7,279 research outputs found
Partially Schur-constant models
In this paper, we introduce a new multivariate dependence model that generalizes the standard Schur-constant model. The difference is that the random vector considered is partially exchangeable, instead of exchangeable, whence the term partially Schur-constant. Its advantage is to allow some heterogeneity of marginal distributions and a more flexible dependence structure, which broadens the scope of potential applications. We first show that the associated joint survival function is a monotonic multivariate function. Next, we derive two distributional representations that provide an intuitive understanding of the underlying dependence. Several other properties are obtained, including correlations within and between subvectors. As an illustration, we explain how such a model could be applied to risk management for insurance networks
An efficient null space inexact Newton method for hydraulic simulation of water distribution networks
Null space Newton algorithms are efficient in solving the nonlinear equations
arising in hydraulic analysis of water distribution networks. In this article,
we propose and evaluate an inexact Newton method that relies on partial updates
of the network pipes' frictional headloss computations to solve the linear
systems more efficiently and with numerical reliability. The update set
parameters are studied to propose appropriate values. Different null space
basis generation schemes are analysed to choose methods for sparse and
well-conditioned null space bases resulting in a smaller update set. The Newton
steps are computed in the null space by solving sparse, symmetric positive
definite systems with sparse Cholesky factorizations. By using the constant
structure of the null space system matrices, a single symbolic factorization in
the Cholesky decomposition is used multiple times, reducing the computational
cost of linear solves. The algorithms and analyses are validated using medium
to large-scale water network models.Comment: 15 pages, 9 figures, Preprint extension of Abraham and Stoianov, 2015
(https://dx.doi.org/10.1061/(ASCE)HY.1943-7900.0001089), September 2015.
Includes extended exposition, additional case studies and new simulations and
analysi
Nonnormal amplification in random balanced neuronal networks
In dynamical models of cortical networks, the recurrent connectivity can
amplify the input given to the network in two distinct ways. One is induced by
the presence of near-critical eigenvalues in the connectivity matrix W,
producing large but slow activity fluctuations along the corresponding
eigenvectors (dynamical slowing). The other relies on W being nonnormal, which
allows the network activity to make large but fast excursions along specific
directions. Here we investigate the tradeoff between nonnormal amplification
and dynamical slowing in the spontaneous activity of large random neuronal
networks composed of excitatory and inhibitory neurons. We use a Schur
decomposition of W to separate the two amplification mechanisms. Assuming
linear stochastic dynamics, we derive an exact expression for the expected
amount of purely nonnormal amplification. We find that amplification is very
limited if dynamical slowing must be kept weak. We conclude that, to achieve
strong transient amplification with little slowing, the connectivity must be
structured. We show that unidirectional connections between neurons of the same
type together with reciprocal connections between neurons of different types,
allow for amplification already in the fast dynamical regime. Finally, our
results also shed light on the differences between balanced networks in which
inhibition exactly cancels excitation, and those where inhibition dominates.Comment: 13 pages, 7 figure
Facial behaviour of analytic functions on the bidisk
We prove that if is an analytic function bounded by 1 on the bidisk
and is a point in a face of the bidisk at which satisfies
Caratheodory's condition then both and the angular gradient
exist and are constant on the face. Moreover, the class of all with
prescribed and can be parametrized in terms of
a function in the two-variable Pick class. As an application we solve an
interpolation problem with nodes that lie on faces of the bidisk.Comment: 18 pages. We have replaced an erroneous proof of Theorem 5.4(1) by a
valid proo
Polytopes from Subgraph Statistics
Polytopes from subgraph statistics are important in applications and
conjectures and theorems in extremal graph theory can be stated as properties
of them. We have studied them with a view towards applications by inscribing
large explicit polytopes and semi-algebraic sets when the facet descriptions
are intractable. The semi-algebraic sets called curvy zonotopes are introduced
and studied using graph limits. From both volume calculations and algebraic
descriptions we find several interesting conjectures.Comment: Full article, 21 pages, 8 figures. Minor expository update
A Caratheodory theorem for the bidisk via Hilbert space methods
If \ph is an analytic function bounded by 1 on the bidisk \D^2 and
\tau\in\tb is a point at which \ph has an angular gradient
\nabla\ph(\tau) then \nabla\ph(\la) \to \nabla\ph(\tau) as \la\to\tau
nontangentially in \D^2. This is an analog for the bidisk of a classical
theorem of Carath\'eodory for the disk.
For \ph as above, if \tau\in\tb is such that the of
(1-|\ph(\la)|)/(1-\|\la\|) as \la\to\tau is finite then the directional
derivative D_{-\de}\ph(\tau) exists for all appropriate directions
\de\in\C^2. Moreover, one can associate with \ph and an analytic
function in the Pick class such that the value of the directional
derivative can be expressed in terms of
Variational Theory and Domain Decomposition for Nonlocal Problems
In this article we present the first results on domain decomposition methods
for nonlocal operators. We present a nonlocal variational formulation for these
operators and establish the well-posedness of associated boundary value
problems, proving a nonlocal Poincar\'{e} inequality. To determine the
conditioning of the discretized operator, we prove a spectral equivalence which
leads to a mesh size independent upper bound for the condition number of the
stiffness matrix. We then introduce a nonlocal two-domain variational
formulation utilizing nonlocal transmission conditions, and prove equivalence
with the single-domain formulation. A nonlocal Schur complement is introduced.
We establish condition number bounds for the nonlocal stiffness and Schur
complement matrices. Supporting numerical experiments demonstrating the
conditioning of the nonlocal one- and two-domain problems are presented.Comment: Updated the technical part. In press in Applied Mathematics and
Computatio
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