Non-linear stability problems formulated in terms of non-linear partial
differential equations have only recently begun to attract attention and
it will probably take some time before our understanding of those
problems reaches some degree of maturity. The passage from the more
classical linear analysis to a non-linear analysis increases the
mathematical complexity of the stability theory to a point where it may
become discouraging, while some of the more usual mathematical methods
lose their applicability. Although considerable progress has been made
in recent years, notably in the field of fluid mechanics, much still
remains to be done before a more permanent outline of the subject can be
established. I have not tried to present in this monograph an account of
what has been accomplished, since the rapidly changing features of the
field make the periodical literature a more appropriate place for such a
review. The aim of this book is to present one particular line of
research, originally developed in a series of papers published in
'Journal de Mecanique' 1962-1963, in which I attempted to construct a
mathematical theory for certain classes of non-linear stability
problems, and to gain some understanding of the non-linear phenomena
which are involved. The opportunity to collect the material in this
volume has permitted a more coherent presentation, while various points
of the analysis have been developed in greater detaiL I hope that a more
unified form of the theory has thus been achieved.