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Mathematica Slovaca 65 (2015) 1509-1516
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Applied Mathematics Letters 25 (2012) 843-846
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Rendiconti del Circolo Matematico di Palermo 60 (2011) 323-327
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Applied Mathematics Letters 24 (2011) 1037-1040
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Applied Mathematics Letters 23 (2010) 498-502
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