An Empirical Study of Price Correlations: 2. The Decrease in Price Correlation with Distance and the Concept of Correlation Length
The dependence of the correlation between wheat price fluctuations on different markets with respect to the distance between those markets is investigated. It is shown that the decrease in the correlation is exponential and is governed by a characteristic distance which is called the correlation length for wheat prices. This is a measure of the level of market integration in a given area. The magnitude of the correlation length is compared for different cereals: wheat, rye, and oats, and the evolution of the correlation length during the 19th century is studied. In particular, it is observed that subsistence crises are characterised by a collective behaviour of the economy which results in a sudden peak in the correlation length. Last, the part played by the spatial correlation between precipitations is investigated. It appears to be rather small; consequently, the price-correlation length is primarily the result of economic factors.
Evaluating the friction of distance parameter in gravity models
Cliff A. D., Martin R. L. and Ord J. K. (1974) Evaluating the friction of distance parameter in gravity models, Reg. Studies 8, 281–286. In this paper we consider whether map pattern (spatial autocorrelation) among the population values in constrained and unconstrained gravity models calibrated by least squares regression hinders interpretation of the coefficient of the distance term. It is shown that, except in certain cases which are most likely to occur in intra-, rather than inter-, urban models, no real problem of interpretation should arise. This finding differs to some extent from those of Curry (1972) and Johnston (1973). The conclusions are supported by both theoretical and empirical results.
Miniband formulation in Ge/Si quantum dot array
In this work, we estimate the coupling effect of miniband structure and density of states (DoS) resulting from different qunatum-dot physical parameters in a well-aligned Ge/Si quantum dot (QD) array fabricated by neutral beam etching technology. The density of QDs dominates the coupling effect and miniband’s bandwidth, the radius of QDs affects the magnitude of energy levels and miniband bandwidth, and the thickness of QDs has a great impact on the magnitude of energy levels. Among the different shapes of Ge/Si QDs, discoid QDs exhibit the most band crossing and broadest bandwith under the same physical parameters.