1. Mykolas Romeris University, Ateities g. 20, LT-08303 Vilnius, Lithuania; Institute of Mathematics and Informatics, Akademijos g. 4, LT-08663 Vilnius, Lithuania
2. Institute of Mathematics and Informatics, Akademijos g. 4, LT-08663 Vilnius, Lithuania
3. Carl Bro a/s, Granskoven 8, DK-2600 Glostrup, Denmark