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Submitted on: 24 December 2008.
Revised on: 13 February 2009.
Accepted on: 30 March 2009.
___________________ADVANCES IN GEOMETRY---WALTER DE GRUYTER GMBH
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Submitted on: 09 January 2023.
Revised on: 29 March 2023.
Accepted on: 22 April 2023.
___________________ADVANCES IN GEOMETRY---WALTER DE GRUYTER GMBH