The original version of the article was published in Central European Journal of Physics 9, 45–48 (2011), DOI: 10.2478/s11534-010-0061-5. Unfortunately, due to an editorial processing error the original version of this article contains a mistake in Eq. (5). Here we display the corrected version of this equation.
 G. Rosen, Cent. Eur. J. Phys. 9, 45 (2011) http://dx.doi.org/10.2478/s11534-010-0061-5[Crossref]
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