Submitted on: 28 March 2021.
Revised on: 04 June 2021.
Accepted on: 22 June 2021.
___________________Izvestiya of Saratov University Mathematics Mechanics Informatics
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Submitted on: 11 July 2018.
Revised on: 13 October 2018.
Accepted on: 18 November 2018.
___________________Izvestiya of Saratov University Mathematics Mechanics Informatics
Submitted on: 05 January 2025.
Revised on: 06 February 2025.
Accepted on: 16 March 2025.
___________________Izvestiya of Saratov University Mathematics Mechanics Informatics
Submitted on: 15 June 2024.
Revised on: 26 July 2024.
Accepted on: 12 August 2024.
___________________Izvestiya of Saratov University Mathematics Mechanics Informatics
Submitted on: 26 January 2010.
Revised on: 14 May 2010.
Accepted on: 20 June 2010.
___________________Izvestiya of Saratov University Mathematics Mechanics Informatics
Submitted on: 07 October 2014.
Revised on: 19 December 2014.
Accepted on: 07 February 2015.
___________________Izvestiya of Saratov University Mathematics Mechanics Informatics
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