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Submitted on: 16 December 2004.
Revised on: 25 January 2005.
Accepted on: 09 February 2005.
Latvian Journal of Physics and Technical Sciences
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Submitted on: 02 March 2017.
Revised on: 04 April 2017.
Accepted on: 30 April 2017.
Latvian Journal of Physics and Technical Sciences
Submitted on: 24 March 2019.
Revised on: 03 May 2019.
Accepted on: 19 May 2019.
Latvian Journal of Physics and Technical Sciences
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