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Higher-order Erdös: Szekeres theorems

Published: 17 June 2012 Publication History

Abstract

Let P=(p1,p2,...,pN) be a sequence of points in the plane, where pi=(xi,yi) and x1<x2...xN. A famous 1935 Erdos-Szekeres theorem asserts that every such P contains a monotone subsequence S of √N points. Another, equally famous theorem from the same paper implies that every such P contains a convex or concave subsequence of Ω(log N) points.
Monotonicity is a property determined by pairs of points, and convexity concerns triples of points. We propose a generalization making both of these theorems members of an infinite family of Ramsey-type results. First we define a (k+1)-tuple K ⊆ P to be positive if it lies on the graph of a function whose kth derivative is everywhere nonnegative, and similarly for a negative (k+1)-tuple. Then we say that S ⊆ P is kth-order monotone if its (k+1)-tuples are all positive or all negative.
We investigate quantitative bound for the corresponding Ramsey-type result (i.e., how large kth-order monotone subsequence can be guaranteed in every N-point P). We obtain an Ω(log(k-1)N) lower bound ((k-1)-times iterated logarithm). This is based on a quantitative Ramsey-type theorem for what we call transitive colorings of the complete (k+1)-uniform hypergraph; it also provides a unified view of the two classical Erdos-Szekeres results mentioned above.
For k=3, we construct a geometric example providing an O(log log N) upper bound, tight up to a multiplicative constant. As a consequence, we obtain similar upper bounds for a Ramsey-type theorem for order-type homogeneous subsets in R3, as well as for a Ramsey-type theorem for hyperplanes in R4 recently used by Dujmovic and Langerman.

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V. Dujmovic and S. Langerman. A center transversal theorem for hyperplanes and applications to graph drawing. In Proc. 27th ACM Symposium on Computational Geometry, pages 117--124, 2011. Full version arXiv:1012.0548.
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P. Erdos and R. Rado. Combinatorial theorems on classifications of subsets of a given set. Proc. London Math. Soc., 3:417--439, 1952.
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J. Fox, J. Pach, B. Sudakov, and A. Suk. Erdos-Szekeres-type theorem for monotone paths and convex bodies. Arxiv preprint 1105.2097v1, 2011. Proc. London Math. Soc., in press.
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M. J. Steele. Variations on the monotone subsequence theme of Erdos and Szekeres. In D. Aldous et al., editors, Discrete Probability and Algorithms, IMA Volumes in Mathematics and its Applications 72, pages 111--131. Springer, Berlin etc., 1995.
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  • (2013)Ramsey-type results for semi-algebraic relationsProceedings of the twenty-ninth annual symposium on Computational geometry10.1145/2462356.2462399(309-318)Online publication date: 17-Jun-2013

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    cover image ACM Conferences
    SoCG '12: Proceedings of the twenty-eighth annual symposium on Computational geometry
    June 2012
    436 pages
    ISBN:9781450312998
    DOI:10.1145/2261250
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    Published: 17 June 2012

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    Author Tags

    1. Erdos--Szekeres theorem
    2. combinatorial geometry
    3. order type
    4. ramsey theory

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    SoCG '12
    SoCG '12: Symposium on Computational Geometry 2012
    June 17 - 20, 2012
    North Carolina, Chapel Hill, USA

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    • (2013)Ramsey-type results for semi-algebraic relationsProceedings of the twenty-ninth annual symposium on Computational geometry10.1145/2462356.2462399(309-318)Online publication date: 17-Jun-2013

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