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In **FGDP**
I used *R*(*Z*) to denote a reflection seismogram,
an impedance function,
and a spectrum with autocorrelation coefficients *r*_{t}.
I liked this classic notation,
which was used by the mathematicians
Wiener and Levinson.
It is confusing, however, to use in one equation
*r*_{t} both for the causal, one-sided, reflection seismogram
and for the two-sided autocorrelation.
Thus I have introduced *S*, which is a natural notation for spectrum,
although *s* is admittedly less natural for autocorrelation.

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Stanford Exploration Project

10/21/1998