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This paper has been withdrawn by the authors. Significantly revised versions of the results of this paper are now available in arXiv:0707.0487v2 and arXiv:0808.3169v1.

This paper has been withdrawn by the authors, because of a crucial gap in the proof of the main theorem.

This paper has been withdrawn by the authors.

This paper has been withdrawn by the authors due to a crucial error.

This paper has been withdrawn. The authors realized that the obtained results were not new.

This paper has been withdrawn by the authors due to an error.

This paper has been withdrawn by the author. It will be replaced, substantially modified, by sections of the author's completed PhD thesis.

This paper is withdrawn because the results in the paper are included in a paper to be published in Mathematical and Computer Modelling.

This paper has been withdrawn by the author(s) in the light of several other works available and due to a misunderstanding in the authorships.

This paper has been withdrawn by the author [arXiv admin].

This paper has been withdrawn by the author(s)

This paper has been withdrawn by the author.

This paper has been withdrawn.

This paper has been withdrawn by the author, due to a crucial error in page 5.

This paper has been withdrawn by the author.

This paper has been withdrawn because the content has been substantially improved in a later paper, arXiv:0806.1165.

The paper was withdrawn because of its significant overlap with a paper appeared recently.

This paper has been withdrawn by the author.

The paper is withdrawn by the authors and replaced be an improved and extended version arxiv: 0812.2968

This paper has been withdrawn by the authors due to new experimental results.