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Submitted on: 24 December 2002.
Revised on: 20 March 2003.
Accepted on: 25 April 2003.
___________________Bulletin of the Karaganda University-Mathematics
Submitted on: 15 February 2009.
Revised on: 14 March 2009.
Accepted on: 06 April 2009.
___________________Bulletin of the Karaganda University-Mathematics
Submitted on: 08 January 2018.
Revised on: 15 February 2018.
Accepted on: 02 March 2018.
___________________Bulletin of the Karaganda University-Mathematics
Submitted on: 08 November 2018.
Revised on: 07 February 2019.
Accepted on: 26 February 2019.
___________________Bulletin of the Karaganda University-Mathematics
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Submitted on: 29 April 2014.
Revised on: 10 June 2014.
Accepted on: 30 July 2014.
___________________Bulletin of the Karaganda University-Mathematics
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Submitted on: 28 November 1998.
Revised on: 24 December 1998.
Accepted on: 21 February 1999.
___________________Bulletin of the Karaganda University-Mathematics
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