@article {Bulg. J. Phys. 41,
	journal = {Bulg. J. Phys.},
	volume = {41},
	year = {2015},
	month = {Oct},
	pages = {387},
	publisher = {Heron Press Ltd.},
	url = {http://www.bjp-bg.com/paper2.php?id=774},
	author = {F.R. Xu, X.M. Fu, W.Y. Liang,}
}
