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This paper has been withdrawn by the author, since some results of the paper have been already obtained in [J.~Lewandowski, Class. Quantum Grav. 9 (1992), no. 10, L147--L151.]

This paper has been withdrawn by the author due a few mistakes in the paper.

This paper has been withdrawn.

This paper has been withdrawn by the author. Much simpler proof of the main result was obtained which led to major changes in the presentation.

This paper has been withdrawn by the author.

This paper has been withdrawn by the author becouse the conjecture presented in this paper is false. The correct study of metrics in interpolation spaces for and applcation to nonharmonic Fourier series will be published in S.Petersburg…

This paper has been withdrawn at the author's request.

This paper has been withdrawn by the author.

The paper has been withdrawn because the proof of part (b) of the main theorem is incomplete.

This paper has been withdrawn by the author, due to a crucial error in the proof of Thm.1

The paper has been withdrawn by the author.

This paper has been withdrawn by the author because it needs to be rewritten completely.

Paper withdrawn by the author.

This paper has been withdrawn by the authors, because of a crucial gap in the proof of the main theorem.

This paper has been withdrawn by the authors due to an error.

This article has been withdrawn due to an error in a proof of the main result.

The paper has been withdrawn by the author due to a crucial error.

This paper has been withdrawn by the author.

This paper has been withdrawn by the author, due to an error in the proof of Theorem 3.8.

This paper has been withdrawn by the author, due to a crucial error in page 5.