Generating conjecture and Einstein-Maxwell field of plane symmetry

For the plane symmetry we have found the electro-vacuum exact solutions of the Einstein-Maxwell equations and we have shown that one of them is equivalent to the McVittie solution of a charged infinite thin plane. The analytical extension has been accomplished and the Penrose conformal diagram has been obtained as well.Comment: 11 pages, 6 figure

A charged rotating cylindrical shell

We give an example of a spacetime having an infinite thin rotating cylindrical shell constituted by a charged perfect fluid as a source. As the interior of the shell the Bonnor--Melvin universe is considered, while its exterior is represented by Datta--Raychaudhuri spacetime. We discuss the energy conditions and we show that our spacetime contains closed timelike curves. Trajectories of charged test particles both inside and outside the cylinder are also examined. Expression for the angular velocity of a circular motion inside the cylinder is given.Comment: 14 pages, 4 figures, minor corrections, to appear in Gen.Rel.Gra

The Internalization of Advertising Services: An Inter-IndustryAnalysis

The common perception appears to be that vertical integration of advertising services is more the exception than the rule in the U.S. advertising industry. This study investigates the extent of such outsourcing and examines inter-industry variation in the use of in-house rather than independent advertising agencies by U.S. advertisers. While the vast majority of large advertisers employ outside agencies, it comes as a surprise to find that when advertisers of all sizes are considered, about half operate some form of in-house agency. Internalization of advertising services is much more widespread than has hitherto been appreciated and varies widely across industries. To explain this variation, we draw on concepts from research on scale economies and transaction costs to develop a set of hypotheses which we test in cross sectional analyses of data covering 69 two digit SIC industries at two points in time, 1991 and 1999. Across industries, we find that the likelihood of internalization of advertising services decreases as the size of advertising outlays increase but increases as advertising intensity and technological intensity increase and is greater for "creative" industries.Advertising Agencies, In-House, Vertical Integration, Make or Buy