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Advances in Difference Equations > 2013 > 2013 > 1 > 1-11
Nonlinear Analysis > 2012 > 75 > 1 > 177-181
Mediterranean Journal of Mathematics > 2010 > 7 > 1 > 1-18
Acta Mathematica Sinica, English Series > 2008 > 24 > 1 > 95-106