51,737 research outputs found
Linear-in- Lower Bounds in the LOCAL Model
By prior work, there is a distributed algorithm that finds a maximal
fractional matching (maximal edge packing) in rounds, where
is the maximum degree of the graph. We show that this is optimal:
there is no distributed algorithm that finds a maximal fractional matching in
rounds.
Our work gives the first linear-in- lower bound for a natural graph
problem in the standard model of distributed computing---prior lower bounds for
a wide range of graph problems have been at best logarithmic in .Comment: 1 + 21 pages, 10 figure
Federated Linear Contextual Bandits with User-level Differential Privacy
This paper studies federated linear contextual bandits under the notion of
user-level differential privacy (DP). We first introduce a unified federated
bandits framework that can accommodate various definitions of DP in the
sequential decision-making setting. We then formally introduce user-level
central DP (CDP) and local DP (LDP) in the federated bandits framework, and
investigate the fundamental trade-offs between the learning regrets and the
corresponding DP guarantees in a federated linear contextual bandits model. For
CDP, we propose a federated algorithm termed as \robin and show that it is
near-optimal in terms of the number of clients and the privacy budget
by deriving nearly-matching upper and lower regret bounds when
user-level DP is satisfied. For LDP, we obtain several lower bounds, indicating
that learning under user-level -LDP must suffer a regret
blow-up factor at least { or
} under different conditions.Comment: Accepted by ICML 202
Constraining primordial non-Gaussianity with cosmological weak lensing: shear and flexion
We examine the cosmological constraining power of future large-scale weak
lensing surveys on the model of \emph{Euclid}, with particular reference to
primordial non-Gaussianity. Our analysis considers several different estimators
of the projected matter power spectrum, based on both shear and flexion, for
which we review the covariances and Fisher matrices. The bounds provided by
cosmic shear alone for the local bispectrum shape, marginalized over
, are at the level of . We consider
three additional bispectrum shapes, for which the cosmic shear constraints
range from (equilateral shape) up to (orthogonal shape). The competitiveness of cosmic
flexion constraints against cosmic shear ones depends on the galaxy intrinsic
flexion noise, that is still virtually unconstrained. Adopting the very high
value that has been occasionally used in the literature results in the flexion
contribution being basically negligible with respect to the shear one, and for
realistic configurations the former does not improve significantly the
constraining power of the latter. Since the flexion noise decreases with
decreasing scale, by extending the analysis up to
cosmic flexion, while being still subdominant, improves the shear constraints
by when added. However on such small scales the highly non-linear
clustering of matter and the impact of baryonic physics make any error
estimation uncertain. By considering lower, and possibly more realistic, values
of the flexion intrinsic shape noise results in flexion constraining power
being a factor of better than that of shear, and the bounds on
and being improved by a factor of upon
their combination. (abridged)Comment: 30 pages, 4 figures, 4 tables. To appear on JCA
Robust pricing and hedging of double no-touch options
Double no-touch options, contracts which pay out a fixed amount provided an
underlying asset remains within a given interval, are commonly traded,
particularly in FX markets. In this work, we establish model-free bounds on the
price of these options based on the prices of more liquidly traded options
(call and digital call options). Key steps are the construction of super- and
sub-hedging strategies to establish the bounds, and the use of Skorokhod
embedding techniques to show the bounds are the best possible.
In addition to establishing rigorous bounds, we consider carefully what is
meant by arbitrage in settings where there is no {\it a priori} known
probability measure. We discuss two natural extensions of the notion of
arbitrage, weak arbitrage and weak free lunch with vanishing risk, which are
needed to establish equivalence between the lack of arbitrage and the existence
of a market model.Comment: 32 pages, 5 figure
Optimal Power Flow with Step-Voltage Regulators in Multi-Phase Distribution Networks
This paper develops a branch-flow based optimal power flow (OPF) problem for
multi-phase distribution networks that allows for tap selection of wye,
closed-delta, and open-delta step-voltage regulators (SVRs). SVRs are assumed
ideal and their taps are represented by continuous decision variables. To
tackle the non-linearity, the branch-flow semidefinite programming framework of
traditional OPF is expanded to accommodate SVR edges. Three types of
non-convexity are addressed: (a) rank-1 constraints on non-SVR edges, (b)
nonlinear equality constraints on SVR power flows and taps, and (c) trilinear
equalities on SVR voltages and taps. Leveraging a practical phase-separation
assumption on the SVR secondary voltage, novel McCormick relaxations are
provided for (c) and certain rank-1 constraints of (a), while dropping the
rest. A linear relaxation based on conservation of power is used in place of
(b). Numerical simulations on standard distribution test feeders corroborate
the merits of the proposed convex formulation.Comment: This manuscript has been submitted to IEEE Transactions on Power
System
Whitham Averaged Equations and Modulational Stability of Periodic Traveling Waves of a Hyperbolic-Parabolic Balance Law
In this note, we report on recent findings concerning the spectral and
nonlinear stability of periodic traveling wave solutions of
hyperbolic-parabolic systems of balance laws, as applied to the St. Venant
equations of shallow water flow down an incline. We begin by introducing a
natural set of spectral stability assumptions, motivated by considerations from
the Whitham averaged equations, and outline the recent proof yielding nonlinear
stability under these conditions. We then turn to an analytical and numerical
investigation of the verification of these spectral stability assumptions.
While spectral instability is shown analytically to hold in both the Hopf and
homoclinic limits, our numerical studies indicates spectrally stable periodic
solutions of intermediate period. A mechanism for this moderate-amplitude
stabilization is proposed in terms of numerically observed "metastability" of
the the limiting homoclinic orbits.Comment: 27 pages, 5 figures. Minor changes throughou
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