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Journal of the Egyptian Mathematical Society > 2017 > 25 > 1 > 43-47
Studia Mathematica > 1998 > 131 > 2 > 143-148
Journal of the Egyptian Mathematical Society > 2017 > 25 > 1 > 43-47
Studia Mathematica > 1998 > 131 > 2 > 143-148