Discussiones Mathematicae - General Algebra and Applications
Discussiones Mathematicae - General Algebra and Applications > 2000 > 20 > 2 > 255-265
Discussiones Mathematicae - General Algebra and Applications > 2002 > 22 > 2 > 107-117
Discussiones Mathematicae - General Algebra and Applications > 2003 > 23 > 2 > 125-137