489 research outputs found
Revisiting the Nystrom Method for Improved Large-Scale Machine Learning
We reconsider randomized algorithms for the low-rank approximation of
symmetric positive semi-definite (SPSD) matrices such as Laplacian and kernel
matrices that arise in data analysis and machine learning applications. Our
main results consist of an empirical evaluation of the performance quality and
running time of sampling and projection methods on a diverse suite of SPSD
matrices. Our results highlight complementary aspects of sampling versus
projection methods; they characterize the effects of common data preprocessing
steps on the performance of these algorithms; and they point to important
differences between uniform sampling and nonuniform sampling methods based on
leverage scores. In addition, our empirical results illustrate that existing
theory is so weak that it does not provide even a qualitative guide to
practice. Thus, we complement our empirical results with a suite of worst-case
theoretical bounds for both random sampling and random projection methods.
These bounds are qualitatively superior to existing bounds---e.g. improved
additive-error bounds for spectral and Frobenius norm error and relative-error
bounds for trace norm error---and they point to future directions to make these
algorithms useful in even larger-scale machine learning applications.Comment: 60 pages, 15 color figures; updated proof of Frobenius norm bounds,
added comparison to projection-based low-rank approximations, and an analysis
of the power method applied to SPSD sketche
Fixed-Rank Approximation of a Positive-Semidefinite Matrix from Streaming Data
Several important applications, such as streaming PCA and semidefinite
programming, involve a large-scale positive-semidefinite (psd) matrix that is
presented as a sequence of linear updates. Because of storage limitations, it
may only be possible to retain a sketch of the psd matrix. This paper develops
a new algorithm for fixed-rank psd approximation from a sketch. The approach
combines the Nystrom approximation with a novel mechanism for rank truncation.
Theoretical analysis establishes that the proposed method can achieve any
prescribed relative error in the Schatten 1-norm and that it exploits the
spectral decay of the input matrix. Computer experiments show that the proposed
method dominates alternative techniques for fixed-rank psd matrix approximation
across a wide range of examples
Precise expressions for random projections: Low-rank approximation and randomized Newton
It is often desirable to reduce the dimensionality of a large dataset by
projecting it onto a low-dimensional subspace. Matrix sketching has emerged as
a powerful technique for performing such dimensionality reduction very
efficiently. Even though there is an extensive literature on the worst-case
performance of sketching, existing guarantees are typically very different from
what is observed in practice. We exploit recent developments in the spectral
analysis of random matrices to develop novel techniques that provide provably
accurate expressions for the expected value of random projection matrices
obtained via sketching. These expressions can be used to characterize the
performance of dimensionality reduction in a variety of common machine learning
tasks, ranging from low-rank approximation to iterative stochastic
optimization. Our results apply to several popular sketching methods, including
Gaussian and Rademacher sketches, and they enable precise analysis of these
methods in terms of spectral properties of the data. Empirical results show
that the expressions we derive reflect the practical performance of these
sketching methods, down to lower-order effects and even constant factors.Comment: Minor corrections and clarifications of the previous version,
including additional discussion in Appendix A.
Learning with SGD and Random Features
Sketching and stochastic gradient methods are arguably the most common
techniques to derive efficient large scale learning algorithms. In this paper,
we investigate their application in the context of nonparametric statistical
learning. More precisely, we study the estimator defined by stochastic gradient
with mini batches and random features. The latter can be seen as form of
nonlinear sketching and used to define approximate kernel methods. The
considered estimator is not explicitly penalized/constrained and regularization
is implicit. Indeed, our study highlights how different parameters, such as
number of features, iterations, step-size and mini-batch size control the
learning properties of the solutions. We do this by deriving optimal finite
sample bounds, under standard assumptions. The obtained results are
corroborated and illustrated by numerical experiments
Aquatic macroinvertebrate responses to native and non-native predators
Non-native species can profoundly affect native ecosystems through trophic interactions with native species. Native prey may respond differently to non-native versus native predators since they lack prior experience. Here we investigate antipredator responses of two common freshwater macroinvertebrates, Gammarus pulex and Potamopyrgus jenkinsi, to olfactory cues from three predators; sympatric native fish (Gasterosteus aculeatus), sympatric native crayfish (Austropotamobius pallipes), and novel invasive crayfish (Pacifastacus leniusculus). G. pulex responded differently to fish and crayfish; showing enhanced locomotion in response to fish, but a preference for the dark over the light in response to the crayfish. P. jenkinsi showed increased vertical migration in response to all three predator cues relative to controls. These different responses to fish and crayfish are hypothesised to reflect the predators’ differing predation types; benthic for crayfish and pelagic for fish. However, we found no difference in response to native versus invasive crayfish, indicating that prey naiveté is unlikely to drive the impacts of invasive crayfish. The Predator Recognition Continuum Hypothesis proposes that benefits of generalisable predator recognition outweigh costs when predators are diverse. Generalised responses of prey as observed here will be adaptive in the presence of an invader, and may reduce novel predators’ potential impacts
Researching intercultural participatory design
What impact does culture have on tools and techniques that are used to\ud
facilitate cooperation amongst stakeholders in Information Communication\ud
Technology (ICT) design projects? This is a question facing the ICT development\ud
activities at the World Maritime University in Malmö, Sweden. At the university\ud
around 300 staff and students from 90 different countries come together every\ud
year. Continuously finding ways to improve how they can actively participate in\ud
design activities of useful and usable ICT support to benefit their everyday work\ud
is a prioritized area. This short paper presents a case that illustrates the\ud
intertwined and negotiated characteristics of culture when working with tools and\ud
techniques for cooperation in a student ICT design project. Using the case, an\ud
ethnographically based research cycle is explored to make sense of and ultimately\ud
further improve the interactions between the actors in an intercultural application\ud
domain
Streaming Tensor Train Approximation
Tensor trains are a versatile tool to compress and work with high-dimensional
data and functions. In this work we introduce the Streaming Tensor Train
Approximation (STTA), a new class of algorithms for approximating a given
tensor in the tensor train format. STTA accesses
exclusively via two-sided random sketches of the original data, making it
streamable and easy to implement in parallel -- unlike existing deterministic
and randomized tensor train approximations. This property also allows STTA to
conveniently leverage structure in , such as sparsity and various
low-rank tensor formats, as well as linear combinations thereof. When Gaussian
random matrices are used for sketching, STTA is admissible to an analysis that
builds and extends upon existing results on the generalized Nystr\"om
approximation for matrices. Our results show that STTA can be expected to
attain a nearly optimal approximation error if the sizes of the sketches are
suitably chosen. A range of numerical experiments illustrates the performance
of STTA compared to existing deterministic and randomized approaches.Comment: 21 pages, code available at https://github.com/RikVoorhaar/tt-sketc
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