Search results for "Spurious relationship"

Critical comments on EEG sensor space dynamical connectivity analysis

2019

Many different analysis techniques have been developed and applied to EEG recordings that allow one to investigate how different brain areas interact. One particular class of methods, based on the linear parametric representation of multiple interacting time series, is widely used to study causal connectivity in the brain. However, the results obtained by these methods should be interpreted with great care. The goal of this paper is to show, both theoretically and using simulations, that results obtained by applying causal connectivity measures on the sensor (scalp) time series do not allow interpretation in terms of interacting brain sources. This is because (1) the channel locations canno…

Thouless-Valatin moment of inertia and removal of the spurious mode in the linear response theory

2020

Symmetry breaking at the mean-field level leads to an appearance of a symmetry restoring Nambu-Goldstone (NG) mode in the linear response theory. These modes represent a special kind of collective motion of the system. However, they can interfere with the calculated intrinsic physical excitations and, hence, they are often called as spurious modes. I discuss translational and rotational NG mode and the inertia parameter associated with these modes, by using the finite amplitude method formalism. I will also discuss how to remove spurious mode from the calculated transition strength function.

On distributing quarterly national growth among regions

2008

In many countries a very important fraction of public expenditure is managed by regional authorities. However, in a world where economic life has quickened and become more turbulent, subnational institutions rarely have a timely regional macroeconomic picture at their disposal. The authors propose a guide to a method for estimating quarterly accounts of regions from the national quarterly and annual regional accounts, by the use of a temporal structure which eliminates possible spurious jumps. The robustness of the process and suggested practicalities are tested, and the proposal is also shown to produce better estimates than other uniregional methods often used in this framework.

Surrogate Data Analysis for Assessing the Significance of the Coherence Function

2004

In cardiovascular variability analysis, the significance of the coupling between two time series is commonly assessed by setting a threshold level in the coherence function. While traditionally used statistical tests consider only the parameters of the adopted estimator, the required zero-coherence level may be affected by some features of the observed series. In this study, three procedures, based on the generation of surrogate series sharing given properties with the original but being structurally uncoupled, were considered: independent identically distributed (IID), Fourier transform (FT), and autoregressive (AR). IID surrogates maintained the distribution of the original series, while …

Assessing nonlinear structures in real exchange rates using recurrence plot strategies

2002

Purchasing power parity (PPP) is an important theory at the basis of a large number of economic models. However, the implication derived from the theory that real exchange rates must follow stationary processes is not conclusively supported by empirical studies. In a recent paper, Serletis and Gogas [Appl. Finance Econ. 10 (2000) 615] show evidence of deterministic chaos in several OECD exchange rates. As a consequence, PPP rejections could be spurious. In this work, we follow a two-stage testing procedure to test for nonlinearities and chaos in real exchange rates, using a new set of techniques designed by Webber and Zbilut [J. Appl. Physiol. 76 (1994) 965], called recurrence quantificatio…

A method of magnetic storage of ultra-cold neutrons for a precise measurement of the neutron lifetime

1999

The beta-decay lifetime of the free neutron has been determined most precisely using storage of ultra-cold neutrons (UCNs) in material bottles. The quantity measured is the storage time which is substantially smaller than due to spurious losses of UCNs at collisions with the bottle walls. The systematic uncertainty of the correction - to the true lifetime is presently the main obstacle towards higher precision in this measurement. In the alternative magnetic trapping of UCNs, storage conditions can be realized where this correction becomes vanishingly small. This paper gives a method to measure the neutron lifetime with very low systematic errors, using confinement of UCNs in vacuum by magn…

Spurious Singularities in Quasipotential Amplitudes

1973

Modeling electron dynamics coupled to continuum states in finite volumes with absorbing boundaries

2015

arXiv:1409.1689v1

Linear-response theory for Mukherjee's multireference coupled-cluster method: Static and dynamic polarizabilities

2012

The formalism of response theory is applied to derive expressions for static and dynamic polarizabilities within the state-specific multireference coupled-cluster theory suggested by Mukherjee and co-workers (Mk-MRCC) [J. Chem. Phys. 110, 6171 (1998)]. We show that the redundancy problem inherent to Mk-MRCC theory gives rise to spurious poles in the Mk-MRCC response functions, which hampers the reliable calculation of dynamic polarizabilities. Furthermore, we demonstrate that in the case of a symmetry-breaking perturbation a working response theory is obtained only if certain internal excitations are included in the responses of the cluster amplitudes. Exemplary calculations within the sing…

Excited states with internally contracted multireference coupled-cluster linear response theory.

2014

In this paper, the linear response (LR) theory for the variant of internally contracted multireference coupled cluster (ic-MRCC) theory described by Hanauer and Kohn [J. Chem. Phys. 134, 204211 (2011)] has been formulated and implemented for the computation of the excitation energies relative to a ground state of pronounced multireference character. We find that straightforward application of the linear-response formalism to the time-averaged ic-MRCC Lagrangian leads to unphysical second-order poles. However, the coupling matrix elements that cause this behavior are shown to be negligible whenever the internally contracted approximation as such is justified. Hence, for the numerical impleme…