Submitted on: 07 February 2009.
Revised on: 23 March 2009.
Accepted on: 16 May 2009.
___________________Electronic Markets---SPRINGER HEIDELBERG
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Submitted on: 30 September 2005.
Revised on: 17 November 2005.
Accepted on: 23 December 2005.
___________________Electronic Markets---SPRINGER HEIDELBERG
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