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The Journal of Physics A: Mathematical and Theoretical publishing team would like to apologise to the the author of the above paper. Due to a technical error, the printed version of the article was published with typographical mistakes. The online version is unaffected, so we would like to direct readers to this version, which can be found at: iopscience.org/jphysa/44/11/113001.
Emil Prodan (Tue,) studied this question.