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17 results on '"Winn, Andrew"'

1. Revisiting a Cutting Plane Method for Perfect Matchings

Author

Chen, Amber Q, Cheung, Kevin K. H., Kielstra, P. Michael, and Winn, Andrew

Subjects
Mathematics - Combinatorics, Computer Science - Discrete Mathematics, 90C10, 68Q25, 68R10, 90C35
Abstract

In 2016, Chandrasekaran, V\'egh, and Vempala published a method to solve the minimum-cost perfect matching problem on an arbitrary graph by solving a strictly polynomial number of linear programs. However, their method requires a strong uniqueness condition, which they imposed by using perturbations of the form $c(i)=c_0(i)+2^{-i}$. On large graphs (roughly $m>100$), these perturbations lead to cost values that exceed the precision of floating-point formats used by typical linear programming solvers for numerical calculations. We demonstrate, by a sequence of counterexamples, that perturbations are required for the algorithm to work, motivating our formulation of a general method that arrives at the same solution to the problem as Chandrasekaran et al. but overcomes the limitations described above by solving multiple linear programs without using perturbations. We then give an explicit algorithm that exploits are method, and show that this new algorithm still runs in strongly polynomial time., Comment: 19 pages

Published
2019

13. Safety controller synthesis using human generated trajectories: Nonlinear dynamics with feedback linearization and differential flatness.

Author

Julius, A. Agung and Winn, Andrew K.

Abstract

The aim of the safety controller synthesis problem is to synthesize a feedback controller that results in closed-loop trajectories that meet certain criteria, namely, the state or output trajectories terminate in a Goal set without entering an Unsafe set. We propose a formal method for synthesizing such a controller using finitely many human generated trajectories. The main theoretical idea behind our results is the concept of trajectory robustness, which is established using the theory of approximate bisimulation. Approximate bisimulation has been used to establish robustness (in the L∞ sense) of execution trajectories of dynamical systems and hybrid systems, resulting in trajectory-based safety verification procedures. The work reported in this paper builds on our earlier work where the dynamics of the system is assumed to be affine linear. We extend the existing results to special classes of nonlinear dynamical systems, feedback linearizable and differentially flat systems. For both cases, we present some examples where it is possible to synthesize the controller using human generated trajectories, which are obtained through interactive computer programs with graphical interface (computer games). [ABSTRACT FROM PUBLISHER]

Published
2012
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16. Structure--Function Analysis of Friedreich's Ataxia Mutants Reveals Determinants of Frataxin Binding and Activation of the Fe-S Assembly Complex.

Author

Bridwcll-Rabb, Jennifer, Winn, Andrew M., and Barondeau, David P.

Subjects
*FRIEDREICH'S ataxia, *NEURODEGENERATION, *FRATAXIN, *GENETIC carriers, *CYSTEINE desulfurase
Abstract

Friedreich's ataxia (FRDA) is a progressive neurodegenerative disease associated with the loss of function of the protein frataxin (FXN) that results from low FXN levels due to a GAA triplet repeat expansion or, occasionally, from missense mutations in the FXN gene. Here biochemical and structural properties of FXN variants, including three FRDA missense mutations (N146K, Q148R, and R165C) and three related mutants (N146A, Q148G, and Q153A), were determined in an effort to understand the structural basis for the loss of function. In vitro assays revealed that although the three FRDA missense mutations exhibited similar losses of cysteine desulfurase and Fe-S cluster assembly activities, the causes for these activation defects were distinct. The R165C variant exhibited a kcat/KM higher than that of native FXN but weak binding to the NFS1, ISD11, and ISCU2 (SDU) complex, whereas the Q148R variant exhibited the lowest kcat/KM of the six tested FXN variants and only a modest binding deficiency. The order of the FXN binding affinities for the SDU Fe-S assembly complex was as follows: FXN > Q148R > N146A > Q148G > N146K > Q153A > R165C. Four different classes of FXN variants were identified on the basis of their biochemical properties. Together, these structure-function studies reveal determinants for the binding and allosteric activation of the Fe-S assembly complex and provide insight into how FRDA missense mutations are functionally compromised. [ABSTRACT FROM AUTHOR]

Published
2011
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17. In Situ Characterization of Interfaces between Liquid Tin--Vanadium Alloys and Alumina by Neutron Reflection Spectroscopy.

Author

Winn, Andrew J., Derby, Brian, Webster, John R.P., and Holt, Stephen

Subjects
*CERAMICS, *LIQUID metals, *TIN alloys, *ALUMINUM oxide, *SURFACE analysis, *INTERFACES (Physical sciences), *REFLECTANCE spectroscopy
Abstract

The composition and structure of interfaces between single crystal Al2O3 (sapphire) samples and liquid Sn-V alloys, containing 1% and 3% V, have been characterized in situ at high temperature using neutron reflection spectroscopy. Measurements made at 900°C are shown to be consistent with a thin (10-25 nm) AlV2O4 layer forming at the solid/liquid interface, with possibly a thinner layer of enhanced V content adjacent to the liquid, to promote wetting. After longer time exposure to temperature this interface layer roughens and a more complex interface structure occurs. [ABSTRACT FROM AUTHOR]

Published
2004
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