Shielded Base Contraction (Extended Abstract)

In this paper we study a kind of non-prioritized contraction operator on belief bases -known as shielded base contractions. We propose twenty different classes of shielded base contractions and obtain axiomatic characterizations for each one of them. Additionally we thoroughly investigate the interrelations (in the sense of inclusion) among all those classes.

How Many Urban Brownfields are Out There?: An Economic Base Contraction Analysis of 31 U.S. Cities

How many brownfield sites are there in the United States? Although numerous federal and state lists of contaminated lands are known—totaling more than 380,000 sites—there is no comprehensive estimate of unlisted or total brownfield sites. This article uses economic base contraction analysis to provide an estimate of the number and acreage of brownfield sites, by type and as a percentage of the land, in 31 large cities in the United States. This approach recognizes that brownfields are the outcome of years of decline in central-city manufacturing, trade, transportation, and residential uses. Using a moderately restrictive definition of brownfield, there are an estimated 75,000 formerly industrial brownfield sites in these U.S. central cities, on 93,000 acres. This is about 5% of the land area in these communities. Another 20,000 acres are present in these same cities in the form of residential brownfields. These findings imply that the overall number of nonresidential brownfields sites in the United States is at least 500,000 to 600,000 or more.

Theory contraction and base contraction unified

One way to construct a contraction operator for a theory (belief set) is to assign to it a base (belief base) and an operator of partial meet contraction for that base. Axiomatic characterizations are given of the theory contractions that are generated in this way by (various types of) partial meet base contractions.