boundary point), at most <math>n+1</math> smaller copies are needed to cover the body, as [[Friedrich Wilhelm Levi|Levi]] already proved.<ref name="BMP"/>

==See also==

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==References==

* {{citation|title=Resultscite andbook Problems| inlast1=Boltjansky Combinatorial Geometry| first1=VVladimir G. |last1 author-link1 =Boltjansky Vladimir Boltyansky | last2=Gohberg | first2=Israel |last2author-link2=Israel Gohberg |authorlink2 title=IsraelResults and Problems in Combinatorial Geometry Gohberg| publisher=[[Cambridge University Press]] | publication-place=Cambridge | year=1985|contribution=11. Hadwiger's Conjecture |pages=44–46 | isbn=978-0-521-26923-0}}.

*{{citationcite book|first1=Peter|last1=Brass|first2=William|last2=Moser|

first3=János|last3=Pach|author3-link=János Pach|contribution=3.3 Levi–Hadwiger Covering Problem and Illumination|pages=136–142|title=Research Problems in Discrete Geometry|publisher=Springer-Verlag|year=2005| isbn=978-0-387-23815-9}}.

*{{citation|first1=Marcelo|last1=Campos|first2=Peter|last2=van Hintum|first3=Robert|last3=Morris|author3-link=Robert Morris (mathematician)|first4=Marius|last4=Tiba|title=Towards Hadwiger's Conjecture via Bourgain Slicing|journal=[[International Mathematics Research Notices]]|doi=10.1093/imrn/rnad198|year=2023|volume=2024 |issue=10 |pages=8282–8295 |arxiv=2206.11227}}.

*{{citation|first1=Israel Ts.|last1=Gohberg|authorlink1=Israel Gohberg|first2=Alexander S.|last2=Markus|language= Russian|year=1960|title=A certain problem about the covering of convex sets with homothetic ones|journal=Izvestiya Moldavskogo Filiala Akademii Nauk SSSR|volume=10|issue=76|pages=87–90}}.

*{{citation|first=Hugo|last=Hadwiger|authorlink=Hugo Hadwiger|year=1957|title=Ungelöste Probleme Nr. 20|journal=[[Elemente der Mathematik]]|volume=12|pages=121}}.

*{{Citation |lastlast1=Huang |firstfirst1=Han |last2=Slomka |first2=Boaz A. |last3=Tkocz |first3=Tomasz |last4=Vritsiou |first4=Beatrice-Helen |date=2022 |title=Improved bounds for Hadwiger’sHadwiger's covering problem via thin-shell estimates |journal=[[Journal of the European Mathematical Society]] |language=en |volume=24 |issue=4 |pages=1431–1448 |doi=10.4171/jems/1132 |doi-access=free |issn=1435-9855|arxiv=1811.12548 }}.

*{{citation|first=Marek|last=Lassak|year=1988|title=Covering the boundary of a convex set by tiles|journal=[[Proceedings of the American Mathematical Society]]|volume=104|issue=1|pages=269–272|mr=0958081|doi=10.1090/s0002-9939-1988-0958081-7|doi-access=free}}.

*{{citation|first=Friedrich Wilhelm|last=Levi|authorlink=Friedrich Wilhelm Levi|title=Überdeckung eines Eibereiches durch Parallelverschiebungen seines offenen Kerns|journal=[[Archiv der Mathematik]]|volume=6|year=1955|pages=369–370|issue=5|doi=10.1007/BF01900507|doi-access=free|s2cid=121459171}}.

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[[Category:Conjectures]]

[[Category:Unsolved problems in geometry]]

[[Category:Eponyms in geometry]]