\chapter*{Acknowledgment}
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I would like to express my deep sense of gratitude to {\bf Prof. H. Narayanan} and {\bf Prof. Madhav P. Desai}, for their invaluable help and guidance during
the course of project.  I am highly indebted to them for constantly encouraging me by giving their critics on my work. I am grateful to them 
for having given me the support and confidence.
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Gaurav Trivedi\\
May 2000\\
{Indian Institute of Technology, Bombay}\\
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