Submitted on: 16 May 2002.
Revised on: 10 September 2002.
Accepted on: 11 October 2002.
___________________BULLETIN OF THE AUSTRALIAN MATHEMATICAL SOCIETY
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Submitted on: 21 May 2004.
Revised on: 12 June 2004.
Accepted on: 21 July 2004.
___________________BULLETIN OF THE AUSTRALIAN MATHEMATICAL SOCIETY
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Submitted on: 18 November 2007.
Revised on: 08 January 2008.
Accepted on: 16 February 2008.
___________________BULLETIN OF THE AUSTRALIAN MATHEMATICAL SOCIETY
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