For any norms N-1, ... , N-m on R-n and N(x) := N-1(x) + ... + N-m(x), we show there is a sparsified norm (N) over tilde (x) = w(1)N(1)(x) + ... + w(m)N(m)(x) such that vertical bar N(x)-(N) over tilde (x)vertical bar...
详细信息
ISBN:
(纸本)9798350318944
For any norms N-1, ... , N-m on R-n and N(x) := N-1(x) + ... + N-m(x), we show there is a sparsified norm (N) over tilde (x) = w(1)N(1)(x) + ... + w(m)N(m)(x) such that vertical bar N(x)-(N) over tilde (x)vertical bar <= epsilon N(x) for all x is an element of R-n, where w(1), ... , w(m) are non-negative weights, of which only O(epsilon(-2) n log(n/epsilon)(log n)(2.5)) are non-zero. Additionally, we show that such weights can be found with high probability in time O(m(log n)(O(1)) + poly(n))T, where T is the time required to evaluate a norm N-i(x), assuming that N(x) is poly(n)-equivalent to the Euclidean norm. This immediately yields analogous statements for sparsifying sums of symmetric submodular functions. More generally, we show how to sparsify sums of pth powers of norms when the sum is p-uniformly smooth. (1)
We aim to understand the extent to which the noise distribution in a planted signal-plus-noise problem impacts its computational complexity. To that end, we consider the planted clique and planted dense subgraph probl...
详细信息
ISBN:
(纸本)9798350318944
We aim to understand the extent to which the noise distribution in a planted signal-plus-noise problem impacts its computational complexity. To that end, we consider the planted clique and planted dense subgraph problems, but in a different ambient graph. Instead of Erdos-Renyi G(n, p), which has independent edges, we take the ambient graph to be the random graph with triangles (RGT) obtained by adding triangles to G(n, p). We show that the RGT can be efficiently mapped to the corresponding G(n, p), and moreover, that the planted clique (or dense subgraph) is approximately preserved under this mapping. This constitutes the first average-case reduction transforming dependent noise to independent noise. Together with the easier direction of mapping the ambient graph from Erdos-Renyi to RGT, our results yield a strong equivalence between models. In order to prove our results, we develop a new general framework for reasoning about the validity of average-case reductions based on low sensitivity to perturbations.
Search patterns of randomly oriented steps of different lengths have been observed on all scales of the biological world, ranging from microscopic to the ecological, including in protein motors, bacteria, T-cells, hon...
详细信息
ISBN:
(纸本)9783959771405
Search patterns of randomly oriented steps of different lengths have been observed on all scales of the biological world, ranging from microscopic to the ecological, including in protein motors, bacteria, T-cells, honeybees, marine predators, and more, see e.g., [21, 22, 31, 33, 34, 35, 36]. Through different models, it has been demonstrated that adopting a variety in the magnitude of the step lengths can greatly improve the search efficiency. However, the precise connection between the search efficiency and the number of step lengths in the repertoire of the searcher has not been identified. Motivated by biological examples in one-dimensional terrains, a recent paper studied the best cover time on an n-node cycle that can be achieved by a random walk process that uses k step lengths [7]. By tuning the lengths and corresponding probabilities the authors therein showed that the best cover time is roughly n(1+circle minus(1/k)). While this bound is useful for large values of k, it is hardly informative for small k values, which are of interest in biology [2, 4, 25, 30]. In this paper, we provide a tight bound for the cover time of such a walk, for every integer k > 1. Specifically, up to lower order polylogarithmic factors, the cover time is n(1+ 1/2k-1). For k = 2, 3, 4 and 5 the bound is thus n(4/3), n(6/5), n(8/7), and n(10/9), respectively. Informally, our result implies that, as long as the number of step lengths k is not too large, incorporating an additional step length to the repertoire of the process enables to improve the cover time by a polynomial factor, but the extent of the improvement gradually decreases with k.
randomness extractors provide a generic way of converting sources of randomness that are merely unpredictable into almost uniformly random bits. While in general, deterministic randomness extraction is impossible, it ...
详细信息
ISBN:
(纸本)9798350318944
randomness extractors provide a generic way of converting sources of randomness that are merely unpredictable into almost uniformly random bits. While in general, deterministic randomness extraction is impossible, it is possible if the source has some structural constraints. While much of the literature on deterministic extraction has focused on sources with strong independence properties, a natural class where deterministic extraction is possible is sources that can sampled by a polynomial size circuit, Levin [SIAM J Comp'86]. Trevisan and Vadhan [FOCS'00] explicitly constructed deterministic randomness extractors for this class of sources, assuming very strong circuit lower bounds. We suggest that there is perhaps an even more reasonable model of natural sources of randomness than Levin's: sources sampled by polynomial size quantum circuits. Under a suitable circuit lower bound, we show that Trevisan and Vadhan's extractor indeed works for this class. Along the way, we substantially improve their analysis in the classical case, showing that a circuit lower bound against NP-circuits suffice in the classical case (as opposed to a lower bounds on Sigma(5)-circuits, as shown by Trevisan and Vadhan). Moreover, we show that under this assumption, it is possible to handle sources sampled by postselecting circuits (a variant of nondeterministic circuits). We show that this model is sufficient to capture randomness extraction in the presence of efficiently computable leakage.
In this work we ask the following basic question: assume the vertices of an expander graph are labelled by 0, 1. What "test" functions f : {0, 1}(t) -> {0, 1} cannot distinguish t independent samples from...
详细信息
ISBN:
(纸本)9781450380539
In this work we ask the following basic question: assume the vertices of an expander graph are labelled by 0, 1. What "test" functions f : {0, 1}(t) -> {0, 1} cannot distinguish t independent samples from those obtained by a random walk? The expander hitting property due to Ajtai, Komlos and Szemeredi (STOC 1987) is captured by the AND test function, whereas the fundamental expander Chernoff bound due to Gillman (SICOMP 1998), Heally (Computational Complexity 2008) is about test functions indicating whether the weight is close to the mean. In fact, it is known that all threshold functions are fooled by a random walk (Kipnis and Varadhan, Communications in Mathematical Physics 1986). Recently, it was shown that even the highly sensitive PARITY function is fooled by a random walk Ta-Shma (STOC 2017). We focus on balanced labels. Our first main result is proving that all symmetric functions are fooled by a random walk. Put differently, we prove a central limit theorem (CLT) for expander random walks with respect to the total variation distance, significantly strengthening the classic CLT for Markov Chains that is established with respect to the Kolmogorov distance (Kipnis and Varadhan, Communications in Mathematical Physics 1986). Our approach significantly deviates from prior works. We first study how well a Fourier character xs is fooled by a random walk as a function of S. Then, given a test function f, we expand f in the Fourier basis and combine the above with known results on the Fourier spectrum of f. We also proceed further and consider general test functions not necessarily symmetric. As our approach is Fourier analytic, it is general enough to analyze such versatile test functions. For our second result, we prove that random walks on sufficiently good expander graphs fool tests functions computed by AC(0) circuits, read-once branching programs, and functions with bounded query complexity.
We design efficient algorithms for computational problems over strings in several models where the algorithms have limited access to the input. These models, and algorithms developed respecting these constraints, are ...
详细信息
We design efficient algorithms for computational problems over strings in several models where the algorithms have limited access to the input. These models, and algorithms developed respecting these constraints, are becoming increasingly relevant due to the rapidly increasing size of datasets in myriad applications. Our first problem of interest is trace reconstruction. This is an important problem in learning theory and coding theory, and has applications in computational biology. In this problem, the goal is to recover an unknown string given independent samples (traces) of it generated via a probabilistic noise process called the deletion channel. We give state-of-the-art algorithms for this problem in several settings. Then we consider the problem of estimating the longest increasing subsequence (LIS) of a given string in sublinear time, given query access to the string. While the LIS of a string can be computed exactly in near-linear time, the optimal complexity of approximating the LIS length, especially when the LIS is much less than the string length, is still open. We significantly improve upon prior work in terms of both approximation and time complexity in this regime. The runtime of our algorithm essentially matches the trivial query complexity lower bound as a function of the length of the LIS. Finally, we consider the problem of local decoding, or random access, on compressed strings. The Burrows-Wheeler Transform (BWT) is an important preprocessing step in lossless text compression that rearranges a string into runs of identical characters (by exploiting context regularities), resulting in highly compressible strings. However, the decoding process of the BWT is inherently sequential, and prevents fast random access to the original string. We design a succinct data structure for locally decoding short substrings (and answering several other queries) of a given string under its compressed BWT efficiently.
暂无评论