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Open Mathematics > 2007 > 5 > 3 > 484-492
Central European Journal of Mathematics > 2007 > 5 > 3 > 484-492
Discussiones Mathematicae - General Algebra and Applications > 2000 > 20 > 1 > 87-95
Czechoslovak Mathematical Journal > 1997 > 47 > 1 > 163-172