Social Networks and Interactions in Cities

We examine how interaction choices depend on the interplay of social and physical distance, and show that agents who are more central in the social network, or are located closer to the geographic center of interaction, choose higher levels of interactions in equilibrium. As a result, the level of interactivity in the economy as a whole will rise with the density of links in the social network and with the degree to which agents are clustered in physical space. When agents can choose geographic locations, there is a tendency for those who are more central in the social network to locate closer to the interaction center, leading to a form of endogenous geographic separation based on social distance. Finally, we show that the market equilibrium is not optimal because of social externalities. We determine the value of the subsidy to interactions that could support the first-best allocation as an equilibrium and show that interaction effort and the incentives for clustering are higher under the subsidy program.social networks, urban-land use, Bonacich centrality

Sprawl and Blight

The objective of this paper is to show how the same market failures that contribute to urban sprawl also contribute to urban blight. The paper develops a simple dynamic model in which new suburban and older central-city properties compete for mobile residents. The level of housing services generated by older properties depends on current maintenance or reinvest-ment expenditures. In this setting, market failures that reduce the cost of occupying suburban locations, thus leading to excessive suburban development, also depress central-city housing prices and undermine maintenance incentives, leading to deficient levels of central-city rein-vestment. Corrective policies that shift population from the suburbs to the center result in higher levels of reinvestment in central-city housing, therefore reducing blight.urban sprawl, blight, market failures

Sprawl and Blight

The objective of this paper is to show how the same market failures that contribute to urban sprawl also contribute to urban blight. The paper develops a simple dynamic model in which new suburban and older central-city properties compete for mobile residents. The level of housing services generated by older properties depends on current maintenance or reinvestment expenditures. In this setting, market failures that reduce the cost of occupying suburban locations, thus leading to excessive suburban development, also depress central-city housing prices and undermine maintenance incentives, leading to deficient levels of central-city reinvestment. Corrective policies that shift population from the suburbs to the center result in higher levels of reinvestment in central-city housing, therefore reducing blight.Urban sprawl; Urban blight; Market failures

Integrated digital/electric aircraft concepts study

The integrated digital/electrical aircraft (IDEA) is an aircraft concept which employs all electric secondary power systems and advanced digital flight control systems. After trade analysis, preferred systems were applied to the baseline configuration. An additional configuration, the alternate IDEA, was also considered. For this concept the design ground rules were relaxed in order to quantify additional synergistic benefits. It was proposed that an IDEA configuration and technical risks associated with the IDEA systems concepts be defined and the research and development required activities to reduce these risks be identified. The selected subsystems include: power generation, power distribution, actuators, environmental control system and flight controls systems. When the aircraft was resized, block fuel was predicted to decrease by 11.3 percent, with 7.9 percent decrease in direct operating cost. The alternate IDEA shows a further 3.4 percent reduction in block fuel and 3.1 percent reduction in direct operating cost

Heterogeneous Agglomeration

Many prior treatments of agglomeration explicitly or implicitly assume that all industries agglomerate for the same reasons. This paper uses U.K. establishment-level coagglomeration data to document substantial heterogeneity across industries in the microfoundations of agglomeration economies. It finds robust evidence of organizational and adaptive agglomeration forces as discussed by Chinitz (1961), Vernon (1960), and Jacobs (1969). These forces interact with the traditional Marshallian (1890) factors of input sharing, labor pooling, and knowledge spillovers, establishing a previously unrecognized complementarity between the approaches of Marshall and Jacobs, as well as others, to the analysis of agglomeration

Palaeomagnetism and magnetostiatigraphy of Triassic strata in the Sangre de Cristo Mountains and Tucumcari Basin, New Mexico, USA

We report palaeomagnetic data and a composite magnetic polarity sequence for Middle and Upper Triassic rocks assigned to the Anton Chico Member of the Moenkopi Formation and Chinle Group, respectively, exposed along the eastern flank of the Sangre de Cristo Mountains and in the Tucumcari Basin of eastern and northeastern New Mexico. Thermal demagnetization isolates a well-defined, dual polarity, characteristic magnetization, carried in most cases by haematite and interpreted as an early acquired chemical remanent magnetization (CRM). Characteristic magnetizations from 74 palaeomagnetic sites (one site = one bed) are used to define a magnetic polarity sequence, which we correlate with previously published Triassic data obtained from both marine and non-marine rocks. Preliminary correlation suggests that the resolution of magnetostratigraphic data derived from continental strata is not necessarily of lesser quality than that from marine rocks. On the basis of the magnetostratigraphic data, a profound unconformity is believed to separate lower-middle Norian and upper Norian-Rhaetian strata of the Chinle Group. Palaeomagnetic poles derived from selected sites in steeply dipping (> 85°) strata for the Middle Triassic (Anisian, ∼240 Ma: 50°N 121°E; N = 8), late Carman-early Norian (∼225 Ma: 53°N 104°E; N = 16), and late Norian-Rhaetian (∼208 Ma: 59°N 77°E; N = 8) are in relatively good agreement with previously published data for the Moenkopi Formation and Chinle Group and related strata in southwest North America. None the less, comparison with palaeomagnetic poles obtained from gently dipping or flat-lying Triassic strata from this study (Anisian, 46°N 112°E; N = 13; late Carnian, 54°N 87°E; N =12) and previously published Triassic poles in southwest North America suggest that a modest ‘apparent rotation’ not greater than about 5° affects declinations from steeply dipping rocks. The distribution of palaeomagnetic poles indicates ∼25° (angular distance) of apparent polar wander between about 240 and 208 Ma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73972/1/j.1365-246X.1996.tb05646.x.pd