Natural preconditioners for saddle point systems

The solution of quadratic or locally quadratic extremum problems subject to linear(ized) constraints gives rise to linear systems in saddle point form. This is true whether in the continuous or discrete setting, so saddle point systems arising from discretization of partial differential equation problems such as those describing electromagnetic problems or incompressible flow lead to equations with this structure as does, for example, the widely used sequential quadratic programming approach to nonlinear optimization.\ud This article concerns iterative solution methods for these problems and in particular shows how the problem formulation leads to natural preconditioners which guarantee rapid convergence of the relevant iterative methods. These preconditioners are related to the original extremum problem and their effectiveness -- in terms of rapidity of convergence -- is established here via a proof of general bounds on the eigenvalues of the preconditioned saddle point matrix on which iteration convergence depends

The antitriangular factorisation of saddle point matrices

Mastronardi and Van Dooren recently introduced the block antitriangular ("Batman") decomposition for symmetric indefinite matrices. Here we show the simplification of this factorisation for saddle point matrices and demonstrate how it represents the common nullspace method. We show the relation of this factorisation to constraint preconditioning and how it transforms but preserves the block diagonal structure of block diagonal preconditioning

On choice of preconditioner for minimum residual methods for nonsymmetric matrices

Existing convergence bounds for Krylov subspace methods such as GMRES for nonsymmetric linear systems give little mathematical guidance for the choice of preconditioner. Here, we establish a desirable mathematical property of a preconditioner which guarantees that convergence of a minimum residual method will essentially depend only on the eigenvalues of the preconditioned system, as is true in the symmetric case. Our theory covers only a subset of nonsymmetric coefficient matrices but computations indicate that it might be more generally applicable

The Expenditure Composition Hypothesis: Empirical Evidence and Implications for Monetary Policy.

Leão (2005) has recently proposed a new explanation for the short run variability of the velocity of money based on the changes in the composition of the expenditure that occur along the business cycle. This paper presents further empirical evidence in favour of Leão’s Expenditure Composition Hypothesis, and draws new implications of this hypothesis for monetary policy. We use a VAR model to analyze the determinants of the velocity of both M1 and M3 in the USA. The main conclusion is that increases in the weight of investment and durable consumption in total expenditure raise the velocity of both narrow and broad money. This is in line with the Expenditure Composition Hypothesis. Furthermore, we draw a new implication of this hypothesis for monetary policy. The more a central bank’s decisions on the interest rate respond to money growth, the more volatile economic growth will be. In other words, a monetary policy strategy - like that of the ECB – which puts emphasis on money growth is de-stabilizing.Velocity of money; monetary policy; business cycle.

Relationships between nutrient composition of flowers and fruit quality in orange trees grown in calcareous soil

To determine if flower nutrient composition can be used to predict fruit quality, a field experiment was conducted over three seasons (1996-1999) in a commercial orange orchard (Citrus sinensis (L.) Osbeck cv. 'Valencia Late', budded on Troyer citrange rootstock) established on a calcareous soil in southern Portugal. Flowers were collected from 20 trees during full bloom in April and their nutrient composition determined, and fruits were harvested the following March and their quality evaluated. Patterns of covariation in flower nutrient concentrations and in fruit quality variables were evaluated by principal component analysis. Regression models relating fruit quality variables to flower nutrient composition were developed by stepwise selection procedures. The predictive power of the regression models was evaluated with an independent data set. Nutrient composition of flowers at full bloom could be used to predict the fruit quality variables fresh fruit mass and maturation index in the following year. Magnesium, Ca and Zn concentrations measured in flowers were related to fruit fresh mass estimations and N, P, Mg and Fe concentrations were related to fruit maturation index. We also established reference values for the nutrient composition of flowers based on measurements made in trees that produced large (> 76 mm in diameter) fruit.info:eu-repo/semantics/publishedVersio

Senior Tourism: a network science approach over the last twenty years through CiteSpace

Adopting a co-authorship and co-citation network approach, the aim of this paper is to evaluate senior tourism research during the last twenty years and to discover structures in terms of leading papers, journals, authors, countries and institutions. The mapping of bibliometric data use CiteSpace. Data from Scopus build a network of 512 articles. The results reveal a slow increase of research, with the last period including 40.69% of outputs. The most co-cited papers are mainly older, represent 2.73% of the sample and account for 13.24% of citations. The authors with most publications are Jang M and King M, representing 1.95%. The co-cited journals show a core-periphery structure, where Tourism Management is ranked first. The co-authorship network reveals that major collaborative networks are based on geographical and institutional proximity, dominated by the United States. The keyword analysis demonstrates that motivation, attitude, satisfaction, experience, heritage and tourism management are significant areas of emerging research.info:eu-repo/semantics/acceptedVersio

Persistence in Airline Accidents

This paper analyses airline accident data from 1927-2006, through fractional integration. It is shown that airline accidents are persistent and (fractionally) cointegrated with airline traffic. There exists a negative relation between air accidents and airline traffic, with the effect of the shocks to that relationship disappearing in the long run. Policy implications are derived for countering accident events.Accidents; airline; Time series; Persistence; Long memory; Cointegration.

Identification of Segments of European Banks with a Latent Class Frontier Model

This paper analyses technical efficiency of European banks over the period 1996-2003 with unbalanced panel data techniques. A latent class frontier model is used which allows the identification of different segments in the production frontier. We find that there are three statistically significant segments in the sample. Therefore, we conclude that no common banking policy can be effective for all the banks included in the sample, and that banking policies by segments are required instead.European banking, latent class frontier model, technical efficiency

A preconditioned MINRES method for nonsymmetric Toeplitz matrices

Circulant preconditioning for symmetric Toeplitz linear systems is well established; theoretical guarantees of fast convergence for the conjugate gradient method are descriptive of the convergence seen in computations. This has led to robust and highly efficient solvers based on use of the fast Fourier transform exactly as originally envisaged in [G. Strang, Stud. Appl. Math., 74 (1986), pp. 171--176]. For nonsymmetric systems, the lack of generally descriptive convergence theory for most iterative methods of Krylov type has provided a barrier to such a comprehensive guarantee, though several methods have been proposed and some analysis of performance with the normal equations is available. In this paper, by the simple device of reordering, we rigorously establish a circulant preconditioned short recurrence Krylov subspace iterative method of minimum residual type for nonsymmetric (and possibly highly nonnormal) Toeplitz systems. Convergence estimates similar to those in the symmetric case are established

The pinewood nematode, Bursaphelenchus xylophilus, in Madeira Island

The environmental conditions in Madeira Island are favorable for the presence and dissemination of the pinewood nematode (PWN), Bursaphelenchus xylophilus. Five hundred Pinus pinaster wood samples were collected in several forest areas and PWN was detected in 22.8 % of the samples. Bursaphelenchus xylophilus isolates from Madeira Island displayed the species-specific diagnostic characters. A morphological variation in the female tail terminus was detected. In most females, the tail presented a broadly rounded terminus and, occasionally, a digitate terminus with a terminal nipple-like extension resembling a mucro. PCR ITS-RFLP analysis revealed that Madeira Island isolates exhibited patterns specific to the species B. xylophilus and similar to virulent isolates. Amplified ITS regions were further sequenced and no genetic diversity was found for this genomic region among 17 Portuguese isolates (Madeira Island and Continental Portugal). Phylogenetic analysis revealed that Portuguese isolates grouped with isolates from China, Korea and one isolate from Japa