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As Voss and Clarke[50] point out many fluctuating signals can
be characterized by a single correlation time . In which
case, for
time scales much smaller than (which means for frequencies
much larger than ) the signal is correlated and the
power spectrum's slope is close to that of the
line, and in the
regions much bigger than (
) the
spectrum is similar to that of white noise. However,
a signal which behaves like
noise cannot be characterized by a single correlation time.
In fact a spectrum with a slope implies a scale-invariant correlation
between long-term and short-term correlation in the region in which the spectrum
is exhibiting the slope.

Shahrokh Yadegari
2001-03-01