1,353 research outputs found
Numerical Verification of Affine Systems with up to a Billion Dimensions
Affine systems reachability is the basis of many verification methods. With
further computation, methods exist to reason about richer models with inputs,
nonlinear differential equations, and hybrid dynamics. As such, the scalability
of affine systems verification is a prerequisite to scalable analysis for more
complex systems. In this paper, we improve the scalability of affine systems
verification, in terms of the number of dimensions (variables) in the system.
The reachable states of affine systems can be written in terms of the matrix
exponential, and safety checking can be performed at specific time steps with
linear programming. Unfortunately, for large systems with many state variables,
this direct approach requires an intractable amount of memory while using an
intractable amount of computation time. We overcome these challenges by
combining several methods that leverage common problem structure. Memory is
reduced by exploiting initial states that are not full-dimensional and safety
properties (outputs) over a few linear projections of the state variables.
Computation time is saved by using numerical simulations to compute only
projections of the matrix exponential relevant for the verification problem.
Since large systems often have sparse dynamics, we use Krylov-subspace
simulation approaches based on the Arnoldi or Lanczos iterations. Our method
produces accurate counter-examples when properties are violated and, in the
extreme case with sufficient problem structure, can analyze a system with one
billion real-valued state variables
Energy-momentum diffusion from spacetime discreteness
We study potentially observable consequences of spatiotemporal discreteness
for the motion of massive and massless particles. First we describe some simple
intrinsic models for the motion of a massive point particle in a fixed causal
set background. At large scales, the microscopic swerves induced by the
underlying atomicity manifest themselves as a Lorentz invariant diffusion in
energy-momentum governed by a single phenomenological parameter, and we derive
in full the corresponding diffusion equation. Inspired by the simplicity of the
result, we then derive the most general Lorentz invariant diffusion equation
for a massless particle, which turns out to contain two phenomenological
parameters describing, respectively, diffusion and drift in the particle's
energy. The particles do not leave the light cone however: their worldlines
continue to be null geodesics. Finally, we deduce bounds on the drift and
diffusion constants for photons from the blackbody nature of the spectrum of
the cosmic microwave background radiation.Comment: 13 pages, 4 figures, corrected minor typos and updated to match
published versio
Deep Self-Taught Learning for Handwritten Character Recognition
Recent theoretical and empirical work in statistical machine learning has
demonstrated the importance of learning algorithms for deep architectures,
i.e., function classes obtained by composing multiple non-linear
transformations. Self-taught learning (exploiting unlabeled examples or
examples from other distributions) has already been applied to deep learners,
but mostly to show the advantage of unlabeled examples. Here we explore the
advantage brought by {\em out-of-distribution examples}. For this purpose we
developed a powerful generator of stochastic variations and noise processes for
character images, including not only affine transformations but also slant,
local elastic deformations, changes in thickness, background images, grey level
changes, contrast, occlusion, and various types of noise. The
out-of-distribution examples are obtained from these highly distorted images or
by including examples of object classes different from those in the target test
set. We show that {\em deep learners benefit more from out-of-distribution
examples than a corresponding shallow learner}, at least in the area of
handwritten character recognition. In fact, we show that they beat previously
published results and reach human-level performance on both handwritten digit
classification and 62-class handwritten character recognition
Fully-Automated Verification of Linear Systems Using Inner- and Outer-Approximations of Reachable Sets
Reachability analysis is a formal method to guarantee safety of dynamical
systems under the influence of uncertainties. A major bottleneck of all
reachability algorithms is the requirement to adequately tune certain algorithm
parameters such as the time step size, which requires expert knowledge. In this
work, we solve this issue with a fully-automated reachability algorithm that
tunes all algorithm parameters internally such that the reachable set enclosure
satisfies a user-defined accuracy in terms of distance to the exact reachable
set. Knowing the distance to the exact reachable set, an inner-approximation of
the reachable set can be efficiently extracted from the outer-approximation
using the Minkowski difference. Finally, we propose a novel verification
algorithm that automatically refines the accuracy of the outer- and
inner-approximation until specifications given by time-varying safe and unsafe
sets can either be verified or falsified. The numerical evaluation demonstrates
that our verification algorithm successfully verifies or falsifies benchmarks
from different domains without any requirement for manual tuning.Comment: 16 page
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