56,471 research outputs found
Higher-order Erdos--Szekeres theorems
Let P=(p_1,p_2,...,p_N) be a sequence of points in the plane, where
p_i=(x_i,y_i) and x_1<x_2<...<x_N. A famous 1935 Erdos--Szekeres theorem
asserts that every such P contains a monotone subsequence S of
points. Another, equally famous theorem from the same paper implies that every
such P contains a convex or concave subsequence of 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 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 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 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 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 R^3, as well as for a Ramsey-type theorem for hyperplanes in R^4
recently used by Dujmovic and Langerman.Comment: Contains a counter example of Gunter Rote which gives a reply for the
problem number 5 in the previous versions of this pape
Optimal Vertex Fault Tolerant Spanners (for fixed stretch)
A -spanner of a graph is a sparse subgraph whose shortest path
distances match those of up to a multiplicative error . In this paper we
study spanners that are resistant to faults. A subgraph is an
vertex fault tolerant (VFT) -spanner if is a -spanner
of for any small set of vertices that might "fail." One
of the main questions in the area is: what is the minimum size of an fault
tolerant -spanner that holds for all node graphs (as a function of ,
and )? This question was first studied in the context of geometric
graphs [Levcopoulos et al. STOC '98, Czumaj and Zhao SoCG '03] and has more
recently been considered in general undirected graphs [Chechik et al. STOC '09,
Dinitz and Krauthgamer PODC '11].
In this paper, we settle the question of the optimal size of a VFT spanner,
in the setting where the stretch factor is fixed. Specifically, we prove
that every (undirected, possibly weighted) -node graph has a
-spanner resilient to vertex faults with edges, and this is fully optimal (unless the famous Erdos Girth
Conjecture is false). Our lower bound even generalizes to imply that no data
structure capable of approximating similarly can
beat the space usage of our spanner in the worst case. We also consider the
edge fault tolerant (EFT) model, defined analogously with edge failures rather
than vertex failures. We show that the same spanner upper bound applies in this
setting. Our data structure lower bound extends to the case (and hence we
close the EFT problem for -approximations), but it falls to for . We leave it as an open problem to
close this gap.Comment: To appear in SODA 201
From Monge-Ampere equations to envelopes and geodesic rays in the zero temperature limit
Let X be a compact complex manifold equipped with a smooth (but not
necessarily positive) closed form theta of one-one type. By a well-known
envelope construction this data determines a canonical theta-psh function u
which is not two times differentiable, in general. We introduce a family of
regularizations of u, parametrized by a positive number beta, defined as the
smooth solutions of complex Monge-Ampere equations of Aubin-Yau type. It is
shown that, as beta tends to infinity, the regularizations converge to the
envelope u in the strongest possible Holder sense. A generalization of this
result to the case of a nef and big cohomology class is also obtained. As a
consequence new PDE proofs are obtained for the regularity results for
envelopes in [14] (which, however, are weaker than the results in [14] in the
case of a non-nef big class). Applications to the regularization problem for
quasi-psh functions and geodesic rays in the closure of the space of Kahler
metrics are given. As briefly explained there is a statistical mechanical
motivation for this regularization procedure, where beta appears as the inverse
temperature. This point of view also leads to an interpretation of the
regularizations as transcendental Bergman metrics.Comment: 28 pages. Version 2: 29 pages. Improved exposition, references
updated. Version 3: 31 pages. A direct proof of the bound on the
Monge-Amp\`ere mass of the envelope for a general big class has been included
and Theorem 2.2 has been generalized to measures satisfying a
Bernstein-Markov propert
Curvature-dimension inequalities and Li-Yau inequalities in sub-Riemannian spaces
In this paper we present a survey of the joint program with Fabrice Baudoin
originated with the paper \cite{BG1}, and continued with the works \cite{BG2},
\cite{BBG}, \cite{BG3} and \cite{BBGM}, joint with Baudoin, Michel Bonnefont
and Isidro Munive.Comment: arXiv admin note: substantial text overlap with arXiv:1101.359
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