Position générale | EPFL Graph Search

En géométrie algébrique et en géométrie algorithmique, une position générale est une notion de pour un ensemble d'objets géométriques (points, droites, courbes, plans, ...). C'est ce qu'on entend quand on parle du cas général, en opposition aux cas particuliers qui peuvent apparaître, auxquels cas on parlera de position spéciale. Cette expression peut changer de sens selon le contexte. Par exemple, deux droites d'un même plan, dans le cas général, se croisent en un point unique, et on dira alors : "deux droites génériques se croisent en un point", ce qui est derrière la notion de point générique. Les cas particuliers sont ceux où les droites sont parallèles (il n'y a alors aucun point d'intersection) ou coïncidentes (tous les points des droites sont des points d'intersection). De même, trois points génériques du plan ne sont pas colinéaires ; si les trois points sont colinéaires voire deux d'entre eux sont confondus, on parle même de cas dégénéré. Cette notion est importante en mat

On some algebraic and extremal problems in discrete geometry

Seyed Hossein Nassajianmojarrad

In the present thesis, we delve into different extremal and algebraic problems arising from combinatorial geometry. Specifically, we consider the following problems. For any integer

n\ge 3

, we define

e(n)

to be the minimum positive integer such that any set of

e(n)

points in general position in the plane contains

n

points in convex position. In 1935, Erd\H{o}s and Szekeres proved that

e(n) \le {2n-4 \choose n-2}+1

and later in 1961, they obtained the lower bound

2^{n-2}+1 \le e(n)

, which they conjectured to be optimal. We prove that

e(n) \le {2n-5 \choose n-2}-{2n-8 \choose n-3}+2

. In a recent breakthrough, Suk proved that

e(n) \le 2^{n+O\left(n^{2/3}\log n\right)}

. We strengthen this result by extending it to pseudo-configurations and also improving the error term. Combining our results with a theorem of Dobbins et al., we significantly improve the best known upper bounds on the following two functions, introduced by Bisztriczky and Fejes T'{o}th and by Pach and T'{o}th, respectively. Let

c(n)

(and

c'(n)

) denote the smallest positive integer

N

such that any family of

N

pairwise disjoint convex bodies in general position (resp.,

N

convex bodies in general position, any pair of which share at most two boundary points) has an

n

members in convex position. We show that

c(n)\le c'(n)\le 2^{n+O\left(\sqrt{n\log n}\right)}

. Given a point set

P

in the plane, an ordinary circle for

P

is defined as a circle containing exactly three points of

P

. We prove that any set of

n

points in the plane, not all on a line or a circle, determines at least

\frac{1}{4}n^2-O(n)

ordinary circles. We determine the exact minimum number of ordinary circles for all sufficiently large

n

, and characterize all point sets that come close to this minimum. We also consider the orchard problem for circles, where we determine the maximum number of circles containing four points of a given set and describe the extremal configurations. A special case of the Schwartz-Zippel lemma states that given an algebraic curve

C\subset \mathbb{C}^2

of degree

d

and two finite sets

A,B\subset \mathbb{C}

, we have

|C\cap (A\times B)|=O_d(|A|+|B|)

. We establish a two-dimensional version of this result, and prove upper bounds on the size of the intersection

|X\cap (P\times Q)|

for a variety

X\subset \mathbb{C}^4

and finite sets

P,Q\subset \mathbb{C}^2

. A key ingredient in our proofs is a two-dimensional version of a special case of Alon's combinatorial Nullstellensatz. As corollaries, we generalize the Szemer'edi-Trotter point-line incidence theorem and several known bounds on repeated and distinct Euclidean distances. We use incidence geometry to prove some sum-product bounds over arbitrary fields. First, we give an explicit exponent and improve a recent result of Bukh and Tsimerman by proving that

\max \{ |A+A|, |f(A, A)|\}\gg |A|^{6/5}

for any small set

A\subset \mathbb{F}_p

and quadratic non-degenerate polynomial

f(x, y)\in \mathbb{F}_p[x, y]

. This generalizes the result of Roche-Newton et al. giving the best known lower bound for the term

\max \{ |A+A|, |A \cdot A |\}

. Secondly, we improve and generalize the sum-product results of Hegyv'{a}ri and Hennecart on

\max\{ |A+B|, |f(B,C)|\}

, for a specific type of function

f

. Finally, we prove that the number of distinct cubic distances generated by any small set

A\times A\subset \mathbb{F}_p^2

\Omega(|A|^{8/7})

, which improves a result of Yazici et al.

EPFL2017

Algebraic and topological methods in combinatorics

Adrian Claudiu Valculescu

The present thesis deals with problems arising from discrete mathematics, whose proofs make use of tools from algebraic geometry and topology. The thesis is based on four papers that I have co-authored, three of which have been published in journals, and one has been submitted for publication (and also appeared as a preprint on the arxiv, and as an extendend abstract in a conference). Specifically, we deal with the following four problems: \begin{enumerate} \item We prove that if

M\in \mathbb{C}^{2\times2}

is an invertible matrix, and

B_M:\mathbb{C}^2\times\mathbb{C}^2\to\mathbb{C}

is a bilinear form

B_M(p,q)=p^TMq

, then any finite set

S

contained in an irreducible algebraic curve

C

of degree

d

\mathbb{C}^2

determines

\Omega_d(|S|^{4/3})

distinct values of

B_M

, unless

C

is a line, or is linearly equivalent to a curve defined by an equation of the form

x^k=y^l

, with

k,l\in\mathbb{Z}\backslash\\{0\\}

, and

\gcd(k,l)=1

. \item We show that if we are given

m

points and

n

lines in the plane, then the number of distinct distances between the points and the lines is

\Omega(m^{1/5}n^{3/5})

, as long as

m^{1/2}\le n\le m^2

. Also, we show that if we are given

m

points in the plane, not all collinear, then the number of distances between these points and the lines that they determine is

\Omega(m^{4/3})

. We also study three-dimensional versions of the distinct point-line distances problem. \item We prove the lower bound

\Omega(|S|^4)

on the number of ordinary conics determined by a finite point set

S

\mathbb{R}^2

, assuming that

S

is not contained in a conic, and at most

c|S|

points of

S

lie on the same line (for some $0

EPFL2017

Near equipartitions of colored point sets

Andreas Fukami Holmsen, Jan Kyncl, Adrian Claudiu Valculescu

Suppose that nk points in general position in the plane are colored red and blue, with at least n points of each color. We show that then there exist n pairwise disjoint convex sets, each of them containing k of the points, and each of them containing points of both colors. We also show that if P is a set of n(d + 1) points in general position in R-d colored by d colors with at least n points of each color, then there exist n pairwise disjoint d-dimensional simplices with vertices in P, each of them containing a point of every color. These results can be viewed as a step towards a common generalization of several previously known geometric partitioning results regarding colored point sets. (C) 2017 Elsevier B.V. All rights reserved.

Elsevier Science Bv2017

Algebraic and topological methods in combinatorics

Adrian Claudiu Valculescu

M\in \mathbb{C}^{2\times2}

is an invertible matrix, and

B_M:\mathbb{C}^2\times\mathbb{C}^2\to\mathbb{C}

is a bilinear form

B_M(p,q)=p^TMq

, then any finite set

S

contained in an irreducible algebraic curve

C

of degree

d

\mathbb{C}^2

determines

\Omega_d(|S|^{4/3})

distinct values of

B_M

, unless

C

is a line, or is linearly equivalent to a curve defined by an equation of the form

x^k=y^l

, with

k,l\in\mathbb{Z}\backslash\\{0\\}

, and

\gcd(k,l)=1

. \item We show that if we are given

m

points and

n

lines in the plane, then the number of distinct distances between the points and the lines is

\Omega(m^{1/5}n^{3/5})

, as long as

m^{1/2}\le n\le m^2

. Also, we show that if we are given

m

points in the plane, not all collinear, then the number of distances between these points and the lines that they determine is

\Omega(m^{4/3})

. We also study three-dimensional versions of the distinct point-line distances problem. \item We prove the lower bound

\Omega(|S|^4)

on the number of ordinary conics determined by a finite point set

S

\mathbb{R}^2

, assuming that

S

is not contained in a conic, and at most

c|S|

points of

S

lie on the same line (for some $0

EPFL2017

On some algebraic and extremal problems in discrete geometry

Seyed Hossein Nassajianmojarrad

In the present thesis, we delve into different extremal and algebraic problems arising from combinatorial geometry. Specifically, we consider the following problems. For any integer

n\ge 3

, we define

e(n)

to be the minimum positive integer such that any set of

e(n)

points in general position in the plane contains

n

points in convex position. In 1935, Erd\H{o}s and Szekeres proved that

e(n) \le {2n-4 \choose n-2}+1

and later in 1961, they obtained the lower bound

2^{n-2}+1 \le e(n)

, which they conjectured to be optimal. We prove that

e(n) \le {2n-5 \choose n-2}-{2n-8 \choose n-3}+2

. In a recent breakthrough, Suk proved that

e(n) \le 2^{n+O\left(n^{2/3}\log n\right)}

c(n)

(and

c'(n)

) denote the smallest positive integer

N

such that any family of

N

pairwise disjoint convex bodies in general position (resp.,

N

convex bodies in general position, any pair of which share at most two boundary points) has an

n

members in convex position. We show that

c(n)\le c'(n)\le 2^{n+O\left(\sqrt{n\log n}\right)}

. Given a point set

P

in the plane, an ordinary circle for

P

is defined as a circle containing exactly three points of

P

. We prove that any set of

n

points in the plane, not all on a line or a circle, determines at least

\frac{1}{4}n^2-O(n)

ordinary circles. We determine the exact minimum number of ordinary circles for all sufficiently large

n

C\subset \mathbb{C}^2

of degree

d

and two finite sets

A,B\subset \mathbb{C}

, we have

|C\cap (A\times B)|=O_d(|A|+|B|)

. We establish a two-dimensional version of this result, and prove upper bounds on the size of the intersection

|X\cap (P\times Q)|

for a variety

X\subset \mathbb{C}^4

and finite sets

P,Q\subset \mathbb{C}^2

\max \{ |A+A|, |f(A, A)|\}\gg |A|^{6/5}

for any small set

A\subset \mathbb{F}_p

and quadratic non-degenerate polynomial

f(x, y)\in \mathbb{F}_p[x, y]

. This generalizes the result of Roche-Newton et al. giving the best known lower bound for the term

\max \{ |A+A|, |A \cdot A |\}

. Secondly, we improve and generalize the sum-product results of Hegyv'{a}ri and Hennecart on

\max\{ |A+B|, |f(B,C)|\}

, for a specific type of function

f

. Finally, we prove that the number of distinct cubic distances generated by any small set

A\times A\subset \mathbb{F}_p^2

\Omega(|A|^{8/7})

, which improves a result of Yazici et al.

EPFL2017