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DISCRETE AND CONTINUOUS DYNAMICAL SYSTEMS, vol. 23, no. 3, page. 867 - 885, 2009-03

COMMUNICATIONS IN MATHEMATICAL SCIENCES, vol. 7, no. 1, page. 189 - 209, 2009-03

MATHEMATICAL AND COMPUTER MODELLING, vol. 47, no. 11-12, page. 1314 - 1322, 2008-06

JOURNAL OF THE KOREAN MATHEMATICAL SOCIETY, vol. 42, no. 1, page. 65 - 83, 2005-11