The effect of a certain sampling pattern in the frequency analysis is
particularly transparent for the power spectrum. Let *s* be the
sampling function taking on the value 1 at the (unevenly spaced) times
of the observations observation and 0 elsewhere. The power spectrum of
the sampling function

= | (12.11) |

is an ordinary, non-random function called the spectral window function. The discrete observations are the product of

(12.12) |

In the opposite case of strong interference, ghost patterns may arise
in the power spectrum due to interference of window function patterns
belonging to positive as well as negative frequencies. The ghost
patterns produced at frequencies nearby or far from the true frequency
are called *aliases* and *power leaks*, respectively.