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Submitted on: 11 July 2005.
Revised on: 19 August 2005.
Accepted on: 25 August 2005.
Quaestiones Mathematicae
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Submitted on: 20 April 2006.
Revised on: 24 May 2006.
Accepted on: 10 June 2006.
Quaestiones Mathematicae---TAYLOR & FRANCIS LTD
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