Bull. Korean Math. Soc. 2013; 50(3): 1021-1027
Printed May 31, 2013
https://doi.org/10.4134/BKMS.2013.50.3.1021
Copyright © The Korean Mathematical Society.
Dragan S. Djordjevi\' c and Milica Z. Kolund\v zija
University of Ni\v s, University of Ni\v s
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