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Aequationes mathematicae > 2015 > 89 > 1 > 161-167
Aequationes mathematicae > 2015 > 89 > 3 > 937-948
Acta Mathematica Hungarica > 2013 > 141 > 1-2 > 68-77
Journal of Mathematical Analysis and Applications > 2011 > 384 > 2 > 273-283