695 research outputs found
Root-finding Approaches for Computing Conformal Prediction Set
Conformal prediction constructs a confidence set for an unobserved response
of a feature vector based on previous identically distributed and exchangeable
observations of responses and features. It has a coverage guarantee at any
nominal level without additional assumptions on their distribution. Its
computation deplorably requires a refitting procedure for all replacement
candidates of the target response. In regression settings, this corresponds to
an infinite number of model fit. Apart from relatively simple estimators that
can be written as pieces of linear function of the response, efficiently
computing such sets is difficult and is still considered as an open problem. We
exploit the fact that, \emph{often}, conformal prediction sets are intervals
whose boundaries can be efficiently approximated by classical root-finding
algorithm. We investigate how this approach can overcome many limitations of
formerly used strategies and we discuss its complexity and drawbacks
Conformalization of Sparse Generalized Linear Models
Given a sequence of observable variables , the conformal prediction method estimates a confidence set for
given that is valid for any finite sample size by merely
assuming that the joint distribution of the data is permutation invariant.
Although attractive, computing such a set is computationally infeasible in most
regression problems. Indeed, in these cases, the unknown variable can
take an infinite number of possible candidate values, and generating conformal
sets requires retraining a predictive model for each candidate. In this paper,
we focus on a sparse linear model with only a subset of variables for
prediction and use numerical continuation techniques to approximate the
solution path efficiently. The critical property we exploit is that the set of
selected variables is invariant under a small perturbation of the input data.
Therefore, it is sufficient to enumerate and refit the model only at the change
points of the set of active features and smoothly interpolate the rest of the
solution via a Predictor-Corrector mechanism. We show how our path-following
algorithm accurately approximates conformal prediction sets and illustrate its
performance using synthetic and real data examples.Comment: ICML 202
Discretized conformal prediction for efficient distribution-free inference
In regression problems where there is no known true underlying model,
conformal prediction methods enable prediction intervals to be constructed
without any assumptions on the distribution of the underlying data, except that
the training and test data are assumed to be exchangeable. However, these
methods bear a heavy computational cost-and, to be carried out exactly, the
regression algorithm would need to be fitted infinitely many times. In
practice, the conformal prediction method is run by simply considering only a
finite grid of finely spaced values for the response variable. This paper
develops discretized conformal prediction algorithms that are guaranteed to
cover the target value with the desired probability, and that offer a tradeoff
between computational cost and prediction accuracy
New Dimensions for Wound Strings: The Modular Transformation of Geometry to Topology
We show, using a theorem of Milnor and Margulis, that string theory on
compact negatively curved spaces grows new effective dimensions as the space
shrinks, generalizing and contextualizing the results in hep-th/0510044.
Milnor's theorem relates negative sectional curvature on a compact Riemannian
manifold to exponential growth of its fundamental group, which translates in
string theory to a higher effective central charge arising from winding
strings. This exponential density of winding modes is related by modular
invariance to the infrared small perturbation spectrum. Using self-consistent
approximations valid at large radius, we analyze this correspondence explicitly
in a broad set of time-dependent solutions, finding precise agreement between
the effective central charge and the corresponding infrared small perturbation
spectrum. This indicates a basic relation between geometry, topology, and
dimensionality in string theory.Comment: 28 pages, harvmac big. v2: references and KITP preprint number added,
minor change
Defect Formation and Critical Dynamics in the Early Universe
We study the nonequilibrium dynamics leading to the formation of topological
defects in a symmetry-breaking phase transition of a quantum scalar field with
\lambda\Phi^4 self-interaction in a spatially flat, radiation-dominated
Friedmann-Robertson-Walker Universe. The quantum field is initially in a
finite-temperature symmetry-restored state and the phase transition develops as
the Universe expands and cools. We present a first-principles, microscopic
approach in which the nonperturbative, nonequilibrium dynamics of the quantum
field is derived from the two-loop, two-particle-irreducible closed-time-path
effective action. We numerically solve the dynamical equations for the
two-point function and we identify signatures of topological defects in the
infrared portion of the momentum-space power spectrum. We find that the density
of topological defects formed after the phase transition scales as a power law
with the expansion rate of the Universe. We calculate the equilibrium critical
exponents of the correlation length and relaxation time for this model and show
that the power law exponent of the defect density, for both overdamped and
underdamped evolution, is in good agreement with the "freeze-out" scenario of
Zurek. We introduce an analytic dynamical model, valid near the critical point,
that exhibits the same power law scaling of the defect density with the quench
rate. By incorporating the realistic quench of the expanding Universe, our
approach illuminates the dynamical mechanisms important for topological defect
formation. The observed power law scaling of the defect density with the quench
rate, observered here in a quantum field theory context, provides evidence for
the "freeze-out" scenario in three spatial dimensions.Comment: 31 pages, RevTex, 8 figures in EPS forma
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