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Journal of Mathematics and Applications > 2018 > Vol. 41 > 39--51
Fasciculi Mathematici > 2009 > Nr 42 > 85-89
Demonstratio Mathematica > 2009 > Vol. 42, nr 3 > 581-598
Fasciculi Mathematici > 2002 > Nr 33 > 27-35
Fasciculi Mathematici > 2001 > Nr 31 > 5-22