In the last two years or so, I was most fortunate in being given
opportunities of lecturing on a new methodology to a variety of
audiences in Britain, China, Finland, France and Spain. Despite my
almost Confucian attitude of preferring talking (i.e. a transient
record) to writing (i.e. a permanent record), the warm encouragement of
friends has led to the ensuing notes. I am also only too conscious of
the infancy of the methodology introduced in these notes. However, it is
my sincere hope that exposure to a wider audience will accelerate its
maturity. Readers are assumed to be familiar with the basic theory of
time series analysis. The book by Professor M.B. Priestley (1981) may be
used as a general reference. Chapter One is addressed to the general
question: "why do we need non-linear time series models?" After
describing some significant advantages of linear models, it singles out
several major limitations of linearity. Of course, the selection
reflects my personal view on the subject, which is only at its very
beginning, although there does seem to be a general agreement in the
literature that time irr'eversibility and limit cycles are among the
most obvious.