1,908 research outputs found
Robust risk aggregation with neural networks
We consider settings in which the distribution of a multivariate random
variable is partly ambiguous. We assume the ambiguity lies on the level of the
dependence structure, and that the marginal distributions are known.
Furthermore, a current best guess for the distribution, called reference
measure, is available. We work with the set of distributions that are both
close to the given reference measure in a transportation distance (e.g. the
Wasserstein distance), and additionally have the correct marginal structure.
The goal is to find upper and lower bounds for integrals of interest with
respect to distributions in this set. The described problem appears naturally
in the context of risk aggregation. When aggregating different risks, the
marginal distributions of these risks are known and the task is to quantify
their joint effect on a given system. This is typically done by applying a
meaningful risk measure to the sum of the individual risks. For this purpose,
the stochastic interdependencies between the risks need to be specified. In
practice the models of this dependence structure are however subject to
relatively high model ambiguity. The contribution of this paper is twofold:
Firstly, we derive a dual representation of the considered problem and prove
that strong duality holds. Secondly, we propose a generally applicable and
computationally feasible method, which relies on neural networks, in order to
numerically solve the derived dual problem. The latter method is tested on a
number of toy examples, before it is finally applied to perform robust risk
aggregation in a real world instance.Comment: Revised version. Accepted for publication in "Mathematical Finance
Quantitative Stability of Regularized Optimal Transport and Convergence of Sinkhorn's Algorithm
We study the stability of entropically regularized optimal transport with
respect to the marginals. Lipschitz continuity of the value and H\"older
continuity of the optimal coupling in -Wasserstein distance are obtained
under general conditions including quadratic costs and unbounded marginals. The
results for the value extend to regularization by an arbitrary divergence. As
an application, we show convergence of Sinkhorn's algorithm in Wasserstein
sense, including for quadratic cost. Two techniques are presented: The first
compares an optimal coupling with its so-called shadow, a coupling induced on
other marginals by an explicit construction. The second transforms one set of
marginals by a change of coordinates and thus reduces the comparison of
differing marginals to the comparison of differing cost functions under the
same marginals.Comment: Forthcoming in 'SIAM Journal on Mathematical Analysis
Estimating the Rate-Distortion Function by Wasserstein Gradient Descent
In the theory of lossy compression, the rate-distortion (R-D) function
describes how much a data source can be compressed (in bit-rate) at any given
level of fidelity (distortion). Obtaining for a given data source
establishes the fundamental performance limit for all compression algorithms.
We propose a new method to estimate from the perspective of optimal
transport. Unlike the classic Blahut--Arimoto algorithm which fixes the support
of the reproduction distribution in advance, our Wasserstein gradient descent
algorithm learns the support of the optimal reproduction distribution by moving
particles. We prove its local convergence and analyze the sample complexity of
our R-D estimator based on a connection to entropic optimal transport.
Experimentally, we obtain comparable or tighter bounds than state-of-the-art
neural network methods on low-rate sources while requiring considerably less
tuning and computation effort. We also highlight a connection to
maximum-likelihood deconvolution and introduce a new class of sources that can
be used as test cases with known solutions to the R-D problem.Comment: Accepted as conference paper at NeurIPS 202
Robust pricing and hedging of options on multiple assets and its numerics
We consider robust pricing and hedging for options written on multiple assets
given market option prices for the individual assets. The resulting problem is
called the multi-marginal martingale optimal transport problem. We propose two
numerical methods to solve such problems: using discretisation and linear
programming applied to the primal side and using penalisation and deep neural
networks optimisation applied to the dual side. We prove convergence for our
methods and compare their numerical performance. We show how adding further
information about call option prices at additional maturities can be
incorporated and narrows down the no-arbitrage pricing bounds. Finally, we
obtain structural results for the case of the payoff given by a weighted sum of
covariances between the assets.Comment: Forthcoming in SIAM Journal on Financial Mathematic
Real-time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T
Purpose To explore the impact of temporal motion-induced coil sensitivity changes on CEST-MRI at 7T and its correction using interleaved volumetric EPI navigators, which are applied for real-time motion correction. Methods Five healthy volunteers were scanned via CEST. A 4-fold correction pipeline allowed the mitigation of (1) motion, (2) motion-induced coil sensitivity variations, Delta B1-, (3) motion-induced static magnetic field inhomogeneities, Delta B-0, and (4) spatially varying transmit RF field fluctuations, Delta B1+. Four CEST measurements were performed per session. For the first 2, motion correction was turned OFF and then ON in absence of voluntary motion, whereas in the other 2 controlled head rotations were performed. During post-processing Delta B1- was removed additionally for the motion-corrected cases, resulting in a total of 6 scenarios to be compared. In all cases, retrospective increment B-0 and -Delta B1+ corrections were performed to compute artifact-free magnetization transfer ratio maps with asymmetric analysis (MTRasym). Results Dynamic Delta B1- correction successfully mitigated signal deviations caused by head motion. In 2 frontal lobe regions of volunteer 4, induced relative signal errors of 10.9% and 3.9% were reduced to 1.1% and 1.0% after correction. In the right frontal lobe, the motion-corrected MTRasym contrast deviated 0.92%, 1.21%, and 2.97% relative to the static case for Delta omega = 1, 2, 3 +/- 0.25 ppm. The additional application of Delta B1- correction reduced these deviations to 0.10%, 0.14%, and 0.42%. The fully corrected MTRasym values were highly consistent between measurements with and without intended head rotations. Conclusion Temporal Delta B1- cause significant CEST quantification bias. The presented correction pipeline including the proposed retrospective Delta B1- correction significantly reduced motion-related artifacts on CEST-MRI.Peer reviewe
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