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A. Sokolovs
Riga Technical University 1 Kalku Str., Riga, LV-1658, LATVIA
L. Grigans
Riga Technical University 1 Kalku Str., Riga, LV-1658, LATVIA
Institute of Physical Energetics 21 Aizkraukles Str., Riga, LV-1006, LATVIA
E. Kamolins
Riga Technical University 1 Kalku Str., Riga, LV-1658, LATVIA
J. Voitkans
Riga Technical University 1 Kalku Str., Riga, LV-1658, LATVIA