6 research outputs found
Covering Metric Spaces by Few Trees
A tree cover of a metric space (X,d) is a collection of trees, so that every pair x,y in X has a low distortion path in one of the trees. If it has the stronger property that every point x in X has a single tree with low distortion paths to all other points, we call this a Ramsey tree cover. Tree covers and Ramsey tree covers have been studied by [Yair Bartal et al., 2005; Anupam Gupta et al., 2004; T-H. Hubert Chan et al., 2005; Gupta et al., 2006; Mendel and Naor, 2007], and have found several important algorithmic applications, e.g. routing and distance oracles. The union of trees in a tree cover also serves as a special type of spanner, that can be decomposed into a few trees with low distortion paths contained in a single tree; Such spanners for Euclidean pointsets were presented by [S. Arya et al., 1995].
In this paper we devise efficient algorithms to construct tree covers and Ramsey tree covers for general, planar and doubling metrics. We pay particular attention to the desirable case of distortion close to 1, and study what can be achieved when the number of trees is small. In particular, our work shows a large separation between what can be achieved by tree covers vs. Ramsey tree covers
Optimality of the Johnson-Lindenstrauss Dimensionality Reduction for Practical Measures
It is well known that the Johnson-Lindenstrauss dimensionality reduction
method is optimal for worst case distortion. While in practice many other
methods and heuristics are used, not much is known in terms of bounds on their
performance. The question of whether the JL method is optimal for practical
measures of distortion was recently raised in BFN19 (NeurIPS'19). They provided
upper bounds on its quality for a wide range of practical measures and showed
that indeed these are best possible in many cases. Yet, some of the most
important cases, including the fundamental case of average distortion were left
open. In particular, they show that the JL transform has average
distortion for embedding into -dimensional Euclidean space, where
, and for more general -norms of distortion, , whereas tight lower bounds were
established only for large values of via reduction to the worst case.
In this paper we prove that these bounds are best possible for any
dimensionality reduction method, for any and , where
is the size of the subset of Euclidean space.
Our results imply that the JL method is optimal for various distortion
measures commonly used in practice such as stress, energy and relative error.
We prove that if any of these measures is bounded by then
for any , matching
the upper bounds of BFN19 and extending their tightness results for the full
range moment analysis.
Our results may indicate that the JL dimensionality reduction method should
be considered more often in practical applications, and the bounds we provide
for its quality should be served as a measure for comparison when evaluating
the performance of other methods and heuristics
Barriers for Faster Dimensionality Reduction
The Johnson-Lindenstrauss transform allows one to embed a dataset of
points in into while preserving the pairwise
distance between any pair of points up to a factor ,
provided that . The transform has found an
overwhelming number of algorithmic applications, allowing to speed up
algorithms and reducing memory consumption at the price of a small loss in
accuracy. A central line of research on such transforms, focus on developing
fast embedding algorithms, with the classic example being the Fast JL transform
by Ailon and Chazelle. All known such algorithms have an embedding time of
, but no lower bounds rule out a clean embedding time.
In this work, we establish the first non-trivial lower bounds (of magnitude
) for a large class of embedding algorithms, including in
particular most known upper bounds